QuikSCAT Publications

Morey2006

Modeling studies of the upper ocean response to a tropical cyclone
S. L. Morey and M. A. Bourassa and D. S. Dukhovskoy and J. J. O'Brien
OCEAN DYNAMICS  56  594--606  (2006)

A coupled ocean and boundary layer flux numerical modeling system is used to study the upper ocean response to surface heat and momentum fluxes associated with a major hurricane, namely, Hurricane Dennis (July 2005) in the Gulf of Mexico. A suite of experiments is run using this modeling system, constructed by coupling a Navy Coastal Ocean Model simulation of the Gulf of Mexico to an atmospheric flux model. The modeling system is forced by wind fields produced from satellite scatterometer and atmospheric model wind data, and by numerical weather prediction air temperature data. The experiments are initialized from a data assimilative hindcast model run and then forced by surface fluxes with no assimilation for the time during which Hurricane Dennis impacted the region. Four experiments are run to aid in the analysis: one is forced by heat and momentum fluxes, one by only momentum fluxes, one by only heat fluxes, and one with no surface forcing. An equation describing the change in the upper ocean hurricane heat potential due to the storm is developed. Analysis of the model results show that surface heat fluxes are primarily responsible for widespread reduction (0.5 degrees-1.5 degrees C) of sea surface temperature over the inner West Florida Shelf 100-300 km away from the storm center. Momentum fluxes are responsible for stronger surface cooling (2 degrees C) near the center of the storm. The upper ocean heat loss near the storm center of more than 200 MJ/m(2) is primarily due to the vertical flux of thermal energy between the surface layer and deep ocean. Heat loss to the atmosphere during the storm's passage is approximately 100-150 MJ/m(2). The upper ocean cooling is enhanced where the preexisting mixed layer is shallow, e.g., within a cyclonic circulation feature, although the heat flux to the atmosphere in these locations is markedly reduced.

Song2006

Wind vector retrieval using dual polarization imagery of ASAR
G. T. Song and Y. J. Hou and P. Qi
PROGRESS IN NATURAL SCIENCE  16  1183--1187  (2006)

A new algorithm for retrieving wind vector using dual polarization imagery of advanced synthetic aperture radar (ASAR) is developed and tested. Based on the combination of co-polarization and cross-polarization, this new algorithm effectively eliminates the 180 degrees ambiguity which occurs when using single imagery of the ASAR to retrieve wind vector. This algorithm also solves the problem that the retrieval results will break clown on very small spatial scales. The results retrieved from dual polarization imagery of ASAR show that the wind speed and direction are in agreement with the data from Quikscat and buoy measurements. The root mean square errors of wind direction and speed between the retrieved results and data from Quikscat are 2.21 degrees and 0.53 m/s, respectively. Comparisons between the new method and the traditional method show that the new methods fit the buoy data and Quikscat data a little better than the traditional method in this image.

Song2006

Surface wind response to oceanic fronts
Q. T. Song and P. Cornillon and T. Hara
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  111    (2006)

[1] The response of surface winds to ocean fronts characterized by sharp gradients in both sea surface temperature (SST) and ocean currents was analyzed using scatterometer (NSCAT and QuikSCAT) wind data and Gulf Stream path positions in conjunction with simulations made with the Pennsylvania State University (PSU)-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). All match-ups, between each scatterometer pass and the Gulf Stream path, were visually examined and only those for which the wind field was free of atmospheric fronts or large curvature over a reasonably straight segment of the Gulf Stream were selected. Ten match-ups met these criteria for the period studied from 16 September 1996 to 29 June 1997 for NSCAT and from 24 July 1999 to 31 December 2000 for QuikSCAT. Changes in the modeled surface wind field across the front in each of the ten cases agree well with changes in the observed winds. Our findings suggest that the perturbation pressure gradient resulting from the thermal forcing by the front accounts for the decrease in wind speed when moving from warm to cold water and the increase observed in the converse. In the cases examined, the adjustment of the surface wind to the front occurred as a result of the vertical motion induced by horizontal divergence/convergence and advection in the marine atmospheric boundary layer (MABL). The dynamical forcing associated with strong surface currents is also shown to modify scatterometer-derived winds. Finally the numerical simulations suggest that the dynamical and thermal effects are very nearly additive.

Risien2006

A satellite-derived climatology of global ocean winds
C. M. Risien and D. B. Chelton
REMOTE SENSING OF ENVIRONMENT  105  221--236  (2006)

An interactive Climatology of Global Ocean Winds (COGOW) is presented based on 5 years (August 1999-July 2004) of QuikSCAT satellite measurements of wind speed and direction 10 m above the sea surface. This climatology provides the first high spatial resolution, observationally based, online atlas of ocean winds. Users can retrieve climatological wind maps and wind statistics, both in tabular and graphical form, from the COGOW web-based atlas. The global coverage of these data provides highly accurate information about the wind statistics in regions of the world ocean that are sparsely sampled by ships and buoys. A case study of the recovery of the vessel Ehime Maru off the Hawaiian Island of Oahu is presented to demonstrate the usage and value of COGOW. Evidence of air sea interactions, one of many wind phenomena visible within COGOW, is discussed to further familiarize users with COGOW. Finally, the utility of COGOW with regard to various operational and research communities is summarized. (c) 2006 Elsevier Inc. All rights reserved.

Walker2006

Arctic sea ice type and concentration mapping using passive and active microwave sensors
N. P. Walker and K. C. Partington and M. L. Van Woert and T. L. T. Street
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  3574--3584  (2006)

The mapping of ice type concentrations in the Arctic is important for commercial operations and for climate-related research. Algorithms based on moderate-resolution passive microwave sensors for mapping first-year ice and multiyear ice concentrations suffer from a number of known problems. In this paper, it has been shown that QuikSCAT scatterometer data can add complimentary information to that available from passive microwave, which can assist in separating different ice classes. Specifically, we identify a class of ice that exhibits a passive microwave signature which is characteristic of first-year ice, but has a scatterometer signature which is typical of multiyear ice. We track the evolution. and distribution of this new ice class throughout the Arctic during the winter season of 2003-2004 and compare the results against the U.S. National Ice Center (NIC) ice charts. It has been found that the new ice class is predominantly multiyear ice and is especially prevalent in the Fram Strait and the high Arctic regions north of the islands Franz Josef Land and Severnaya Zemlya. A simple algorithm has been proposed that enables a passive microwave-based partial ice concentration algorithm (for example, the NT algorithm based on Special Sensor Microwave/Imager data) to be adapted using QuikSCAT seatterometer data, so that the new ice class is corrected from the first-year ice class to the multiyear ice category. The algorithm performance is measured against the NIC ice charts. We provide a discussion regarding the possible physical causes of the effects that have been observed and described.

Bordoni2006

Principal component analysis of the summertime winds over the Gulf of California: A gulf surge index
S. Bordoni and B. Stevens
MONTHLY WEATHER REVIEW  134  3395--3414  (2006)

A principal component analysis of the summertime near-surface Quick Scatterometer (QuikSCAT) winds is used to identify the leading mode of synoptic-scale variability of the low-level flow along the Gulf of California during the North American monsoon season. A gulf surge mode emerges from this analysis as the leading EOF, with the corresponding principal component time series interpretable as an objective index for gulf surge occurrence. This index is used as a reference time series for regression analysis and compositing meteorological fields of interest, to explore the relationship between gulf surges and precipitation over the core and marginal regions of the monsoon, as well as the manifestation of these transient events in the large-scale circulation. It is found that, although seemingly mesoscale features confined over the Gulf of California, gulf surges are intimately linked to patterns of large-scale variability of the eastern Pacific ITCZ and greatly contribute to the definition of the northward extent of the monsoonal rains.

Klinger2006

Monthly climatologies of oceanic friction velocity cubed
B. A. Klinger and B. H. Huang and B. Kirtman and P. Schopf and J. D. Wang
JOURNAL OF CLIMATE  19  5700--5708  (2006)

Different measures of wind influence the ocean in different ways. In particular, the time-averaged mixed layer turbulent energy production rate is proportional to < u(*)(3)>, where u(*) is the ``oceanic friction velocity'' that is based on wind stress. Estimating < u(*)(3)> from monthly averages of wind stress or wind speed may introduce large biases due to the day-to-day variability of the direction and magnitude of the wind. The authors create monthly climatologies of < u(*)(3)> from daily wind stress measurements obtained from the Goddard Satellite-based Surface Turbulent Fluxes version 2 (GSSTF2; based on satellite microwave measurements), the Quick Scatterometer (QuikSCAT; based on satellite scatterometry measurements), and the National Centers for Environmental Prediction (NCEP) reanalysis wind. The differences among zonal averages of these climatologies and of a similar climatology based on the da Silva version of the Comprehensive Ocean-Atmosphere Data Set (COADS) have a complex dependence on latitude. These differences are typically 10\%-30\% of the climatological values. The GSSTF2 data confirm that < u(*)(3)> is much larger than estimates from monthly averaged wind stress or wind speed, especially outside the Tropics.

Simon2006

Conditions leading to the onset of the Indian monsoon: A satellite perspective
B. Simon and S. H. Rahman and P. C. Joshi
METEOROLOGY AND ATMOSPHERIC PHYSICS  93  201--210  (2006)

The summer monsoon onset-2004 over the Kerala Coast (Southern tip of the Indian Peninsula) was monitored in real-time, using the Tropical Rainfall Measuring Mission (TRMM)/TMI derived total precipitable water vapor, wind speed and sea surface temperature (SST), National Centre for Environmental Prediction (NCEP) and QuikScat wind data. The 2004 onset was of a gradual type, with an early start (24 May), followed by slow growth to full strength (10 June). Hence, the unambiguous forecasting of such onsets becomes very difficult. The water vapor build up over the western Arabian Sea is one of the necessary conditions that gives us a lead time of two and half weeks for the onset of monsoon. The strength of the Hadley cell (monitored using NCEP meridional wind), which is associated with a large convective heat source is also used as a predictive parameter with a lead-time of two weeks. The other dynamical conditions considered are the early May propagation of the Madden Julian Oscillation (MJO) followed by a second MJO, which began in the Western Indian Ocean (WIO) and the kinetic energy over the South East Arabian Sea, with an early start around 24 May (50 m(2)/s(2)) and strengthening around 10 June (80 m2/s2). The setting of large-scale monsoon current using various satellite derived parameters and the distinct features for the year 2004 have been delineated.

Portabella2006

Scatterometer backscatter uncertainty due to wind variability
M. Portabella and A. Stoffelen
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  3356--3362  (2006)

Wind retrieval from scatterometer backscafter measurements is not trivial. A good assessment of the different measurement uncertainties inherent in scatterometer systems is very important for successful wind retrieval and quality control. One source of these uncertainties, i.e., geophysical noise, is dominated by the subcell wind variability. Although the latter is known to dominate the total measurement noise at low winds, no attempt to fully model such effect has yet been performed. In this paper, a simple method to derive a model of geophysical noise for the European Remote Sensing Satellite (ERS) scatterometer is proposed. It is assumed that this noise is mainly due to the spatial distribution of the backscatter footprints and the wind variability within the wind vector cell. In a simulation experiment these parameters were varied, and the values for which the simulation compares best to real data in the three-dimensional measurement space were selected. The resulting geophysical noise model is dependent on wind speed and across subsatellite track location. The empirical method presented here is straightforward and could be applied to other scatterometer systems.

Shen2006

A new hurricane wind retrieval algorithm for SAR images
H. Shen and W. Perrie and Y. J. He
GEOPHYSICAL RESEARCH LETTERS  33    (2006)

A new algorithm for hurricane wind retrieval is developed based on the primary characteristics of hurricane processes. By assuming constant wind speeds and equal-distant wind directions in three neighboring sub-image blocks of any specific concentric circle around the hurricane eye, both wind speed and wind direction are resolved without any external information. To evaluate the accuracy of the new algorithm, a Radarsat-1 synthetic aperture radar image of hurricane Isabel is studied. The retrieved wind speed is compared with QuikSCAT scatterometer winds and in situ winds. Good consistency is shown. Since the new algorithm doesn't rely on any external wind information, fine resolution wind structures can be retrieved without adding errors due to linear interpolation of the externally-derived coarse-resolution wind direction.

Frolking2006

Evaluation of the SeaWinds scatterometer for regional monitoring of vegetation phenology
S. Frolking and T. Milliman and K. McDonald and J. Kimball and M. S. Zhao and M. Fahnestock
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES  111    (2006)

[ 1] Phenology, or the seasonality of recurring biological events such as vegetation canopy development and senescence, is a primary constraint on global carbon, water and energy cycles. We analyzed multiseason Ku-band radar backscatter measurements from the SeaWinds-on-QuikSCAT scatterometer to determine canopy phenology and growing season vegetation dynamics from 2000 to 2002 at 27 sites representing major global land cover classes and regionally across most of North America. We compared these results with similar information derived from the MODIS leaf area index (LAI) data product (MOD-15A2). In site-level linear regression analysis, the correspondence between radar backscatter and LAI was significant ( p < 0.05) at most but not all sites and was generally higher (R-2 > 0.5) for sites with relatively low LAI or where the seasonal range in LAI was large (e.g., > 3 m(2) m(-2)). The SeaWinds instrument also detected generally earlier onset of vegetation canopy growth in spring than the optical/near-infrared (NIR) based LAI measurements from MODIS, though the timing of canopy senescence and the end of the growing season were more similar. Over North America, the correlation between the two time series was stratified largely by land cover class, with higher correlations (R similar to 0.7 - 0.9) for most cropland, deciduous broadleaf forest, crop/natural vegetation mosaic land cover, and some grassland. Lower correlations were observed for open shrubland and evergreen needleleaf forest. Overall, the results indicate that SeaWinds backscatter is sensitive to growing season canopy dynamics across a range of broadleaf vegetation types and provides a quantitative view that is independent of optical/NIR remote sensing instruments.

Kunz2006

Melt detection in Antarctic ice shelves using scatterometers and microwave radiometers
L. B. Kunz and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  2461--2469  (2006)

Ku-band dual-polarization radar backscatter measurements from the SeaWinds-on-QuikSCAT scatterometer are used to determine periods of surface freeze and melt in the Antarctic ice shelves. The normalized horizontal-polarization radar backscatter (sigma(o)) and backscatter polarization ratio are used in maximum-likelihood estimation of the ice state. This method is used to infer the daily ice-surface conditions for 25 study locations located on the Ronne, Ross, Larsen, Amery, Shackleton, and other ice shelves. The temporal and spatial variations of the radar response are observed for various neighborhood sizes surrounding each given location during the study period. Criteria for determining the dates of melt onset and freeze-up for each Austral summer are presented. Validation of the ice-state and melt-onset date estimates is performed by analyzing the corresponding brightness temperature (T-b) measurements from Special Sensor Microwave/Imager (SSM/I) radiometers. QuikSCAT sigma(o) measurements from 1999 to 2003 are analyzed and found to be effective in determining periods of melt in Antarctic ice sheets at high temporal and spatial resolutions. These estimates can be used in studies of the climatic effects of the seasonal and interannual melting of the Antarctic ice sheets.

VonAhn2006

Operational impact of QuikSCAT winds at the NOAA Ocean Prediction Center
J. M. Von Ahn and J. M. Sienkiewicz and P. S. Chang
WEATHER AND FORECASTING  21  523--539  (2006)

The NASA Quick Scatterometer (QuikSCAT) has revolutionized the analysis and short-term forecasting of winds over the oceans at the NOAA Ocean Prediction Center (OPC). The success of QuikSCAT in OPC operations is due to the wide 1800-km swath width, large retrievable wind speed range ( 0 to in excess of 30 m s(-1)), ability to view QuikSCAT winds in a comprehensive form in operational workstations, and reliable near-real-time delivery of data. Prior to QuikSCAT, marine forecasters at the OPC made warning and forecast decisions over vast ocean areas based on a limited number of conventional observations or on the satellite presentation of a storm system. Today, QuikSCAT winds are a heavily used tool by OPC forecasters. Approximately 10\% of all short-term wind warning decisions by the OPC are based on QuikSCAT winds. When QuikSCAT is available, 50\%-68\% of all weather features on OPC surface analyses are placed using QuikSCAT. QuikSCAT is the first remote sensing instrument that can consistently distinguish extreme hurricane force conditions from less dangerous storm force conditions in extratropical cyclones. During each winter season (October-April) from 2001 to 2004, 15-23 extratropical cyclones reached hurricane force intensity over both the North Atlantic and North Pacific Oceans. Due to QuikSCAT, OPC forecasters are now more likely to anticipate the onset of hurricane force conditions. QuikSCAT has also revealed significant wind speed gradients in the vicinity of strong sea surface temperature (SST) differences near the Gulf Stream and shelfbreak front of the western North Atlantic. These wind speed gradients are most likely due to changes in low-level stability of the boundary layer across the SST gradients. OPC forecasters now use a variety of numerical guidance based tools to help predict boundary layer stability and the resultant near-surface winds.

Luis2006

Characteristic patterns of QuikScat-based wind stress and turbulent heat flux in the tropical Indian Ocean
A. J. Luis and O. Isoguchi and H. Kawamura
REMOTE SENSING OF ENVIRONMENT  103  398--407  (2006)

Using QuikScat-based vector wind data for 1999-2003, surface wind stress and turbulent heat (Q) have been mapped for the tropical Indian Ocean (10) to understand their seasonal variability. During July wind stress is enhanced by similar to 70\% in the Arabian Sea compared to that during January. The Arabian Sea experiences a large Q loss (150-200 W/m(2)) during the summer and winter monsoons, which is nearly 1.3 times of that in the Bay of Bengal. The southeasterlies are strengthened during the southern hemisphere winter. Empirical Orthogonal Function analysis captures different phases of monsoon-induced variability in wind stress and Q, ranging from seasonal to high-frequency perturbations. Coherency between time coefficients of EOF-1 for wind stress and Q suggests that former leads the latter with a temporal lag of 20-40 days for period > 322 days. At high frequencies (< 21 days) Q leads wind stress with a temporal lag of 2 days. Possible explanation for wind stress leading Q over an annual time scale is offered based on the marine atmospheric boundary layer physics and pre-conditioned ocean surface, while on shorter time scales (21 days) ocean thermodynamics through mixed layer processes cause Q to lead wind stress. (c) 2006 Elsevier Inc. All rights reserved.

Alvarez2006

Use of MeteoGalicia wind data to monitor oil spills off the Galician coast: Comparison with QuikSCAT data
I. Alvarez and M. Gomez-Gesteira and M. deCastro and C. Moreira and E. M. Novoa and B. Diaz and J. L. G. Gesteira and C. F. Balseiro and R. Prego
CIENCIAS MARINAS  32  351--360  (2006)

The Galician coast has suffered the periodic impact of oil spills. Predicting the spill extension, the most probable points of impact and the arrival time is a process that should be carried out immediately to decrease the effect on marine life. In these situations, the use of particle tracking models is very important because they allow a fast execution taking into account different scenarios. These models, where the main forcing is the wind provided by meteorological models, depend to a great extent on the accuracy of meteorological predictions. The prediction provided by MeteoGalicia is compared with real data measured by the QuikSCAT satellite, showing a high correlation between both databases, although QuikSCAT data are slightly higher (about 15\%) in magnitude.

Fernandez2006

Dual-polarized C- and Ku-band ocean backscatter response to hurricane-force winds
D. E. Fernandez and J. R. Carswell and S. Frasier and P. S. Chang and P. G. Black and F. D. Marks
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  111    (2006)

[ 1] Airborne ocean backscatter measurements at C- and Ku-band wavelengths and H and V polarizations at multiple incidence angles obtained in moderate to very high wind speed conditions (25 - 65 m s(-1)) during missions through several tropical cyclones are presented. These measurements clearly show that the normalized radar cross sections (NRCS) response stops increasing at hurricane-force winds for both frequency bands and polarizations except for high incidence angles at C- band and H polarization. The results also show the mean NRCS departing from a power law behavior for all the presented frequency bands, polarizations, and incidence angles, suggesting a reduction in the drag coefficient. The overall flattening of the azimuthal response of the NRCS is also very apparent in all cases. A new set of geophysical model functions (GMFs) at C- and Kuband are developed from these direct ocean backscatter observations for ocean surface winds ranging from 25 to 65 m s(-1). The developed GMFs provide a much more accurate characterization of the NRCS versus wind speed and direction, and their implementation in operational retrieval algorithms from satellite-based scatterometer observations would result in better wind fields. The differences between these measurements and other currently available GMFs, such as QuikSCAT, NSCAT2, CMOD4, and CMOD5, are reported. The implementation of these GMFs in retrieval algorithms will result in better wind fields from satellite-based scatterometers measurements.

Chelton2006

On the use of QuikSCAT scatterometer measurements of surface winds for marine weather prediction
D. B. Chelton and M. H. Freilich and J. M. Sienkiewicz and J. M. Von Ahn
MONTHLY WEATHER REVIEW  134  2055--2071  (2006)

The value of Quick Scatterometer (QuikSCAT) measurements of 10-m ocean vector winds for marine weather prediction is investigated from two Northern Hemisphere case studies. The first of these focuses on an intense cyclone with hurricane-force winds that occurred over the extratropical western North Pacific on 10 January 2005. The second is a 17 February 2005 example that is typical of sea surface temperature influence on low-level winds in moderate wind conditions in the vicinity of the Gulf Stream in the western North Atlantic. In both cases, the analyses of 10-m winds from the NCEP and ECMWF global numerical weather prediction models considerably underestimated the spatial variability of the wind field on scales smaller than 1000 km compared with the structure determined from QuikSCAT observations. The NCEP and ECMWF models both assimilate QuikSCAT observations. While the accuracies of the 10-m wind analyses from these models measurably improved after implementation of the QuikSCAT data assimilation, the information content in the QuikSCAT data is underutilized by the numerical models. QuikSCAT data are available in near - real time in the NOAA/NCEP Advanced Weather Interactive Processing System (N-AWIPS) and are used extensively in manual analyses of surface winds. The high resolution of the QuikSCAT data is routinely utilized by forecasters at the NOAA/NCEP Ocean Prediction Center, Tropical Prediction Center, and other NOAA weather forecast offices to improve the accuracies of wind warnings in marine forecasts.

Howell2006

Application of a SeaWinds/QuikSCAT sea ice melt algorithm for assessing melt dynamics in the Canadian Arctic Archipelago
S. E. L. Howell and A. Tivy and J. J. Yackel and R. K. Scharien
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  111    (2006)

[ 1] A remotely sensed sea ice melt algorithm utilizing SeaWinds/QuikSCAT (QuikSCAT) data is developed and applied to sea ice the Canadian Arctic Archipelago (CAA) from 2000 to 2004. The extended advanced very high resolution radiometer Polar Pathfinder (APP-x) data set is used to identify spatially coupled relationships between sea ice melt and radiative forcings. In situ data from the Collaborative Interdisciplinary Cryospheric Experiment (C-ICE) (2000, 2001, and 2002) and the Canadian Arctic Shelf Exchange Study ( CASES) ( 2004) are used to validate APP-x data during the melt period. QuikSCAT-detected maps of melt onset, pond onset, and drainage are created from 2000 to 2004, and results indicate considerable interannual variability of melt dynamics within the CAA. In some years, melt stages are positively spatially autocorrelated, whereas other years exhibit a negative or no spatial autocorrelation. QuikSCAT-detected stages of melt are found to be influenced by interannual varying amounts and timing of radiative forcing making prediction difficult. The spatiotemporal variability of ice melt also influences the distribution of ice within the CAA. The lower-latitude regions of the CAA are shown to have accumulated increasing concentrations of multiyear ice from 2000 to 2005. This paper concludes with a discussion of the interplay between thermodynamic and dynamic sea ice processes likely to have contributed to this trend.

Feng2006

Assessment of wind-forcing impact on a global wind-wave model using the TOPEX altimeter
H. Feng and D. Vandemark and Y. Quilfen and B. Chapron and B. Beckley
OCEAN ENGINEERING  33  1431--1461  (2006)

The study presents assessment of an operational wave model (Wavewatch III), focusing upon the model sensitivity to wind-forcing products. Four wind fields are used to drive the model, including the NCEP/NCAR reanalysis and three other products that assimilate various satellite wind measurements having high spatial resolution, including the QuikSCAT scatterometer. Three wave field statistics: significant wave height, mean zero-crossing wave period, and mean square slope are compared with collocated TOPEX altimeter derivatives to gauge the relative skill of differing wind-forced model runs, as well as to demonstrate an extended use of the altimeter beyond simply supplying wave height for wave model validation and assimilation. Results suggest that model output is critically sensitive to choice of the wind field product. Higher spatial resolution in the wind fields does lead to improved agreement for the higher-order wave statistics. (C) 2005 Elsevier Ltd. All rights reserved.

Bartalis2006

Azimuthal anisotropy of scatterometer measurements over land
Z. Bartalis and K. Scipal and W. Wagner
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  2083--2092  (2006)

Studies of the Earth's land surface involving scatterometers; are becoming an increasingly important application field of microwave remote sensing. Similarly to scatterometer observations of ocean waves, the backscattering coefficient (sigma(o)) response of land surfaces depends on both the incidence and azimuth angle under which the observations are made. In order to retrieve geophysical parameters from scatterometer data, it is necessary to account for azimuthal-modulation effects of the backscattered signal. In the present study, this paper localizes the regions affected by a strong azimuthal signal dependence when observed with the European Remote Sensing Satellite Scatterometer and the SeaWinds Scatterometer on QuikSCAT (QSCAT). The possible physical reasons for the azimuthal effects, relating the very detailed QSCAT azimuthal response to the spatial orientation of special topographic features and land cover within the sensor footprint, were then discussed. Different methods for normalizing the backscattering coefficient with respect of observation azimuth angle were also proposed and evaluated. First, the mean local incidence angle of the sensor footprint using the shuttle radar topography mission digital elevation model (DEM) were modeled and concluded that the resolution of the DEM is too coarse to characterize most of the observed azimuthal effects. A more effective way of normalizing the backscatter with respect to azimuth is then found to be by using historical backscatter observations to statistically determine the expected backscatter at each observation azimuth and incidence angle as well as time of the year. The efficiency of this method is limited to the availability of past measurements for each location on the Earth.

Dwivedi2006

Influence of northeasterly trade winds on intensity of winter bloom in the Northern Arabian Sea
R. M. Dwivedi and M. Raman and S. Parab and S. G. P. Matondkar and S. Nayak
CURRENT SCIENCE  90  1397--1406  (2006)

Chlorophyll and wind pattern retrieved from remote sensing data have been used to study biological activity in the oceanic waters of Northern Arabian Sea (NAS) during February-March 2002-05. Occurrence of algal bloom in these waters during this period was noticed with the help of ship observations in the past. The same was detected from OCEANSAT I/OCM with time series chlorophyll images for January-March 2000. Occurrence of this bloom was later re-confirmed using OCM data in the subsequent years also. The time-series chlorophyll images established that the bloom develops every year during February-March. This period happens to coincide with the presence of northeasterly trade winds over the NAS. Two ship cruises were conducted with the help of research vessels FORV Sagar Sampada (SS-212 during 26 February-7 March 2003 and SS-222 during 21 February-11 March 2004) during this period at the bloom site. The aim was species identification of the bloom and to study various environmental parameters associated with the bloom. Two diverse situations in the context of biological activity were observed while collecting in situ data in 2003 and 2004. Distribution of the bloom was found uniform over a large area and concentration of phytoplankton was relatively higher in 2003. Compared to this, it was observed during the same period in 2004 that phytoplankton was distributed in scattered and small patches and its concentration was relatively less. Corresponding to this observation, it was noticed from the ship data that wind strength was significantly weaker and the oceanic waters were less turbulent in 2004 compared to the same in 2003. In light of this elementary observation, an attempt was made to observe variations in the wind pattern during 2003 and 2004 using QuikSCAT/SeaWinds scatterometer data. It could be established that occurrence of the bloom as well as the observed inter annual variability in chlorophyll pattern were coupled with prevailing trade winds. It was found that density of surface water increased (inversion) during this period, which could result in convective action and the observed bloom. The vertical density gradient revealed an increasing pattern with increase in wind speed. Moreover, it was observed that response of chlorophyll to acting wind force is delayed by one to two weeks. This led to an important inference that wind can be treated as a precursor to predict variations in chlorophyll pattern in the context of the observed event of the bloom.

Ashcraft2006

Comparison of methods for melt detection over Greenland using active and passive microwave measurements
I. S. Ashcraft and D. G. Long
INTERNATIONAL JOURNAL OF REMOTE SENSING  27  2469--2488  (2006)

Microwave measurements have been used in various studies to detect melt based on their sensitivity to liquid water present in snow. To contrast different melt detection methods used with different sensors, six different melt detection method/sensor combinations are compared using data for the summer of 2000. The sensors include the Special Spectral Microwave Imager (SSM/I), SeaWinds on QuikSCAT (QSCAT), and the European Remote Sensing (ERS) Advanced Microwave Instrument (AMI) in scatterometer mode. Existing melt detection methods are compared with melt detection based on a simple physical model. The model relates the moisture content and depth of a surface melt layer of wet snow to a single channel melt detection threshold. The model can be applied to both active and passive sensors and improves the consistency between brightness temperature (Tb) and normalized radar backscatter (su) based detection of melt. Model-based melt estimates from different sensors are highly correlated and do not exhibit the unnatural phenomenon observed with previous methods. Relative merits and limitations of the various methods are discussed.

Sasaki2006

Far-reaching Hawaiian Lee Countercurrent driven by wind-stress curl induced by warm SST band along the current
H. Sasaki and M. Nonaka
GEOPHYSICAL RESEARCH LETTERS  33    (2006)

[1] Two eddy-resolving ocean simulations are carried out to study local air-sea interaction associated with the Hawaiian Lee Countercurrent (HLCC), one forced by the QuikSCAT satellite wind field (QSCAT run) and the other by the NCEP/NCAR reanalysis (NCEP run). The simulated HLCC in the QSCAT run extends west-southwestward from the Hawaiian Islands much farther than that in the NCEP run. We attribute this difference to difference in the wind fields. In the QSCAT wind field, there exist band-like structures of positive and negative wind stress curls induced by warm sea surface temperature band associated with the HLCC; however, these features are not apparent in the reanalysis wind field. The QSCAT run provides a good example of local two-way air-sea interactions in association with the HLCC. Furthermore, interannual variations are suggested in both the simulated HLCC and wind fields over the HLCC.

deCastro2006

Hydrographic and atmospheric analysis of an autumnal upwelling event in the Ria of Vigo (NW Iberian Peninsula)
M. deCastro and A. W. Dale and M. Gomez-Gesteira and R. Prego and I. Alvarez
ESTUARINE COASTAL AND SHELF SCIENCE  68  529--537  (2006)

An autumnal upwelling event was observed in the Ria of Vigo (NW Iberian Peninsula) on 15th November 2001. This event was analyzed by means of thermohaline variables measured at CTD stations located in the study area, satellite sea surface temperature and wind data provided by QuikSCAT. Salinity and temperature distributions revealed that the upwelled water mass was Eastern North Atlantic Central Water (ENACW), typically observed during summer upwelling events. However, previous to the upwelling event, the characteristic autumnal body of water was recorded on 31st October 2001. Subsequent sea surface temperature and upwelling index corroborated the presence of an autumnal upwelling event of approximately 15 days duration. The probability of upwelling-favorable winds (from 1999 to 2004) was lower during autumn-winter than during spring-summer, although they may occur at any time of the year. Probabilities of similar to 45\% were calculated for February and November, with the highest probability (65\%) corresponding to July. (c) 2006 Elsevier Ltd. All rights reserved.

Caruso2006

Interannual variability of the Kuroshio intrusion in the South China Sea
M. J. Caruso and G. G. Gawarkiewicz and R. C. Beardsley
JOURNAL OF OCEANOGRAPHY  62  559--575  (2006)

The interannual variability of intrusions of the Kuroshio into the South China Sea (SCS) is investigated using satellite remote sensing data supported by in-situ measurements. The mesoscale circulation of the SCS is predominantly wind-forced by the northeast winter and southwest summer monsoons. Although the region has been studied extensively, considerable uncertainty remains about the annual and interannual mesoscale nature of the circulation. The frequency and characteristics of Kuroshio intrusions and their effect on circulation patterns in the northeast SCS are also not well understood. Satellite observations of Sea Surface Temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM) and the Advanced Very High Resolution Radiometer (AVHRR) and Sea Surface Height Anomalies (SSHA) from TOPEX/Poseidon for the period 1997-2005 are used here to analyze the annual and interannual variability in Kuroshio intrusions and their effects on the region. Analysis of SST and SSHA shows the formation and characteristics of intrusions vary considerably each year. Typically, the intrusion occurs in the central region of Luzon Strait and results in an anticyclonic circulation in the northeastern SCS. However, in some years, the intrusion is located in the northern portion of Luzon Strait and a cyclonic intrusion results. Wind stress and wind stress curl derived from the National Aeronautics and Space Administration (NASA) QuikSCAT satellite scatterometer are used to evaluate the relationship between wind stress or wind stress curl and the presence of winter Kuroshio intrusions into the SCS.

Luger2006

CO2 fluxes in the subtropical and subarctic North Atlantic based on measurements from a volunteer observing ship
H. Luger and R. Wanninkhof and D. W. R. Wallace and A. Kortzinger
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  111    (2006)

[ 1] Surface seawater pCO(2) and related parameters were measured at high frequency onboard the volunteer observing ship M/V Falstaff in the North Atlantic Ocean between 36 degrees and 52 degrees N. Over 90,000 data points were used to produce monthly CO2 fluxes for 2002/2003. The air-sea CO2 fluxes calculated by two different averaging schemes were compared. The first approach used gas transfer velocity determined from wind speed retrieved at the location of the ship and called colocated winds, while for the second approach a monthly averaged gas transfer velocity was calculated from the wind for each grid pixel including the variability in wind. The colocated wind speeds determined during the time of passage do not capture the monthly wind speed variability of the grid resulting in fluxes that were 47\% lower than fluxes using the monthly averaged wind products. The Falstaff CO2 fluxes were in good agreement with a climatology using averaged winds. Over the entire region they differed by 2 - 5\%, depending on the time-dependent correction scheme to account for the atmospheric in increase in pCO(2). However, locally the flux differences between the ship measurements and the climatology were greater, especially in regions north of 45 degrees N, like the eastern sector. A comparison of two wind speed products showed that the annual CO2 sink is 4\% less when using 6 hourly NCEP/NCAR wind speeds compared to the QuikSCAT wind speed data.

Maloney2006

An assessment of the sea surface temperature influence on surface wind stress in numerical weather prediction and climate models
E. D. Maloney and D. B. Chelton
JOURNAL OF CLIMATE  19  2743--2762  (2006)

The ability of six climate models to capture the observed coupling between SST and surface wind stress in the vicinity of strong midlatitude SST fronts is analyzed. The analysis emphasizes air-sea interactions associated with ocean meanders in the eastward extensions of major western boundary current systems such as the Gulf Stream, Kuroshio, and Agulhas Current. Satellite observations of wind stress from the SeaWinds scatterometer on NASA's Quick Scatterometer and SST from the Advanced Microwave Scanning Radiometer clearly indicate the influence of SST on surface wind stress on scales smaller than about 30 longitude x 10 latitude. Spatially high-pass-filtered SST and wind stress variations are linearly related, with higher SST associated with higher wind stress. The influence of SST on wind stress is also clearly identifiable in the ECMWF operational forecast model, having a grid resolution of 0.35 x 0.35 degrees (T511). However, the coupling coefficient between wind stress and SST, as indicated by the slope of the linear least squares fit, is only half as strong as for satellite observations. The ability to simulate realistic air-sea interactions is present to varying degrees in the coupled climate models examined. The Model for Interdisciplinary Research on Climate 3.2 (MIROC3.2) high-resolution version (HIRES) (1.1 degrees x 1.1 degrees, T106) and the NCAR Community Climate System Model 3.0 (1.4 degrees x 1.4 degrees, T85) are the highest-resolution models considered and produce the most realistic air-sea coupling associated with midlatitude current systems. Coupling coefficients between SST and wind stress in MIROC3.2\_HIRES and the NCAR model are at least comparable to those in the ECMWF operational model. The spatial scales of midlatitude SST variations and SST-induced wind perturbations in MIROC3.2\_HIRES are comparable to those of satellite observations. The spatial scales of SST variability in the NCAR model are larger than those in the ECMWF model and satellite observations, and hence the spatial scales of SST-induced perturbations in the wind fields are larger. It is found that the ability of climate models to simulate air-sea interactions degrades with decreasing grid resolution. SST anomalies in the GFDL Climate Model 2.0 (CM2.0) (2.0 degrees x 2.5 degrees), Met Office Third Hadley Centre Coupled Ocean-Atmosphere General Circulation Model (HadCM3) (2.5 degrees x 3.8 degrees), and MIROC3.2 medium-resolution version (MEDRES) (2.8 degrees x 2.8 degrees, T42) have larger spatial scales and are more geographically confined than in the higher-resolution models. The GISS Model E20/Russell (4.0 degrees x 5.0 degrees) is unable to resolve the midlatitude ocean eddies that produce prominent air-sea interaction. Notably, MIROC3.2\_MEDRES exhibits much weaker coupling between wind stress and SST than does the higher vertical and horizontal resolution version of the same model. GFDL CM2.0 and Met Office HadCM3 exhibit a linear relationship between SST and wind stress. However, coupling coefficients for the Met Office model are significantly weaker than in the GFDL and higher-resolution models. In addition to model grid resolution (both vertical and horizontal), deficiencies in the parameterization of boundary layer processes may be responsible for some of these differences in air-sea coupling between models and observations.

Miyama2006

Dynamics of biweekly oscillations in the equatorial Indian Ocean
T. Miyama and J. P. McCreary and D. Sengupta and R. Senan
JOURNAL OF PHYSICAL OCEANOGRAPHY  36  827--846  (2006)

Variability of the wind field over the equatorial Indian Ocean is spread throughout the intraseasonal (10 - 60 day) band. In contrast, variability of the near-surface nu field in the eastern, equatorial ocean is concentrated at biweekly frequencies and is largely composed of Yanai waves. The excitation of this biweekly variability is investigated using an oceanic GCM and both analytic and numerical versions of a linear, continuously stratified (LCS) model in which solutions are represented as expansions in baroclinic modes. Solutions are forced by Quick Scatterometer (QuikSCAT) winds ( the model control runs) and by idealized winds having the form of a propagating wave with frequency sigma and wavenumber k(w). The GCM and LCS control runs are remarkably similar in the biweekly band, indicating that the dynamics of biweekly variability are fundamentally linear and wind driven. The biweekly response is composed of local (nonradiating) and remote ( Yanai wave) parts, with the former spread roughly uniformly along the equator and the latter strengthening to the east. Test runs to the numerical models separately forced by the tau(x) and tau(y) components of the QuikSCAT winds demonstrate that both forcings contribute to the biweekly signal, the response forced by tau(y) being somewhat stronger. Without mixing, the analytic spectrum for Yanai waves forced by idealized winds has a narrowband ( resonant) response for each baroclinic mode: Spectral peaks occur whenever the wavenumber of the Yanai wave for mode n is sufficiently close to kw and they shift from biweekly to lower frequencies with increasing modenumber n. With mixing, the higher-order modes are damped so that the largest ocean response is restricted to Yanai waves in the biweekly band. Thus, in the LCS model, resonance and mixing act together to account for the ocean's favoring the biweekly band. Because of the GCM's complexity, it cannot be confirmed that vertical mixing also damps its higher-order modes; other possible processes are nonlinear interactions with near-surface currents, and the model's low vertical resolution below the thermocline. Test runs to the LCS model show that Yanai waves from several modes superpose to form a beam ( wave packet) that carries energy downward as well as eastward. Reflections of such beams from the near- surface pycnocline and bottom act to maintain near- surface energy levels, accounting for the eastward intensification of the near- surface, equatorial nu field in the control runs.

Kwok2006

ICESat over Arctic sea ice: Interpretation of altimetric and reflectivity profiles
R. Kwok and G. F. Cunningham and H. J. Zwally and D. Yi
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  111    (2006)

We provide an assessment of the ICESat altimeter for studying the Arctic Ocean and examine the magnitude of the large- and small-scale expressions of geophysical processes embedded in the elevation profiles. This analysis includes data from six surveys. At the large scale the response of the ice-covered ocean to atmospheric loading is near ideal (i.e., approximately -1 cm/hPa). After removal of the inverted barometer effects and best available geoid the elevation signal is still dominated by unresolved geoid residuals (similar to 0.4 m) that can be seen in the similarity of the remaining spatial patterns. Seasonal differences in elevations over multiyear ice are consistent with snow depth climatology; the broad differential spatial patterns are indicative of interannual differences in multiyear ice coverage associated with advection. Patterns in the derived surface roughness fields correspond to the seasonal and perennial ice zones seen in QuikSCAT data. At the small scale, near-coincident RADARSAT imagery provides a spatial context for understanding the signature of the observed elevations, waveforms, and reflectivity, in particular, those associated with thin ice, open water, multiyear ice, and ridges. The precision of the elevation estimates measured over relatively flat sea ice, identified in synthetic aperture radar (SAR) imagery, is similar to 2 cm. The unambiguous identification of ridged areas in coupled ICESat/RADARSAT analysis could be used to enhance the utility of SAR imagery for examining ridge distributions. Over a 20 day period we monitored the evolution of the reflectivity of a newly opened lead. The steep increase in reflectivity due to snow coverage suggests that dips in ICESat reflectivity are likely areas of thin ice and could serve as a basis for selection of tie points for use as sea level reference. Identification of these tie points is crucial for accurate estimation of sea ice freeboard.

Susanto2006

Ocean color variability in the Indonesian Seas during the SeaWiFS era
R. D. Susanto and T. S. Moore and J. Marra
GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS  7    (2006)

More than 6 years of satellite-derived ocean color (SeaWiFS) and 7 years of sea surface temperature (AVHRR) and sea surface wind (ERS1/2, NSCAT, and QuikSCAT) are investigated for the Indonesian Seas. Harmonic analysis and monthly means in ocean color indicate that during the southeast Asia-Australia monsoon southeasterly wind from Australia generates upwelling and brings cooler and nutrient-rich water near the surface, enhancing productivity and increasing ocean color in the Banda Sea and the southern coasts of Jawa (Java)-Sumatra. Conditions are reversed during the northwest monsoon. The northwest wind induces downwelling and produces a weaker biological response in terms of ocean color. Anomalous winds associated with the 1997-1998 El Nino/La Nina events coinciding with the Indian Ocean Dipole (IOD) produced significant departures from the 6-year monthly mean in both magnitude and timing of the seasonal response to the southeast monsoon. Ocean color intensified in the upwelling region along the southern coast of Jawa-Sumatra, and the area of increased amplitude extended westward and prolonged the southeast monsoon period. In addition, localized minimum values of ocean color are observed along the exit pathways of the Indonesian Throughflow.

Stoffelen2006

On Bayesian scatterometer wind inversion
A. Stoffelen and M. Portabella
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  1523--1533  (2006)

In a quest for a generic unbiased scatterometer wind inversion method, the different inversion procedures currently in use are revisited in this paper. A careful examination of both the errors in the wind and in the measurement domain, combined with the nonlinear shape of the geophysical model,function (GMF), leads to a generic and novel Bayesian wind retrieval approach in the measurement domain. In this approach the shape,of the GMF solution manifold in measurement space is more,important than the specified noise. This shape,is related to the system wind direction sensitivity, and when this sensitivity is uniform, realistic and precise wind direction distributions are retrieved, even when measurements lie far from the GMF manifold. A simplified measurement space transformation that produces such uniform sensitivity for the, European Remote Sensing Satellite (ERS) scatterometer is presented and shown to have reduced wind direction bias,compared to the more traditional (measurement-noise normalized) inversion for ERS. Moreover, the simplified, wind inversion reveals a similar performance to the current operational ERS wind inversion, but is potentially more generally applicable. The simplified method is then applied to SeaWinds but is ineffective. In this case the instrument geometry results in a low sensitivity to wind direction at a few specific directions. As a consequence, certain wind direction solutions remain favored in the SeaWinds inversion.

Park2006

Relationship between satellite-observed cold water along the Primorye coast and sea ice in the East Sea (the Sea of Japan)
K. A. Park and K. Kim and P. C. Cornillon and J. Y. Chung
GEOPHYSICAL RESEARCH LETTERS  33    (2006)

The relationship between cold surface water along the Primorye coast and sea ice in the Tatar Strait of the northern East Sea ( the Sea of Japan) is examined by analyzing sea surface temperatures (SST) from NOAA/AVHRR, sea ice concentrations from SSM/I, wind vectors from QuikSCAT, and position data from satellite-tracked drifting buoys. Relatively low SSTs along the Primorye coast in spring and early summer are attributed to sea ice melted water advected to the southwest as the Liman Cold Current (LCC). Monthly to year-to-year variations of SSTs along the Primorye coast in spring and early summer are negatively correlated with those of the sea ice concentration in the Tatar Strait during the previous winter. Translational speeds from the surface drifter and SST anomalies demonstrate that the LCC significantly varies by 2 - 18 km/day both in space and time.

Li2006

Tropical cyclogenesis associated with Rossby wave energy dispersion of a preexisting typhoon. Part I: Satellite data analyses
T. Li and B. Fu
JOURNAL OF THE ATMOSPHERIC SCIENCES  63  1377--1389  (2006)

The structure and evolution characteristics of Rossby wave trains induced by tropical cyclone (TC) energy dispersion are revealed based on the Quick Scatterometer (QuikSCAT) and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) data. Among 34 cyclogenesis cases analyzed in the western North Pacific during 2000-01 typhoon seasons, six cases are associated with the Rossby wave energy dispersion of a preexisting TC. The wave trains are oriented in a northwest-southeast direction, with alternating cyclonic and anticyclonic vorticity circulation. A typical wavelength of the wave train is about 2500 km. The TC genesis is observed in the cyclonic circulation region of the wave train, possibly through a scale contraction process. The satellite data analyses reveal that not all TCs have a Rossby wave train in their wakes. The occurrence of the Rossby wave train depends to a certain extent on the TC intensity and the background flow. Whether or not a Rossby wave train can finally lead to cyclogenesis depends on large-scale dynamic and thermodynamic conditions related to both the change of the seasonal mean state and the phase of the tropical intraseasonal oscillation. Stronoer low-level convergence and cyclonic vorticity. weaker vertical shear, and greater midtropospheric moisture are among the favorable large-scale conditions. The rebuilding process of a conditional unstable stratification is important in regulating the frequency of TC genesis.

Ebuchi2006

Evaluation of marine surface winds observed by SeaWinds and AMSR on ADEOS-II
N. Ebuchi
JOURNAL OF OCEANOGRAPHY  62  293--301  (2006)

Marine surface winds observed by two microwave sensors, SeaWinds and Advanced Microwave Scanning Radiometer (AMSR), on the Advanced Earth Observing Satellite-II (ADEOS-II) are evaluated by comparison with off-shore moored buoy observations. The wind speed and direction observed by SeaWinds are in good agreement with buoy data with root-mean-squared (rms) differences of approximately 1 m s(-1) and 20 degrees, respectively. No systematic biases depending on wind speed or cross-track wind vector cell location are discernible. The effects of oceanographic and atmospheric environments on the scatterometry are negligible. Though the wind speed observed by AMSR also showed agreement with buoy observations with rms difference of 1.27 m s(-1), the AMSR wind speed is systematically lower than the buoy data for wind speeds lower than 5 m s(-1). The AMSR wind seems to have a discontinuous trend relative to the buoy data at wind speeds of 5-6 m s(-1). Similar results have been obtained in an intercomparison of wind speeds globally observed by SeaWinds and AMSR on the same orbits. A global wind speed histogram of the AMSR wind shows skewed features in comparison with those of SeaWinds and European Centre for Medium-range Weather Forecasts (ECMWF) analyses.

Shibata2006

Features of ocean microwave emission changed by wind at 6 GHz
A. Shibata
JOURNAL OF OCEANOGRAPHY  62  321--330  (2006)

Ocean microwave emissions changed by the ocean wind at 6 GHz were investigated by combining data of the Advanced Microwave Scanning Radiometer (AMSR) and SeaWinds, both aboard the Advanced Earth Observation Satellite-II (ADEOS-II). This study was undertaken to improve the accuracy of the sea surface temperature (SST) retrieved from the AMSR 6 GHz data. Two quantities, 6V*(H*), were defined by the brightness temperature of the AMSR at 6 GHz with two polarizations (V-pol and H-pol), adjusted for atmospheric effects and with a calm ocean surface emission removed. These quantities represent a microwave emission change due to the ocean wind at 6 GHz. 6V* does not change in a region where 6H* is less than around 4 K (referred to as z0). Both 6V* and 6H* increase above z0. The 6V* to 6H* ratio, sp, varies with the relative wind directions. Furthermore, the sp values vary with the SST, between the northern and southern hemisphere, and seasonally. By specifying appropriate values for z0 and sp, the SST error between AMSR and buoy measurement became flat against 6H*, which is related to the ocean wind. Two extreme cases were observed: the Arabian Sea in summer and the Northwestern Atlantic Ocean in winter. The air-sea temperature difference in the former case was largely positive, while it was largely negative in the latter. The 6V* and 6H* relations differed from global conditions in both cases, which resulted in incorrect SSTs in both areas when global coefficients were applied.

Hosoda2006

Difference characteristics of sea surface temperature observed by GLI and AMSR aboard ADEOS-II
K. Hosoda and H. Murakami and A. Shibata and F. Sakaida and H. Kawamura
JOURNAL OF OCEANOGRAPHY  62  339--350  (2006)

This study compares infrared and microwave measurements of sea surface temperature (SST) obtained by a single satellite. The simultaneous observation from the Global Imager (GLI: infrared) and the Advanced Microwave Scanning Radiometer (AMSR: microwave) aboard the Advanced Earth Observing Satellite-II (ADEOS-II) provided an opportunity for the intercomparison. The GLI- and AMSR-derived SSTs from April to October 2003 are analyzed with other ancillary data including surface wind speed and water vapor retrieved by AMSR and SeaWinds on ADEOS-II. We found no measurable bias (defined as GLI minus AMSR), while the standard deviation of difference is less than VC. In low water vapor conditions, the GLI SST has a positive bias less than 0.2 degrees C, and in high water vapor conditions, it has a negative (positive) bias during the daytime (nighttime). The low spatial resolution of AMSR is another factor underlying the geographical distribution of the differences. The cloud detection problem in the GLI algorithm also affects the difference. The large differences in high-latitude region during the nighttime might be due to the GLI cloud-detection algorithm. AMSR SST has a negative bias during the daytime with low wind speed (less than 7 ms(-1)), which might be related to the correction for surface wind effects in the AMSR SST algorithm.

Shibata2006

A wind speed retrieval algorithm by combining 6 and 10 GHz data from Advanced Microwave Scanning Radiometer: Wind speed inside hurricanes
A. Shibata
JOURNAL OF OCEANOGRAPHY  62  351--359  (2006)

A wind speed retrieval algorithm was developed using 6 and 10 GHz h-pol (6H and 10H) data of the Advanced Microwave Scanning Radiometer (AMSR) aboard the Advanced Earth Observation Satellite-II (ADEOS-II) and AMSR-E aboard AQUA, for the purpose of retrieving wind speed inside rainstorms, primarily hurricanes and typhoons. The h-pol was used rather than the v-pol, because the brightness temperature sensitivity to the ocean wind at h-pol is larger than v-pol. The microwave emission change of 6H and 10H corresponding to ocean wind was evaluated in no-rain areas by combining AMSR and SeaWinds data aboard the ADEOS-II (SeaWinds was NASA's scatterometer), and it was found that the ratio of the two 6H to 10H increments due to ocean wind is 0.9. Assuming that this result also holds with higher wind speeds and under rainy conditions, the brightness temperatures at 6H and 10H were simulated using a microwave radiative transfer model. A parameter W6 (unit; Kelvin) was then defined, representing an increment at 6H due to ocean wind. W6 is applicable to rainy areas, and to all ranges of sea surface temperature. W6 was compared with wind speed reported by the National Hurricanes Center for several hurricanes in the Western Atlantic Ocean during three years (2002 to 2004). W6 averaged around centers of hurricanes was found to exhibit a sensitivity to wind speed, such as increasing from 22 K to 65 K as the wind speed rose from 65 to 140 knots (33 to 72 m/s), and an empirical relationship relating the averaged W6 to wind speed in hurricanes was derived.

Hilburn2006

Correcting active scatterometer data for the effects of rain using passive radiometer data
K. A. Hilburn and F. J. Wentz and D. K. Smith and P. D. Ashcroft
JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY  45  382--398  (2006)

A model for the effects of rain on scatterometer data is proposed. Data from the Advanced Microwave Scanning Radiometer (AMSR) and the SeaWinds scatterometer, both on the Midori-H satellite, are used. The model includes three basic rain effects: attenuation, rain roughening of the sea surface, and volumetric backscatter. Attenuation is calculated directly from the radiometer data and beam filling is explicitly addressed. The model simultaneously solves for both the rain roughening and volumetric backscatter. Fitting the coefficients of the model requires an estimate of the radar cross section because of wind roughening, and NCEP Global Data Assimilation System (GDAS) wind vectors are used for this purpose. Both the derived rain roughening and volumetric backscatter are similar to results in published work, but the values are slightly smaller, especially for vertical polarization. Drop size distribution variability is accounted for by formulating the radar equation in terms of the parameters of the radiative transfer equation and using additional radiometric information. Explicit inclusion of vertical profile variability results in an underdetermined problem, but it is implicitly included in fitting the model to the data. The correction makes large improvements in wind speeds and modest improvements in wind directions. Wind statistics and specific examples are shown to illustrate the nature of the improvements. The correction is limited, however, by measurement mismatch issues and the nonlinear nature of the wind retrieval.

Bessho2006

Tropical cyclone wind retrievals from the advanced microwave sounding unit: application to surface wind analysis
K. Bessho and M. DeMaria and J. A. Knaff
JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY  45  399--415  (2006)

Horizontal winds at 850 hPa from tropical cyclones retrieved using the nonlinear balance equation, where the mass field was determined from Advanced Microwave Sounding Unit (AMSU) temperature soundings, are compared with the surface wind fields derived from NASA's Quick Scatterometer (QuikSCAT) and Hurricane Research Division H*Wind analyses. It was found that the AMSU-dcrived wind speeds at 850 hPa have linear relations with the surface wind speeds from QuikSCAT or H*Wind. There are also characteristic biases of wind direction between AMSU and QuikSCAT or H*Wind. Using this information to adjust the speed and correct for the directional bias, a new algorithm was developed for estimation of the tropical cyclone surface wind field from the AMSU-derived 850-hPa winds. The algorithm was evaluated in two independent cases from Hurricanes Floyd (1999) and Michelle (2001), which were observed simultaneously by AMSU, QuikSCAT, and H*Wind. In this evaluation the AMSU adjustment algorithm for wind speed worked well. Results also showed that the bias correction algorithm for wind direction has room for improvement.

Liu2006

Spacebased observations of oceanic influence on the annual variation of South American water balance
W. T. Liu and X. S. Xie and W. Q. Tang and V. Zlotnicki
GEOPHYSICAL RESEARCH LETTERS  33    (2006)

The mass change of South America (SA) continent measured by the Gravity Recovery and Climate Experiment (GRACE) imposes a constraint on the uncertainties in estimating the annual variation of rainfall measured by Tropical Rain Measuring Mission (TRMM) and ocean moisture influx derived from QuikSCAT data. The approximate balance of the mass change rate with the moisture influx less climatological river discharge, in agreement with the conservation principle, bolsters not only the credibility of the spacebased measurements, but supports the characterization of ocean's influence on the annual variation of continental water balance. The annual variation of rainfall is found to be in phase with the mass change rate in the Amazon and the La Plata basins, and the moisture advection across relevant segments of the Pacific and Atlantic coasts agrees with the annual cycle of rainfall in the two basins and the Andes mountains.

Park2006

Modification of surface winds near ocean fronts: Effects of Gulf Stream rings on scatterometer (QuikSCAT, NSCAT) wind observations
K. A. Park and P. Cornillon and D. L. Codiga
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  111    (2006)

Modifications to surface winds by currents and sea surface temperature (SST) gradients near frontal boundaries of Gulf Stream rings are analyzed using satellite SST and scatterometer (NASA's Quick Scatterometer (QuikSCAT), NASA scatterometer (NSCAT)) wind observations. A component of scatterometer wind approximately equal and opposite to the surface current vector is observed and attributed to the fact that scatterometers detect relative motion of water and air. Warm-core ring (WCR) SSTs act to destabilize the marine atmospheric boundary layer (MABL), increasing surface wind magnitude by 10-15\% and decreasing veering angle by 5-15 degrees relative to large-scale mean winds. Cold-core ring (CCR) SSTs cause impacts of similar magnitude and opposite sense. Magnitudes and directions of modifications are accounted for by MABL dynamics of a nonlinear planetary boundary layer model forced by air-sea temperature differences. Wind modifications occur within tens of kilometers of SST fronts, implying a wind response timescale of order 1 hour. By contrast, uniformity of modified winds across the larger area within rings suggests the response time for the MABL to return to equilibrium downstream from a front exceeds 10 hours. Over WCRs, strong divergence (convergence) occurs on the upwind (downwind) side; curl is strongly negative (positive) to the right (left) side facing downwind. Opposite patterns are generally seen over CCRs. Divergence (curl) peaks where winds blow perpendicular (parallel) to SST fronts. SST image analysis indicates enhanced cloudiness occurs with downwind convergence over WCRs. Wind stress curl due to ring modifications causes dipolar Ekman pumping sufficient to influence ring translation and decay processes.

Monahan2006

The probability distribution of sea surface wind speeds. Part 1: Theory and SeaWinds observations
A. H. Monahan
JOURNAL OF CLIMATE  19  497--520  (2006)

The probability distribution of sea surface wind speeds, w, is considered. Daily SeaWinds scatterometer observations are used for the characterization of the moments of sea surface winds on a global scale. These observations confirm the results of earlier studies, which found that the two-parameter Weibull distribution provides a good (but not perfect) approximation to the probability density function of w. In particular, the observed and Weibull probability distributions share the feature that the skewness of w is a concave upward function of the ratio of the mean of w to its standard deviation. The skewness of w is positive where the ratio is relatively small (such as over the extratropical Northern Hemisphere), the skewness is close to zero where the ratio is intermediate (such as the Southern Ocean), and the skewness is negative where the ratio is relatively large (such as the equatorward flank of the subtropical highs). An analytic expression for the probability density function of w, derived from a simple stochastic model of the atmospheric boundary layer, is shown to be in good qualitative agreement with the observed relationships between the moments of w. Empirical expressions for the probability distribution of w in terms of the mean and standard deviation of the vector wind are derived using Gram-Charlier expansions of the joint distribution of the sea surface wind vector components. The significance of these distributions for improvements to calculations of averaged air-sea fluxes in diagnostic and modeling studies is discussed.

Monahan2006

The probability distribution of sea surface wind speeds. Part II: Dataset intercomparison and seasonal variability
A. H. Monahan
JOURNAL OF CLIMATE  19  521--534  (2006)

The statistical structure of sea surface wind speeds is considered, both in terms of the leading-order moments (mean, standard deviation, and skewness) and in terms of the parameters of a best-fit Weibull distribution. An intercomparison is made of the statistical structure of sea surface wind speed data from four different datasets: SeaWinds scatterometer observations, a blend of Special Sensor Microwave Imager (SSM/I) satellite observations with ECMWF analyses, and two reanalysis products [NCEP-NCAR and 40-yr ECMWF Re-Analysis (ERA-40)]. It is found that while the details of the statistical structure of sea surface wind speeds differs between the datasets, the leading-order features of the distributions are consistent. In particular, it is found in all datasets that the skewness of the wind speed is a concave upward function of the ratio of the mean wind speed to its standard deviation, such that the skewness is positive where the ratio is relatively small (such as over the extratropical Northern Hemisphere), the skewness is close to zero where the ratio is intermediate (such as the Southern Ocean), and the skewness is negative where the ratio is relatively large (such as the equatorward flank of the subtropical highs). This relationship between moments is also found in buoy observations of sea surface winds. In addition, the seasonal evolution of the probability distribution of sea surface wind speeds is characterized. It is found that the statistical structure on seasonal time scales shares the relationships between moments characteristic of the year-round data. Furthermore, the seasonal data are shown to depart from Weibull behavior in the same fashion as the year-round data, indicating that non-Weibull structure in the year-round data does not arise due to seasonal nonstationarity in the parameters of a strictly Weibull time series.

Brown2006

An emissivity-based wind vector retrieval algorithm for the WindSat polarimetric radiometer
S. T. Brown and C. S. Ruf and D. R. Lyzenga
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  611--621  (2006)

The Naval Research Laboratory WindSat polarimetric radiometer was launched on January 6, 2003 and is the first fully polarimetric radiometer to be flown in space. WindSat has three fully polarimetric channels at 10.7, 18.7, and 37.0 GHz and vertically and horizontally polarized channels at 6.8 and 23.8 GHz. A first-generation wind vector retrieval algorithm for the WindSat polarimetric radiometer is developed in this study. An atmospheric clearing algorithm is used to estimate the surface emissivity from the measured WindSat brightness temperature at each channel. A specular correction factor is introduced in the radiative transfer equation to account for excess reflected atmospheric brightness, compared to the specular assumption, as a function wind speed. An empirical geophysical model function relating the surface emissivity to the wind vector is derived using coincident QuikSCAT scatterometer wind vector measurements. The confidence in the derived harmonics for the polarimetric channels is high and should be considered suitable to validate analytical surface scattering models for polarized ocean surface emission. The performance of the retrieval algorithm is assessed with comparisons to Global Data Assimilation System (GDAS) wind vector outputs. The root mean square (RMS) uncertainty of the closest wind direction ambiguity is less than 20 degrees for wind speeds greater than 6 m/s and less than 15 degrees at 10 m/s and greater. The retrieval skill, the percentage of retrievals in which the first-rank solution is the closest to the GDAS reference, is 75\% at 7 m/s and 85\% or higher above 10 m/s. The wind speed is retrieved with an RMS uncertainty of 1.5 m/s.

Freilich2006

The accuracy of preliminary WindSat vector wind measureinrients: Comparisons with NDBC buoys and QuikSCAT
M. H. Freilich and B. A. Vanhoff
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  622--637  (2006)

Two preliminary, six-month long global WindSat vector wind datasets are validated using buoys and OuikSCAT measurements. Buoy comparisons yield speed and direction root mean square accuracies of 1.4 m/s and 25 degrees for the ``NESDIS0'' product and 1.3 m/s and 23 degrees for the more recently produced ``B1'' product from the Naval Research Laboratory, WindSat along- and across-wind random component errors of 0.7-1.0 and 2.6-2.8 m/s (respectively) are larger than those calculated for QuikSCAT in the same period. Global WindSat-OuikSCAT comparisons generally confirmed the buoy analyses. While simple rain flags based directly on WindSat brightness temperature measurements alone are shown to overflag for rain systematically, the advanced ``Environmental Data Record'' rain Hag in the B1 product matches well with Special Sensor Microwave/Imager rain detection frequency and preserves the accuracy of the unflagged vector wind measurements.

Monaldo2006

Evaluation of WindSat wind vector performance with respect to QuikSCAT estimates
F. M. Monaldo
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  638--644  (2006)

The WindSat instrument was launched on January 6, 2003 as part of a risk reduction effort to assess the potential of using spaceborne fully polarimetric radiometry to measure the marine wind vector. Microwave radiometry on the Special Sensor Microwave/Imager onboard the Defense Meteorological Satellite Program satellites has long provided wind speed measurements. Fully polarimetric radiometry offers the additional possibility of obtaining wind direction as well. By contrast, the QuikSCAT satellite uses active microwave measurements to estimate the wind vector from space. It represents the most comprehensive satellite dataset against which to compare WindSat measurements. In this paper, we systematically compare temporally and spatially coincident WindSat and QuikSCAT wind vector measurements against the design goals of the WindSat instrument, taking into consideration expected differences related to instrument. precision and the spatial and temporal variability of the wind field.

English2006

An evaluation of the potential of polarimetric radiometry for numerical weather prediction using QuikSCAT
S. J. English and B. Candy and A. Jupp and D. Bebbington and S. Smith and A. Holt
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  668--675  (2006)

It has been proposed that wind vector information derived from passive microwave radiometry. may provide an impact on numerical weather forecasts of similar magnitude to that achieved by scatterometers. Polarimetric radiometers have a lower sensitivity to wind direction than scatterometers at low wind speed but comparable sensitivity at high windspeed. In this paper, we describe an experiment which aimed to determine if an observing system only capable of providing wind direction information at wind speeds over 8 ms(-1) can provide comparable impact to one providing wind vectors at wind speeds over 2 ms(-1). The QuikSCAT dataset used in the experiments has a wide swath and is used operationally by several forecast centers. The results confirm that assimilation of wind vectors from QuikSCAT only for wind speeds above 8 ms(-1) gives similar analysis increments and forecast impacts to assimilating wind vectors at all wind speeds above 2 ms(-1). Measurements from the WindSat five frequency polarimetric radiometer are compared with calculations from Met Office global forecast fields, and this also confirms that WindSat measurement and radiative transfer model accuracy appears to be sufficiently good to provide useful information for numerical weather prediction.

Han2006

Impact of atmospheric submonthly oscillations on sea surface temperature of the tropical Indian Ocean
W. Q. Han and W. T. Liu and J. L. Lin
GEOPHYSICAL RESEARCH LETTERS  33    (2006)

Impacts of atmospheric intraseasonal oscillations (ISOs) at submonthly periods ( 10 - 30 days) on Indian Ocean sea surface temperature (SST) are studied using satellite observed outgoing long wave radiation, QuikSCAT winds, SST and an ocean general circulation model for the period of 1999 - 2004. The results suggest that submonthly ISOs can cause significant 10 - 30 day SST changes throughout the equatorial basin and northern Bay of Bengal, with an amplitude of as large as 0.5 degrees C and standard deviation of exceeding 0.2 degrees C for a 4-year record. Impact of the submonthly ISO associated with the Indian summer monsoon is separately examined. It is associated with basin-scale SST evolution with distinct spatial structures. The SST variation results mainly from submonthly wind forcing, which causes changes in oceanic processes and surface turbulent heat fluxes. Radiative fluxes can also have large influences in some regions for some ISO events.

Tveter2006

Assimilating ambiguous QuikScat scatterometer observations in HIRLAM 3-D-Var at the Norwegian Meteorological Institute
F. T. Tveter
TELLUS SERIES A-DYNAMIC METEOROLOGY AND OCEANOGRAPHY  58  59--68  (2006)

The Norwegian Meteorological Institute has implemented ambiguous QuikScat observations in the HIRLAM three-dimensional variational data assimilation system, by approximating the method that will achieve the best mean squared error verification results according to the Bayesian decision theory. A four-month long impact study of this implementation during the 2003/2004 winter showed that assimilating QuikScat observations had a small positive effect on the average mean sea level pressure forecasting skill.

Castelao2006

Upwelling around Cabo Frio, Brazil: The importance of wind stress curl
R. M. Castelao and J. A. Barth
GEOPHYSICAL RESEARCH LETTERS  33    (2006)

Data from the SeaWinds scatterometer on the QuikSCAT satellite are used to estimate upwelling around Cabo Frio, Brazil, due to Ekman transport and Ekman pumping. The region close to shore (up to 200 km from the coast) is characterized by negative wind stress curl (upwelling favorable) year-round, with maximum values during summer, and minimum values during fall. Integrated values from Sao Sebastiao Island to Vitoria reveal that during summer, Ekman pumping and Ekman transport are of the same magnitude in the region. Estimates of Ekman transport are relatively uniform along the coast during summer. Ekman pumping, on the other hand, is strongly enhanced between Sao Sebastiao Island and Vitoria, the region where the coldest water on satellite images is frequently found. This suggests that wind stress curl-driven upwelling is a major contributor to the coldest surface water being found near Cabo Frio.

Quilfen2006

Altimeter dual-frequency observations of surface winds, waves, and rain rate in tropical cyclone Isabel
Y. Quilfen and J. Tournadre and B. Chapron
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  111    (2006)

[1] Extreme weather events such as tropical cyclones are difficult to observe with conventional means. Satellite-based observations provide essential measurements of key parameters governing tropical cyclones. They are critical for short-term forecasting. Radiometers onboard the Defense Meteorological Satellite Program satellite series, WindSat and Tropical Rainfall Measuring Mission satellites, scatterometers onboard the ERS, ADEOS, and QuikScat satellites offer unprecedented synoptic observations of surface wind and atmospheric liquid water content, revealing the storm structures with good accuracy. However, satellite estimates do not provide direct measurements of geophysical parameters and can suffer from limitations linked to the sensors characteristics, such as the signal wavelength and polarization or the measurement incidence angle. For example, measurements at Ku band are strongly affected by rain. Still, each observing system can offer specific information that can be combined with the others. In particular, we highlight the capabilities of dual-frequency altimeter to provide very high resolution measurements of rain rate, surface wind speed, and wave characteristics. A method is proposed to obtain continuous along-track 5 km resolution measurements of these parameters in the tropical cyclone Isabel. The results shows that dual-frequency altimeters can provide useful information to complement and validate the operational fields provided by the atmospheric numerical models and by NOAA observing systems.

Allen2006

Microwave observations of daily Antarctic sea-ice edge expansion and contraction rates
J. R. Allen and D. G. Long
IEEE GEOSCIENCE AND REMOTE SENSING LETTERS  3  54--58  (2006)

Algorithms for estimating sea-ice extent from remotely sensed microwave sensor data can benefit from knowledge of the ``a priori'' distribution of the daily expansion and contraction of the sea-ice pack. To estimate the probability distribution of daily Antarctic sea-ice extent change, two independent sea-ice datasets are analyzed: sea-ice extent derived from the QuikSCAT scatterometer and ice concentration estimates from the Special Sensor Microwave/Imager. The daily sea-ice advance and retreat is tracked over a four-year period. The distribution of the daily sea-ice advance/retreat from each sensor is similar and is approximately double-exponential. Daily ice-pack statistics are presented.

Smith2006

A statistical approach to WindSat ocean surface wind vector retrieval
C. K. Smith and M. Bettenhausen and P. W. Gaiser
IEEE GEOSCIENCE AND REMOTE SENSING LETTERS  3  164--168  (2006)

WindSat is the first space-based polarimetric microwave radiometer. It is designed to evaluate the capability of polarimetric microwave radiometry to measure ocean surface wind vectors from space. The sensor provides risk reduction for the National Polar-orbiting Operational Environmental Satellite System Conical Scanning Microwave Imager/Sounder, which is planned to provide wind vector data operationally starting in 2010. The channel set also enables retrieval of sea surface temperature, and columnar water vapor and cloud liquid water over the oceans. We describe statistical algorithms for retrieval of these parameters, and a combined statistical/maximum-likelihood estimator algorithm for retrieval of wind vectors. We present a quantitative analysis of the initial wind vector retrievals relative to QuikSCAT wind vectors.

Urbano2006

Rediscovering the second core of the Atlantic NECC
D. F. Urbano and M. Jochum and I. C. A. da Silveira
OCEAN MODELLING  12  1--15  (2006)

The North Equatorial Countercurrent (NECC) is investigated at 35 W through a combination of theory, high resolution Ocean General Circulation Model outputs, and observations. Transport from ADCP measurements during four WOCE cruises and from 5-year SeaWinds scatterometer wind data (QuikSCAT satellite) were used to support model results. It is found that the NECC in the annual mean is approximately in Sverdrup balance, but that seasonal changes in the wind stress curl lead the transport expected from the Sverdrup balance by one month, the propagation time of the seasonal Rossby waves from the African coast to 35 W. An investigation of the vertical structure of the NECC shows that there is an eastward core throughout the year, but in spring it is below the westward Ekman flow. Only 60\% of the total transport are above the thermocline. The most interesting result of the present study is that model as well as observations describe a distinct second, northern core of the NECC which has not yet found much attention in the literature. It is shown here that the two cores of the NECC are the direct result of the finite width of the Inter-tropical Convergence Zone (ITCZ). This suggests that observational programs that try to determine the NECC transport have to cover the area up to 15 degrees N. (c) 2005 Elsevier Ltd. All rights reserved.

Kara2005

Black sea mixed layer sensitivity to various wind and thermal forcing products on climatological time scales
A. B. Kara and H. E. Hurlburt and A. J. Wallcraft and M. A. Bourassa
JOURNAL OF CLIMATE  18  5266--5293  (2005)

This study describes atmospheric forcing parameters constructed from different global climatologies, applied to the Black Sea, and investigates the sensitivity of Hybrid Coordinate Ocean Model (HYCOM) simulations to these products. Significant discussion is devoted to construction of these parameters before using them in the eddy-resolving (approximate to 3.2-km resolution) HYCOM simulations. The main goal is to answer how the model dynamics can be substantially affected by different atmospheric forcing products in the Black Sea. Eight wind forcing products are used: four obtained from observation-based climatologies, including one based on measurements from the SeaWinds scatterometer on the Quick Scatterometer (QuikSCAT) satellite, and the rest formed from operational model products. Thermal forcing parameters, including solar radiation, are formed from two operational models: the European Centre for Medium-Range Weather Forecasts (ECMWF) and the Fleet Numerical Meteorology and Oceanography Center (FNMOC) Navy Operational Global Atmospheric Prediction System (NOGAPS). Climatologically forced Black Sea HYCOM simulations (without ocean data assimilation) are then performed to assess the accuracy and sensitivity of the model sea surface temperature (SST) and sea surface circulation to these wind and thermal forcing products. Results demonstrate that the model-simulated SST structure is quite sensitive to the wind and thermal forcing products, especially near coastal regions. Despite this sensitivity, several robust features are found in the model SST in comparison to a monthly 9.3-km-resolution satellite-based Pathfinder SST climatology. Annual mean HYCOM SST usually agreed to within approximate to +/- 0.2 degrees of the climatology in the interior of the Black Sea for any of the wind and thermal forcing products used. The fine-resolution (0.25 degrees x 0.25 degrees) wind forcing from the scatterometer data along with thermal forcing from NOGAPS gave the best SST simulation with a basin-averaged rms difference value of 1.21 degrees C, especially improving model results near coastal regions. Specifically, atmospherically forced model simulations with no assimilation of any ocean data suggest that the basin-averaged rms SST differences with respect to the Pathfinder SST climatology can vary from 1.21 degrees to 2.15 degrees C depending on the wind and thermal forcing product. The latter rms SST difference value is obtained when using wind forcing from the National Centers for Environmental Prediction (NCEP), a product that has a too-coarse grid resolution of 1.875 degrees x 1.875 degrees for a small ocean basin such as the Black Sea. This paper also highlights the importance of using high-frequency (hybrid) wind forcing as opposed to monthly mean wind forcing in the model simulations. Finally, there are large variations in the annual mean surface circulation simulated using the different wind sets, with general agreement between those forced by the model-based products (vector correlation is usually > 0.7). Three of the observation-based climatologies generally yield unrealistic circulation features and currents that are too weak.

Solanki2005

Application of QuikSCAT SeaWinds data to improve remotely sensed Potential Fishing Zones (PFZs) forecast methodology: Preliminary validation results
H. U. Solanki and Y. Pradhan and R. M. Dwivedi and S. Nayak and D. K. Gulati and V. S. Somvamshi
INDIAN JOURNAL OF MARINE SCIENCES  34  441--448  (2005)

In this study, we used chlorophyll concentration and sea surface temperature (SST) images derived from IRS P4-OCM and NOAA- AVHRR, respectively, to delineate the oceanographic features exhibiting different oceanic processes. QuikSCAT-SeaWinds derived wind vectors were used to understand, establish, quantify and to demonstrate the variability of wind induced watermass flow as well as their impacts on features/oceanographic process. Oceanographic features like eddies, rings and fronts were found shifted as per movement and direction of the wind. The movement of water mass due to wind provides insight of environmental factors relevant to dispersal of fishery resources. An algorithm was developed to Compute water mass transport and feature shift. Based on these studies an approach for incorporating QuikSCAT-SeaWinds data to improve PFZs forecast methodology has been developed. The improved PFZs forecast methodology was validated through near real time fishing operations. About 82-85\% success rate was reported during validation experiments carried out during 2004. The improved methodology would prolong the validity of PFZs forecast.

Huang2005

Improvements in typhoon forecasts with assimilated GPS occultation refractivity
C. Y. Huang and Y. H. Kuo and S. H. Chen and F. Vandenberghe
WEATHER AND FORECASTING  20  931--953  (2005)

In this study, the fifth-generation Pennsylvania State University - National Center for Atmospheric Research Mesoscale Model ( MM5) with three-dimensional variational data assimilation ( 3DVAR) is utilized to investigate influences of GPS occultation refractivity on simulations of typhoons past Taiwan. Two recent cases were simulated, including Typhoon Nari in September 2001 and Typhoon Nakri in July 2002. The GPS observation data are taken from the Challenging Minisatellite Payload for Geophysical Research and Application ( CHAMP) and Satilite de Aplicaciones Cientificas-C ( SAC-C) satellites that provide several retrieved refractivity profiles in the simulated domain near the initialization time. Through 3DVAR, the observed refractivity can be quickly ingested into the model initial conditions to recover the information over the ocean. The initial moisture increments from ingested GPS refractivity soundings exhibit a maximum magnitude of about 1.5 g kg(-1) associated with temperature increments of generally less than 0.2 degrees C. The differences between the model local refractivity and the observed refractivity are less than 3\% with a maximum magnitude of about 10 units. Pronounced increments from an occultation point are found within an influential radius of 500 - 600 km only. For the simulation without the assimilation of GPS refractivity ( the no-GPS run), the simulated Typhoon Nari coherently moves southwestward toward Taiwan early in the simulation but then exhibits a westward track along the northwest of Taiwan after landfall. With GPS refractivity assimilated, the simulated westward track in the no-GPS run is closer to the west coast. During landfall, the cloud convection associated with the intense vortex core encounters the Central Mountain Range ( CMR) and produces torrential rainfalls along its northwestern slope. Both the GPS run and the no-GPS run capture the observed feature of very intense rainfall over the southwestern slope base of the CMR later in the simulation, while the intensity as well as the track is improved in the GPS run. In the other case ( Nakri), the simulated rainfall distributions, in general, are similar for both the GPS run and the no-GPS run; however, the GPS run exhibits a more pronounced low to the southeast of Taiwan, which results in more intense rainfall in the northeast of Taiwan as observed. Both GPS runs for Nari and Nakri show improved skills in 24-h accumulated rainfall prediction, in particular, at later stages, as supported by higher threat scores and smaller root-mean-square errors against observations over the island. This positive impact can be attributed largely to the fact that the accumulative effects from assimilation of initial GPS refractivity soundings are instrumental to model performance. A cycling 3DVAR scheme is also explored in the simulation for Nari to investigate the impact of complementary NASA Quick Scatterometer ( QuikSCAT) near-surface wind observations on model prediction. When such observed near-surface wind is assimilated into reinitialization at a later integration time, the track prediction is further improved and thus the prediction for accumulated rainfall is improved as well.

Arguez2005

Detection of the MJO signal from QuikSCAT
A. Arguez and M. A. Bourassa and J. J. O'Brien
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  22  1885--1894  (2005)

Wind data from the SeaWinds instrument on NASA's Quick Scatterometer (QuikSCAT) satellite are investigated to ascertain how well the surface manifestation of the Madden-Julian oscillation (MJO) can be resolved. The MJO signal is detected in nonfiltered gridded data using extended EOF analysis of the zonal wind field, overshadowed by annual, semiannual, and monsoon-related modes. After bandpass filtering with Lanczos weights, MJO signals are clearly detected in several kinematic quantities, including the zonal wind speed, the zonal pseudostress, and the velocity potential. Extraction of the MJO using QuikSCAT winds compares favorably with extraction using NCEP Reanalysis 2, except that the QuikSCAT signal appears to be more robust. In addition, an alternative bandpass-filtering technique using variable filter weights near time series endpoints is presented. The method uses least squares minimization to match newly created frequency response functions in edge zones as closely as possible to the predetermined frequency response function of interior points. This method stands in contrast to the common practice of simply discarding those endpoints where a convolution cannot be computed.

Pulvirenti2006

Retrieval of atmospheric and surface parameters from satellite microwave radiometers over the Mediterranean Sea
L. Pulvirenti and N. Pierdicca
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  90--101  (2006)

A procedure to estimate atmospheric and sea surface parameters in the Mediterranean area from satellite microwave radiometric measurements is described. The method is founded on a simulator of brightness temperatures at the top of the atmosphere. The simulator is based on microwave sea emissivity and scattering model functions, derived from the outputs of the SEAWIND software, which implements a two-scale microwave sea surface model and a radiative transfer scheme in a nonscattering atmosphere. The development of the model functions aims to reduce the SEAWIND computational time, still maintaining its sensitivity to the main geophysical variables. Different adaptations of the simulation model have been performed to better reproduce the radiometric data in the region of interest. A comparison between the simulations and the Special Sensor Microwave/Imager (SSM/1) observations acquired throughout year 2000 over the Mediterranean Sea has permitted us to refine the model functions as well as to assess the whole simulation procedure. As for the inversion problem, a regression analysis has been applied to two different synthetic datasets to retrieve integrated precipitable water vapor, liquid water path and wind speed. The first dataset simulates the observations of SSMA, whilst the second one concerns the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E). Both have been generated by using the ECMWF atmospheric profiles and the measurements of the SeaWinds scatterometer aboard QuickSCAT. The SSN?I data have been used to carry out a statistical validation of the estimators. AMSR-E observations of a Tramontane-Mistral event, typical of the Mediterranean Sea, have been analyzed to evaluate the benefits of its expanded channel capability.

Haarpaintner2006

Arctic-wide operational sea ice drift from enhanced-resolution QuikScat/SeaWinds scatterometry and its validation
J. Haarpaintner
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  44  102--107  (2006)

The near-real-time enhanced-resolution QuikScat/SeaWinds (QS) scatterometry composite data are used for daily automatic 48-h sea ice tracking by maximum cross-correlation over the entire Arctic. A correlation window of 61 x 61 pixels is used for best performance. Both QS polarizations, vertical (W) and horizontal (HH), are used independently, which permits to filter the final results for erroneous vectors by comparing the two drift vectors, (u) over right arrow (VV) and (u) over right arrow (HH), respectively. Additional filtering is performed by setting a minimum correlation coefficient and by considering the spatial consistency of the motion field. The algorithm has been validated with winter 2002/2003 buoy data from the International Arctic Buoy Program showing error standard deviations in the 48-h displacement of 3.1 and 3.2 km in the latitude and longitude direction, respectively. This corresponds to an error standard deviation in ice drift speed of just 2.6 cm/s. Errors are largest in dynamic regions with lower ice concentrations as for example the southern Fram Strait. The enhanced-resolution data improve previous drift results by about 25\%, but are still blurred by the necessary 36-h period to produce the composites.

Moore2005

Tip jets and barrier winds: A QuikSCAT climatology of high wind speed events around Greenland (vol 18, pg 3713, 2005)
G. W. K. Moore and I. A. Renfrew
JOURNAL OF CLIMATE  18  4919--4919  (2005)

Nghiem2005

Rapid formation of a sea ice barrier east of Svalbard
S. V. Nghiem and M. L. Van Woert and G. Neumann
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  110    (2005)

Daily SeaWinds scatterometer images acquired by the QuikSCAT satellite show an elongated sea ice feature that formed very rapidly (similar to 1-2 days) in November 2001 east of Svalbard over the Barents Sea. This sea ice structure, called ``the Svalbard sea ice barrier,'' spanning approximately 10 degrees in longitude and 2 degrees in latitude, restricts the sea route and poses a significant navigation hazard. The secret of its formation appears to lie in the bottom of the sea: A comparison between bathymetry from the International Bathymetric Chart of the Arctic Ocean data and the pattern of sea ice formation from scatterometer data reveals that the sea ice barrier conforms well with and stretches above a deep elongated channel connecting the Franz Josef-Victoria Trough to the Hinlopen Basin between Svalbard and Franz Josef Land. Historic hydrographic data from this area indicate that this sea channel contains cold Arctic water less than 50 m below the surface. Strong and persistent cold northerly winds force strong heat loss from this shallow surface layer, leading to the rapid formation of the sea ice barrier. Heat transfer rates estimated from European Centre for Medium-Range Weather Forecasts temperature and wind data over this region suggest that the surface water along the deep channel can be rapidly cooled to the freezing point. Scatterometer results in 1999-2003 show that sea ice forms in this area between October and December. Understanding the ice formation mechanisms helps to select appropriate locations for deployment of buoys measuring wind and air-sea temperature profile and to facilitate ice monitoring, modeling, and forecasting.

Allen2005

An analysis of SeaWinds-based rain retrieval in severe weather events
J. R. Allen and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  43  2870--2878  (2005)

The Ku-band SeaWinds scatterometer estimates near-surface ocean wind vectors by relating measured backscatter to a geophysical model function for the near-surface vector wind. The conventional wind retrieval algorithm does not explicitly account for SeaWinds' sensitivity to rain, resulting in rain-caused wind retrieval error. A new retrieval method, termed ``simultaneous wind/rain retrieval,'' that estimates both wind and rain from rain-contaminated measurements has been previously proposed and validated with Tropical Rain Measuring Mission data. Here, the accuracy of rains retrieved by the new method is validated through comparison with the Next Generation Weather Radar (NEXRAD) in coastal storm events. The rains detected by both sensors are comparable, though SeaWinds-estimated rains exhibit greater variability. The performance of simultaneous wind/rain retrieval in flagging excessively rain-contaminated winds is discussed and compared to existing methods. A new rain-only retrieval algorithm for use in rain-backscatter-dominated areas is proposed and tested. A simple noise model for SeaWinds rain estimates is developed, and Monte Carlo simulation is employed to verify the model. The model shows that SeaWinds rain estimates have a standard deviation of 2.5 mm/h, which is higher than the NEXRAD measurements. Thresholding SeaWinds rain estimates at 2 mm/h yields a better rain flag than current rain flag algorithms.

Back2005

The relationship between wind speed and precipitation in the Pacific ITCZ
L. E. Back and C. S. Bretherton
JOURNAL OF CLIMATE  18  4317--4328  (2005)

The relationship between wind speed and precipitation in the Pacific ITCZ is analyzed using 4 yr of daily Special Sensor Microwave Imager (SSM/I) and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) satellite passive microwave retrievals averaged over 2.5 degrees boxes. Throughout the ITCZ, at high-column relative humidities (conditions under which deep convection is likely to occur), faster winds are associated with substantially more precipitation, explaining a small, but highly statistically significant fraction of daily rainfall variability. The slope of this relationship varies geographically and rapidly increases as the atmosphere becomes moister. Analysis of other data sources, including vector mean winds computed from QuikSCAT and area-averaged radar-derived precipitation estimates from Kwajalein Island, shows that the wind speed-precipitation correlation is robust. This relation provides a test of large-scale forecast models and insight into conceptual models of deep convection. The observed increases in precipitation are much greater than evaporation changes associated with the increased wind speed; this implies a convergence feedback by which evaporation induces moisture convergence that feeds increases in precipitation. The authors study whether the 40-yr ECMWF Re-Analysis (ERA-40) and NCEP-NCAR reanalysis show the observed wind speed-precipitation correlation and explore mechanisms for convergence feedback using column-integrated moist static energy budgets computed from the reanalyses.

Mostovoy2005

Regional accuracy of QuikSCAT gridded winds
G. V. Mostovoy and P. J. Fitzpatrick and Y. Li
INTERNATIONAL JOURNAL OF REMOTE SENSING  26  4117--4136  (2005)

The quality of gridded 00 UTC and 12 UTC QuikSCAT wind speed fields provided by the Florida State University (FSU) and NASA Jet Propulsion Laboratory (JPL) are analysed over the Bay of Bengal during May-August 2001. Additionally, an examination of these Fields is performed over the Gulf of Mexico for the May-August period from 2001 to 2003. Both 00 UTC and 12 UTC time almost coincide with QuikSCAT sampling times (twice a day) and correspond to either early morning or late evening local time over these regions. The primary restriction for generating accurate maps with a temporal resolution of 12 hours and less is a lack of adequate sanipling. Due to non-uniform spatial-temporal sampling of the scatterometer, interpolation procedures cannot provide proper estimates in data gaps over the regions not covered by a swath. Wind speed autocorrelation coefficients for gridded datasets have been compared with that of original QuikSCAT data and buoy winds. It is shown that the spatial and temporal interpolation used to obtain these datasets results in smoothing of the QuikSCAT wind speeds, reducing their original variance. This smoothing is amplified where substantial diurnal wind variability occurs. A comparison with buoy data shows that FSU and JPL gridded fields Lire unable to reproduce correctly observed low correlations in wind speed between morning and evening breeze flows and diurnal variability of winds, leading to a reduced difference between 00 UTC and 12 UTC values in comparison with buoys and swath QuikSCAT data. Rather, the FSU and JPL maps describe daily mean fields. Another consequence of the spatial-temporal interpolation is that the winds are distorted at a frequency matching the donimant sampling interval (3-4 days) of QuikSCAT measurements over the Bay of Bengal.

Frolking2005

Interannual variability in North American grassland biomass/productivity detected by SeaWinds scatterometer backscatter
S. Frolking and M. Fahnestock and T. Milliman and K. McDonald and J. Kimball
GEOPHYSICAL RESEARCH LETTERS  32    (2005)

We analyzed 2000 - 2004 growing-season SeaWinds Ku-band microwave backscatter and MODIS leaf area index (LAI) data over North America. Large anomalies in mid-growing-season mean backscatter and LAI, relative to 5-year mean values, occurred primarily in the western Great Plains; backscatter and LAI anomalies had similar spatial patterns across this region. Backscatter and LAI time series data for three similar to 10(3) km(2) regions in the western Great Plains were strongly correlated (r(2) similar to 0.6 - 0.8), and variability in mid-growing season values was well-correlated with annual precipitation ( October through September). The results indicate that SeaWinds backscatter is sensitive to interannual variability in grassland biomass/productivity, and can provide an assessment that is completely independent of optical/near-infrared remote sensing instruments.

Rogers2005

Evaluations of global wave prediction at the fleet numerical meteorology and oceanography center
W. E. Rogers and P. A. Wittmann and D. W. C. Wang and R. M. Clancy and Y. L. Hsu
WEATHER AND FORECASTING  20  745--760  (2005)

It is a major challenge to determine whether bias in operational global wave predictions is predominately due to the wave model itself (internal error) or due to errors in wind forcing (an external error). Another challenge is to characterize bias attributable to errors in wave model physics (e.g., input, dissipation, and nonlinear transfer). In this study, hindcasts and an evaluation methodology are constructed to address these challenges. The bias of the wave predictions is evaluated with consideration of the bias of four different wind forcing fields [two of which are supplemented with the NASA Quick Scatterometer (QuikSCAT) measurements]. It is found that the accuracy of the Fleet Numerical Meteorology and Oceanography Center's operational global wind forcing has improved to the point where it is unlikely to be the primary source of error in the center's global wave model (WAVEWATCH-III). The hindcast comparisons are specifically designed to minimize systematic errors from numerics and resolution. From these hindcasts, insight into the physics-related bias in the global wave model is possible: comparison to in situ wave data suggests an overall positive bias at northeast Pacific locations and an overall negative bias at northwest Atlantic locations. Comparison of frequency bands indicates a tendency by the model physics to overpredict energy at higher frequencies and underpredict energy at lower frequencies.

Morey2005

Remotely sensed winds for episodic forcing of ocean models
S. L. Morey and M. A. Bourassa and X. J. J. Davis and J. J. O'Brien and J. Zavala-Hidalgo
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  110    (2005)

A new method is described for forcing regional ocean models with wind stress fields derived from satellite scatterometer data. A variational technique is applied to produce regularly gridded surface wind (stress) fields in time and space using data from the NASA SeaWinds scatterometer aboard the QuikSCAT satellite. Three uniformly gridded wind stress products are produced with satellite scatterometer data for the Gulf of Mexico, one based solely on scatterometer data and the other two constrained to a background field derived from numerical weather prediction (NWP) models. These, along with winds from the Eta-29 NWP model, are used to force a high-resolution ocean model of the Gulf of Mexico. The four wind products are compared to National Data Buoy Center in situ observations. Ocean model data are compared to coastal sea level stations and moored acoustic Doppler current profiler data over the West Florida Shelf. The results show that the satellite products capture much of the variability at atmospheric synoptic scales and show great promise for representing energetic episodic events such as tropical cyclones. The techniques presented in this work are applicable to improving numerical ocean simulations of inaccessible or data-sparse regions of the world.

Rawlins2005

Remote sensing of snow thaw at the pan-Arctic scale using the SeaWinds scatterometer
M. A. Rawlins and K. C. McDonald and S. Frolking and R. B. Lammers and M. Fahnestock and J. S. Kimball and C. J. Vorosmarty
JOURNAL OF HYDROLOGY  312  294--311  (2005)

Remotely sensed estimates of snow thaw offer the potential of more complete spatial coverage across remote, undersampled areas such as the terrestrial Arctic drainage basin. We compared the timing of spring thaw determined from approximately 25 km resolution daily radar backscatter data with observed daily river discharge time series and model simulated snow water content data for 52 basins (5000-10,000 km(2)) across Canada and Alaska for the spring of 2000. Algorithms for identifying critical thaw transitions were applied to daily backscatter time series from the SeaWinds scatterometer aboard NASA QuikSCAT, the observed discharge data, and model snow water from the pan-Arctic Water Balance Model (PWBM). Radar-derived thaw shows general agreement with discharge increases (Mean Absolute Difference, MAD= 21 days, r=0.45), with better agreement (16 days) in basins with moderate-high runoff due to snow thaw. Even better agreement is noted when comparing the scatterometer-derived primary thaw timing with model simulated snow water increase (MAD= 14 days, r=0.75). Good correspondence is found across higher latitude basins in western Canada and Alaska, while the largest discrepancies appear at the driest watersheds with lower snow and daily discharge amounts. Extending this analysis to the entire pan-Arctic drainage basin, we compared scatterometer-derived date of the primary (maximum) thaw with the timing of simulated snow water increases from the PWBM. Good agreement is found across much of the pan-Arctic; discrepancies for over half of the analyzed grid cells are less than one week. MADs are 11.7 days for the Arctic basin in Eurasian and 15.1 days across North America. Mean biases are low; 2.1 and -3.1 days for Eurasia and North America, respectively. Stronger backscatter response (high signal-low noise) is noted with higher seasonal snow accumulation, low to moderate tree cover and low topographic complexity. Although our results show inconsistent performance along coastal regions and warmer southerly parts of the study domain, active radar instruments such as SeaWinds offer the potential for monitoring high-latitude snow thaw at spatial scales appropriate for pan-Arctic applicationsin near real time. Applications include hydrological model verification, analysis of lags between snow thaw and river response, and determination of large-scale snow extent. (c) 2005 Elsevier B.V. All rights reserved.

Moore2005

Tip jets and barrier winds: A QuikSCAT climatology of high wind speed events around Greenland
G. W. K. Moore and I. A. Renfrew
JOURNAL OF CLIMATE  18  3713--3725  (2005)

The high topography of Greenland results in a number of orographically induced high wind speed flows along its coast that are of interest from both a severe weather and climate perspective. Here the surface wind field dataset from the NASA-JPL SeaWinds scatterometer on board the Quick Scatterometer (Quik-SCAT) satellite is used to develop a wintertime climatology of these flows. The high spatial resolution and the twice-daily sampling of the SeaWinds instrument allows for a much more detailed view of the surface winds around Greenland than has been previously possible. Three phenomena stand out as the most distinctive features of the surface wind field during the winter months: the previously identified tip jets and reverse tip jets, as well as the hitherto unrecognized barrier flows along its southeast coast in the vicinity of the Denmark Strait. Peak surface wind speeds associated with these phenomena can be as large as 50 m s(-1) with winds over 25 m s(-1) occurring approximately 10\%-15\% of the time at each location. A compositing technique is used to show that each type of flow is the result of an interaction between a synoptic-scale parent cyclone and the high topography of Greenland. In keeping with previous work, it is argued that tip jets are caused by a combination of conservation of the Bernoulli function during orographic descent and acceleration due to flow splitting as stable air passes around Cape Farewell, while barrier winds are a geostrophic response to stable air being forced against high topography. It is proposed that reverse tip jets occur when barrier winds reach the end of the topographic barrier and move from a geostrophic to a gradient wind balance, becoming supergeostrophic as a result of their anticyclonic curvature.

Gille2005

Statistical characterization of zonal and meridional ocean wind stress
S. T. Gille
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  22  1353--1372  (2005)

Four years of ocean vector wind data are used to evaluate statistics of wind stress over the ocean. Raw swath wind stresses derived from the Quick Scatterometer (QuikSCAT) are compared with five different global gridded wind products, including products based on scatterometer observations, meteorological analysis winds from the European Centre for Medium-Range Weather Forecasts, and reanalysis winds from the National Centers for Environmental Prediction. Buoy winds from a limited number of sites in the Pacific Ocean are also considered. Probability density functions (PDFs) computed for latitudinal bands show that mean wind stresses for the six global products are largely in agreement, while variances differ substantially, by a factor of 2 or more, with swath wind stresses indicating highest variances for meridional winds and for zonal winds outside the Tropics. Higher moments of the PDFs also differ. Kurtoses are large for all wind products, implying that PDFs are not Gaussian. None of the available gridded products fully captures the range of extreme wind events seen in the raw swath data. Frequency spectra for the five gridded products agree with frequency spectra from swath data at low frequencies, but spectral slopes differ at higher frequencies, particularly for frequencies greater than 100 cycles per year (cpy), which are poorly resolved by a single scatterometer. In the frequency range between 10 and 90 cpy that is resolved by the scatterometer, spectra derived from swath data are flatter than spectra from gridded products and are judged to be flatter than omega(-2/3) at all latitudes.

Wang2005

Melt season duration on Canadian Arctic ice caps, 2000-2004
L. Wang and M. J. Sharp and B. Rivard and S. Marshall and D. Burgess
GEOPHYSICAL RESEARCH LETTERS  32    (2005)

The extent and duration of summer melt on ice caps in the Queen Elizabeth Islands (QEI), Nunavut, Canada, in 2000 - 2004 were mapped using enhanced resolution QuikSCAT (QSCAT) scatterometer images. The mean melt duration depends mainly on surface elevation and distance from Baffin Bay. Over most ice caps, inter-annual variations in melt duration and the variation in melt duration with elevation are closely related to variations in the July 500 hPa height over the QEI. Pressure-related variability in the vertical gradient of near-surface air temperature appears to be a major control on the inter-annual variations in average melt duration.

Brown2005

Comments on the synergism between the analytic planetary boundary-layer model and remote sensing data
R. A. Brown
BOUNDARY-LAYER METEOROLOGY  116  187--199  (2005)

This paper is adapted from a presentation at the session of the European Geophysical Society meeting in 2002 honouring Joost Businger. It documents the interaction of the non-linear planetary boundary-layer (PBL) model (UW-PBL) and satellite remote sensing of marine surface winds from verification and calibration studies for the sensor model function to the current state of verification of the model by satellite data. It is also a personal history where Joost Businger had seminal input to this research at several critical junctures. The first scatterometer in space was on SeaSat in 1978, while currently in orbit there are the QuikSCAT and ERS-2 scatterometers and the WindSat radiometer. The volume and detail of data from the scatterometers during the past decade are unprecedented, though the value of these data depends on a careful interpretation of the PBL dynamics. The model functions (algorithms) that relate surface wind to sensor signal have evolved from straight empirical correlation with simple surface-layer 10-m winds to satellite sensor model functions for surface pressure fields. A surface stress model function is also available. The validation data for the satellite model functions depended crucially on the PBL solution. The non-linear solution for the flow of fluid in the boundary layer of a rotating coordinate system was completed in 1969. The implications for traditional ways of measuring and modelling the PBL were huge and continue to this day. Unfortunately, this solution replaced an elegant one by Ekman with a stability/finite perturbation equilibrium solution. Consequently, there has been great reluctance to accept this solution. The verification of model predictions has been obtained from the satellite data.

Lancaster2005

Laser pulse reflectance of the ocean surface from the GLAS satellite lidar

Risien2004

Variability in satellite winds over the Benguela upwelling system during 1999-2000
C. M. Risien and C. J. C. Reason and F. A. Shillington and D. B. Chelton
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  109    (2004)

[ 1] Wind stress variability over the Benguela upwelling system is considered using 16 months ( 01 August 1999 to 29 November 2000) of satellite-derived QuikSCAT wind data. Variability is investigated using a type of artificial neural network, the self-organizing map (SOM), and a wavelet analysis. The SOM and wavelet analysis are applied to an extracted data set to find that the system may be divided into six discrete wind regimes. The wavelet power spectra for these wind regions span a range of frequencies from 4 to 64 days, with each region appearing to contain distinct periodicities. To the north, 10degrees-23.5degreesS, the majority of the power occurs during austral winter, with a 4-16 day periodicity. Further investigation of National Centers for Environmental Prediction reanalysis outgoing longwave radiation data indicates that the winter intensification of wind stress off the Angolan coast is linked with convective activity over equatorial West Africa. The summer activity appears to be linked with the intensification of the Angolan heat low. Convective activity over the Congo basin appears to impact upon wind stress variability, off the Angolan coast, throughout the year. Farther south, 24degrees-35degrees S, the majority of the power occurs in the summer. Here a bimodal distribution occurs, with peaks of 4-12 and 25-50 days. The southernmost regions appear to be forced at higher frequencies by both midlatitude cyclones ( austral winter) and mesoscale coastal lows ( austral summer). At lower frequencies, eastward propagating periodic wind events that originate over eastern South America appear to be important to the forcing of wind stress over the southern Benguela.

Tang2004

Remote sensing oceanography of a harmful algal bloom off the coast of southeastern Vietnam
D. L. Tang and H. Kawamura and H. Doan-Nhu and W. Takahashi
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  109    (2004)

[ 1] Harmful algal blooms (HABs) in the southeastern Vietnamese coastal waters have caused large economic losses in aquacultured and wild fisheries in recent years; however, there have been few oceanographic studies on these HAB events. The present study reports an extensive HAB off southeastern Vietnamese waters during late June to July 2002 with in situ observations and analyzes the oceanographic conditions using satellite remote sensing data. The HAB had high chlorophyll a ( Chl a) concentrations ( up to 4.5 mg m-3) occurring -200 km off the coast and -200 km northeast of the Mekong River mouth for a period of -6 weeks. The bloom was dominated by the harmful algae haptophyte Phaeocystis cf. globosa and caused a very significant mortality of aquacultured fish and other marine life. In the same period, sea surface temperature (SST) imagery showed a cold water plume extending from the coast to the open sea, and QuikScat data showed strong southwesterly winds blowing parallel to the coastline. This study indicated that the HAB was induced and supported by offshore upwelling that brings nutrients from the deep ocean to the surface and from coastal water to offshore water and that the upwelling was driven by strong wind through Ekman transport when winds were parallel to the coastline. This study demonstrated the possibility of utilizing a combination of satellite data of Chl a, SST, and wind velocity together with coastal bathymetric information and in situ observations to give a better understanding of the biological oceanography of HABs.

Draper2004

Evaluating the effect of rain on SeaWinds scatterometer measurements
D. W. Draper and D. G. Long
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  109    (2004)

A simple wind/rain backscatter model is used with co-located precipitation radar ( PR) data from the Tropical Rainfall Measuring Mission (TRMM) satellite to evaluate the effect of rain on SeaWinds on QuikSCAT sigmadegrees observations. The model incorporates wind-induced surface scattering, the surface rain perturbation, and atmospheric rain attenuation and scattering. The co-located PR measurements afford direct computation of SeaWinds-scale averaged rain rate and atmospheric rain attenuation and scattering. An estimate of the wind-induced surface backscatter is computed via numerical weather prediction (NWP) winds. By synergistically combining the SeaWinds, NWP, and PR data, estimates of surface rain perturbation and combined surface/atmospheric scattering are made as a function of PR-derived rain rate. The scattering from rain is dominated mainly by the surface perturbation low rain rates, and by atmospheric scattering at high rain rates. The backscatter model estimates 94\% of the observed rain-contaminated SeaWinds on QuikSCAT sigmadegrees values to within 3 dB. Using the model, the conditions are determined for which it is possible to estimate rain from scatterometer measurements and where wind retrieval is not possible.

Monaldo2004

A systematic comparison of QuikSCAT and SAR ocean surface wind speeds
F. M. Monaldo and D. R. Thompson and W. G. Pichel and P. Clemente-Colon
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  42  283--291  (2004)

We performed a systematic comparison of wind speed measurements from the SeaWinds QuikSCAT scatterometer and wind speeds computed from RADARSAT-1 synthetic aperture radar (SAR) normalized radar cross section measurements. These comparisons were made over in the Gulf of Alaska and extended over a two-year period, 2000 and 2001. The SAR wind speed estimates require a wind direction to initialize the retrieval. Here, we first used wind directions from the Navy Operational Global Atmospheric Prediction System (NOGAPS) model. For these retrievals, the standard deviation between the resulting SAR and QuikSCAT wind speed measurements was 1.78 m/s. When we used the QuikSCAT-measured wind directions to initialize the inversion, comparisons improve to a standard deviation of 1.36 m/s. We used these SAR-scatterometer comparisons to generate a new C-band horizontal polarization model function. With this new model function, the wind speed inversion improves to a standard deviation of 1.24 m/s with no mean bias. These results strongly suggest that SAR and QuikSCAT measurements can be combined to make better high-resolution wind measurements than either instrument could alone in coastal areas.

Portabella2004

A probabilistic approach for SeaWinds data assimilation
M. Portabella and A. Stoffelen
QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY  130  127--152  (2004)

Scatterometer sea surface wind observations are being successfully assimilated into numerical weather prediction models. The quality of the winds retrieved from the new SeaWinds scatterometer (onboard the QuikSCAT satellite) depends on the subsatellite cross-track location. In particular, the poor azimuth separation or diversity between views in the nadir region results in poor quality winds. In the QuikSCAT nadir region, where the local cost-function minima are broad. the use of the standard procedure results in arbitrary and inaccurate winds. A new scheme. Which accounts for broad cost-function minima by allowing more ambiguous wind solutions, i.e. a multiple solution scheme (MSS), is proposed as an alternative to the standard procedure. The probability of every ambiguous solution being the 'true' wind is empirically derived, and used in the ambiguity-removal procedure to make the scheme flexible enough to accept many wind solutions. A comparison between the standard wind retrieval and the MSS procedures at 100 km resolution is then performed, using independent model winds for validation. The MSS turns out to be more in agreement with the model reference than the standard procedure, especially at nadir. Moreover, it shows more spatially consistent and realistic winds by more effectively exploiting the information content of the observations.

Chelton2004

Satellite measurements reveal persistent small-scale features in ocean winds
D. B. Chelton and M. G. Schlax and M. H. Freilich and R. F. Milliff
SCIENCE  303  978--983  (2004)

Four-year averages of 25-kilometer-resolution measurements of near-surface wind speed and direction over the global ocean from the QuikSCAT satellite radar scatterometer reveal the existence of surprisingly persistent small-scale features in the dynamically and thermodynamically important curl and divergence of the wind stress. Air-sea interaction over sea surface temperature fronts throughout the world ocean is evident in both the curl and divergence fields, as are the influences of islands and coastal mountains. Ocean currents such as the Gulf Stream generate distinctive patterns in the curl field. These previously unresolved features have important implications for oceanographic and air-sea interaction research.

Olsen2004

Sea-air flux of CO2 in the Caribbean Sea estimated using in situ and remote sensing data
A. Olsen and J. A. Trinanes and R. Wannitilchof
REMOTE SENSING OF ENVIRONMENT  89  309--325  (2004)

Empirical relationships between sea surface carbon dioxide fugacity (fCO(2)(sw)) and sea surface temperature (SST) were applied to datasets of remotely sensed SST to create fCO(2)(sw) fields in the Caribbean Sea. SST datasets from different sensors were used, as well as the SST fields created by optimum interpolation of bias corrected AVHRR data. Empirical relationships were derived using shipboard fCO(2)(sw) data, in situ SST data, and SST data from the remote sensing platforms. The results show that the application of a relationship based on shipboard SST data, on fields of remotely sensed SST yields biased fCO(2)(sw) values. This bias is reduced if the fCO(2)(sw)-SST relationships are derived using the same SST data that are used to create the SST fields. The fCO(2)(sw) fields found to best reproduce observed fCO(2)(sw) are used in combination with wind speed data from QuikSCAT to create weekly maps of the sea-air CO2 flux in the Caribbean Sea in 2002. The region to the SW of Cuba was a source Of CO2 to the atmosphere throughout 2002, and the region to the NE was a sink during winter and spring and a source during summer and fall. The net uptake of CO2 in the region was doubled when potential skin layer effects on fCO(2)(sw) were taken into account. (C) 2003 Elsevier Inc. All rights reserved.

Weissman2003

Calibrating the quikscat/seawinds radar for measuring rainrate over the oceans
D. E. Weissman and M. A. Bourassa and J. J. O'Brien and J. S. Tongue
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  41  2814--2820  (2003)

This effort continues a study of the effects of rain, over the oceans, on the signal retrieved by the SeaWinds scatterometer. It is determined that the backscatter radar cross section can be used to estimate the volumetric rain rate, averaged horizontally, across the surface resolution cells of the scatterometer. The dual polarization of the radar has a key role in developing this capability. The relative magnitudes of the radar backscatter depends on the volumetric rain rate, the rain column height and surface wind velocity, the viewing angle, as well as the polarization (due to the oblateness of raindrops at the higher rain rates). The approach to calibrating the SeaWinds normalized radar cross section (NRCS) is to collect National Weather Service Next Generation Weather Radar (NEXRAD) radar-derived rain rate measurements (4-km spatial resolution and 6-min rotating cycles) colocated in space (offshore) and time with scatterometer observations. These calibration functions lead to a Z-R relationship, which is then used at mid-ocean locations to estimate the rain rate in 0.25degrees or larger resolution cells, which are compared with Tropical Rainfall Mapping Mission (TRMM) Microwave Imager (TMI) rain estimates. Experimental results to date are in general agreement with simplified theoretical models of backscatter from rain, for this frequency, 14 GHz. These comparisons show very good agreement on a cell-by-cell basis with the TMI estimates for both wide areas (1000 km) and smaller area rain events.

Pickett2003

QuikSCAT satellite comparisons with nearshore buoy wind data off the US West Coast
M. H. Pickett and W. Q. Tang and L. K. Rosenfeld and C. H. Wash
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  20  1869--1879  (2003)

To determine the accuracy of nearshore winds from the QuikSCAT satellite, winds from three satellite datasets ( scientifically processed swath, gridded near-real-time, and gridded science datasets) were compared to those from 12 nearshore and 3 offshore U. S. West Coast buoys. Satellite observations from August 1999 to December 2000 that were within 25 km and 30 min of each buoy were used. Comparisons showed that satellite - buoy wind differences near shore were larger than those offshore. Editing the satellite data by discarding observations recorded in rain and those recorded in light winds improved the accuracy of all three datasets. After removing rain-flagged data and wind speeds less than 3 m s(-1), root-mean-squared differences (satellite minus buoy) for swath data, the best of the three datasets, were 1.4 m s(-1) and 37degrees based on 5741 nearshore comparisons. By removing winds less than 6 m s(-1), these differences were reduced to 1.3 m s(-1) and 26degrees. At the three offshore buoys, the root-mean-squared differences for the swath data, with both rain and winds less than 6 m s(-1) removed, were 1.0 m s(-1) and 15degrees based on 1920 comparisons. Although the satellite's scientifically processed swath data near shore do not match buoy observations as closely as those offshore, they are sufficiently accurate for many coastal studies.

Chan2003

Interannual variations of tropical cyclone size over the western {North Pacific}
J. C. L. Chan and C. K. M. Yip
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

This paper presents the results of an investigation on the interannual variations of tropical cyclone (TC) size, defined as the average radius of 15 m s(-1) surface winds, over the western North Pacific from 1999 to 2002. The wind data are from QuikSCAT. The average TC size is found to increase from 1999 to 2002, with the TC sizes in 1999 and 2000 being significantly smaller than those in both 2001 and 2002. Differences in the environmental flow patterns associated with the tracks and formation positions of the TCs among these years apparently explain such size changes. Since 1999 and 2000 were La Nina years, TCs in these two years formed more westward with fewer recurvers so that they tended to be smaller. On the other hand, with 2002 being an El Nino year, TCs formed further eastward with more recurvers, and thus tended to be larger.

Cardellach2003

Mediterranean Balloon Experiment: ocean wind speed sensing from the stratosphere, using GPS reflections
E. Cardellach and G. Ruffini and D. Pino and A. Rius and A. Komjathy and J. L. Garrison
REMOTE SENSING OF ENVIRONMENT  88  351--362  (2003)

The MEditerranean Balloon EXperiment (MEBEX), conducted in August 99 from the middle-up stratosphere, was designed to assess the wind retrieval sensitivity of Global Navigation Satellite Systems Reflections (GNSSR) technology from high altitudes. Global Positioning System reflected signals (GPSR) collected at altitudes around 37 km with a dedicated receiver have been inverted to mean square slopes (MSS) of the sea surface and wind speeds. The theoretical tool to interpret the geophysical parameters was a bistatic model, which also depends on geometrical parameters. The results have been analyzed in terms of internal consistency, repeatability and geometry-dependent performance. In addition, wind velocities have been compared to independent measurements by QuikSCAT, TOPEX, ERS/RA and a Radio Sonde, with an agreement better than 2 m/s. A Numerical Weather Prediction Model (NWPM, the MM5 mesoscale forecast model) has also been used for comparison with varying results during the experiment. The conclusion of-this study confirms the capability of high altitude GPSR/Delay-map receivers with low gain antennas to infer surface winds. (C) 2003 Elsevier Inc. All rights reserved.

Yueh2003

QuikSCAT wind retrievals for tropical cyclones
S. H. Yueh and B. W. Stiles and W. T. Liu
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  41  2616--2628  (2003)

The use of QuikSCAT data for wind retrievals of tropical cyclones is described. The evidence of QuikSCAT sigma(0) dependence on wind direction for >30-m/s wind speeds is presented. The QuikSCAT sigma(0) s show a peak-to-peak wind direction modulation of similar to1 dB at 35-m/s wind speed, and the amplitude of modulation decreases with increasing wind speed. The decreasing directional sensitivity to wind speed agrees well with the trend of QSCAT1 model function at near 20 m/s. A correction of the QSCAT1 model function for above 23-m/s wind speed is proposed. We explored two microwave radiative transfer models to correct the attenuation and scattering effects of rain for wind retrievals. One is derived from the collocated QuikSCAT and Special Sensor Microwave/Imager (SSM/I) dataset, and the other one is a published parametric model developed for rain radars. These two radiative transfer models account for the effects of volume scattering, scattering from rain-roughened surfaces and rain attenuation. The models suggest that the sigma(0) s of wind-roughened sea surfaces for 40-50-m/s winds are comparable to the sigma(0) s of rain contributions for up to about 10-15 mm/h. Both radiative transfer models have been used to retrieve the ocean wind vectors from the collocated QuikSCAT and SSM/I rain rate data for several tropical cyclones. The resulting wind speed estimates of these tropical cyclones show improved agreement with the wind fields derived from the best track analysis and Holland's model for up to about 15-mm/h SSM/I rain rate. A comparative analysis of maximum wind speed estimates suggests that other rain parameters likely have to be considered for further improvements.

Li2003

Satellite data analysis and numerical simulation of tropical cyclone formation
T. Li and B. Fu and X. Y. Ge and B. Wang and M. Peng
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] Forecast of tropical cyclone ( TC) formation has long been a great challenge owing to lack of reliable observations over vast open oceans. Recent satellite products provide a unique opportunity to reveal detailed atmospheric wave structures prior to TC formation. Using the QuikSCAT surface wind and the Tropical Rainfall Measurement Mission Microwave Image data, we document temporal and spatial structures of Rossby wave trains induced by energy dispersion from a pre- existing TC and easterly wave propagation characteristics prior to cyclogenesis in the western North Pacific. Using a baroclinic model, we further simulate cyclogenesis processes associated with the TC energy dispersion and easterly waves.

Pasch2003

Comments on ``Early detection of tropical cyclones using SeaWinds-derived vorticity''
R. J. Pasch and S. R. Stewart and D. P. Brown
BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY  84  1415--1416  (2003)

Sharp2003

Comments on ``Early detection of tropical cyclones using SeaWinds-derived vorticity'' - Reply
R. J. Sharp and M. A. Bourassa and J. J. O'Brien
BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY  84  1417--1417  (2003)

Castro2003

Evolution and extension of the Santa Ana winds of February 2002 over the ocean, off California and the Baja California Peninsula
R. Castro and A. Pares-Sierra and S. G. Marinone
CIENCIAS MARINAS  29  275--281  (2003)

The evolution and extension of Santa Ana winds over the Pacific Ocean, off the coasts of California and the Baja California Peninsula, is documented from QuikScat satellite observations. The typical wind pattern in this region is towards the southeast, but between 9 and 12 February 2002 it was modified by Santa Ana winds, changing in direction towards the south, southwest and west. The changes were notable at first in southern California, but the largest indices of variation occurred between 26degreesN and 28degreesN off the Baja California Peninsula, with maxima in the Gulf of California, mainly in the northern part. The maximum extension of the winds to the west of the coast was on I I February, reaching 32degreesN and 125degreesW, or similar to700 km; however, towards the southwest the winds extended to 25degreesN and 124degreesW, or similar to1000 km. The topography of the peninsula plays an important role in controlling the winds passing from the Gulf of California to the Pacific, causing plumes of sand and dust in several locations over the ocean.

Hilburn2003

Development of scatterometer-derived surface pressures for the Southern Ocean
K. A. Hilburn and M. A. Bourassa and J. J. O'Brien
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  108    (2003)

[1] High-resolution, research-quality surface pressures are objectively calculated over the Southern Ocean using winds derived from the SeaWinds scatterometer on the QuikSCAT satellite. The pressure fields are validated in comparison to in situ observations. Overall, the scatterometer-derived surface pressures are a small improvement over the National Centers for Environmental Prediction/National Center for Atmospheric Research ( NCEP/NCAR) reanalysis, which is used as the objective technique's background field. This improvement is understated primarily because the comparison data undersample storms. Instances are found where the NCEP/NCAR reanalysis misses storms entirely and the scatterometer-derived pressures are a large improvement ( as much as 20 hPa).

Tournadre2003

Impact of rain cell on scatterometer data: 1. Theory and modeling
J. Tournadre and Y. Quilfen
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  108    (2003)

[1] The two scatterometers currently in operation, the Ku-band NASA Seawinds on the QuikScat satellite and the C-band AMI-Wind on the ERS-2 satellite, are designed to infer the ocean wind vectors from sea surface radar backscatter measurements. They provide excellent coverage of the ocean, and their wind products are of great value for ocean and meteorological communities. However, the presence of rain within scatterometer cells can significantly modify the sea surface backscatter coefficient and hence alter the wind vector retrieval. These perturbations can hamper the analysis of wind fields within atmospheric low-pressure systems or tropical cyclones. Rain perturbations result from volume scattering and attenuation by raindrops in the atmosphere as well as changes of sea surface roughness by impinging drops. For scatterometers operating at Ku-Band, attenuation and volume scattering are strong and one order of magnitude larger than at C-band. The wind retrieval will thus be less affected for the C-band AMI-Wind instrument than for the Ku-band Seawinds. A theoretical model, based on radiative transfer formulation including rain attenuation and scattering, has been developed to quantify the modification by rain of the measured backscatter and of the retrieved wind vectors. Changes in surface roughness, a complex phenomenon not yet fully understood and parameterized, is not considered here although it could be of importance for high rain rates. As a scatterometer cell covers several hundred square kilometers, inhomogeneities of rain within the cell will further modify the measured backscatter, particularly in case of small, intense precipitating rain cells. Using analytical rain cell models and constant wind fields, the effects of partial beam filling by rain is investigated. The model results show that Ku-band scatterometer data are greatly affected by rain and are extremely sensitive to the distribution of rain within scatterometer cells, i.e., to the distance between the rain cell center and the scatterometer resolution cell center. When the scatter from the sea surface is low, the additional volume scattering from rain will have a marked effect leading to an overestimation of the low wind speed actually present. Conversely, when the backscatter is already high (at high winds), attenuation by rain will reduce the signal causing an underestimation of the wind speed. The wind direction is modified in a complex manner and mainly depends on the rain distribution within the scatterometer cell. These results show that, especially at low and moderate wind speed, rain data such as the Special Sensor Microwave/Imager (SSM/I) rain fields are too coarse for correction of Normalized Radar Cross Section (NRCS) and that high-resolution rain data (such as the Tropical Rainfall Mapping Mission (TRMM) ones) are necessary. They also show that a good rain flagging is still an important issue for the operational use of Ku-band scatterometer data. A succeeding paper will present an example of application of the model for the correction of QuikScat data using TRMM rain data within a tropical cyclone.

Senan2003

Intraseasonal ''monsoon jets'' in the equatorial Indian Ocean
R. Senan and D. Sengupta and B. N. Goswami
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] The zonal wind in the equatorial Indian Ocean (EqIO) is westerly almost throughout the year. It has a strong semiannual cycle and drives the spring and fall Wyrtki jets. In addition, high resolution daily satellite winds show ``westerly wind bursts'' lasting 10-40 days, associated with atmospheric convection in the eastern EqIO. These bursts have the potential to produce intraseasonal eastward equatorial jets in the ocean. Using an ocean model driven by QuikSCAT scatterometer winds, we show that strong westerly bursts associated with summer monsoon intraseasonal oscillations can drive ''monsoon jets'' in the eastern EqIO, which have been observed recently. Although there are distinct equatorial wind bursts associated with Madden-Julian oscillations in January March, they do not produce equatorial jets in the ocean. The role of ocean dynamics in producing the selective response of the ocean is discussed.

Lin2003

First evidence for the detection of natural surface films by the QuikSCAT scatterometer
I. I. Lin and W. Alpers and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] For the first time it is demonstrated that with the QuikSCAT scatterometer it is possible to detect natural surface films resulting from enhanced biological activity in the ocean. It is shown for two regions in the Norwegian and Baltic Sea that areas of strongly reduced Normalized Radar Cross Section (NRCS) are associated with areas of enhanced chlorophyll-a concentration as evidenced by quasi-simultaneously acquired SeaWiFS data. This result has two implications. Firstly, it opens up the possibility to map globally natural surface film coverage using QuikSCAT data. Secondly, it demonstrates that in ocean areas with high biological activity the presence of natural surface films can give rise to significant errors in wind vector retrieval when using the current QuikSCAT wind retrieval algorithm.

ONeill2003

Observations of SST-induced perturbations of the wind stress field over the Southern Ocean on seasonal timescales
L. W. O'Neill and D. B. Chelton and S. K. Esbensen
JOURNAL OF CLIMATE  16  2340--2354  (2003)

The surface wind stress response to sea surface temperature (SST) over the latitude range 30degrees-60degreesS in the Southern Ocean is described from the National Aeronautics and Space Administration's QuikSCAT scatterometer observations of wind stress and Reynolds analyses of SST during the 2-yr period August 1999 to July 2001. While ocean-atmosphere coupling at midlatitudes has previously been documented from several case studies, this is the first study to quantify this relation over the entire Southern Ocean. The spatial structures of the surface wind perturbations with wavelengths shorter than 10degrees latitude by 30degrees longitude are closely related to persistent spatial variations of the SST field on the same scales. The wind stress curl and divergence are shown to be linearly related, respectively, to the crosswind and downwind components of the SST gradient. The curl response has a magnitude only about half that of the divergence response. This observed coupling is consistent with the hypothesis that SST modification of marine atmospheric boundary layer (MABL) stability affects vertical turbulent mixing of momentum, inducing perturbations in the surface winds. The nonequivalence between the responses of the curl and divergence to the crosswind and downwind SST gradients suggests that secondary circulations in the MABL may also play an important role by producing significant perturbations in the surface wind field near SST fronts that are distinct from the vertical turbulent transfer of momentum. The importance of the wind stress curl in driving Ekman vertical velocity in the open ocean implies that the coupling between winds and SST may have important feedback effects on upper ocean processes near SST fronts.

Nakamura2003

Variation of underwater noise in the tomography experiments at the Central Pacific
T. Nakamura and H. Fujimori and I. Nakano
JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES \& REVIEW PAPERS  42  3194--3197  (2003)

Tomography experiments using seven 200 Hz transceivers were conducted from January to December 2000 around the date line in the Central Pacific Ocean. The surface buoy for, data communication of transceiver No. 6 was separated from the mooring system and began to drift on September 12, 2000. In this report, variation of underwater noise is investigated around that day. As a result, an increase of noise level more than 15 dB was found on September 11 in comparison to the stationary level. To investigate the reason for the extraordinary noise level, wind data by ``QuikSCAT'' of NASA were examined. Wind speed was approximately 10 m/s at the area and did not affect the noise level. As many line spectra were observed in the underwater noise and a steep slope was shown in the inclination data of the mooring on that day, separation of the buoy may have occurred by an artificial phenomenon.

Hu2003

Oceanic thermal and biological responses to Santa Ana winds
H. Hu and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] Ocean surface wind vectors with improved spatial resolutions were derived from the National Aeronautics and Space Administration's (NASA's) Quick Scatterometer (QuikSCAT) satellite. They allow us to examine the details and extent of oceanic influence of a Santa Ana event - a strong offshore and downslope wind in southern California that may spread wide fires, damage properties, and endanger aviation. The oceanic thermal and biological responses to the surface wind jets were observed with other spaceborne sensors. It is found that surface wind jets reduce sea surface temperatures and increase biological productivities. Spacebased measurements demonstrate the inadequacy of current operational numerical weather prediction (NWP) models to accurately and consistently predict the characteristics of Santa Ana winds over the coastal ocean.

Patoux2003

Global pressure fields from scatterometer winds
J. Patoux and R. C. Foster and R. A. Brown
JOURNAL OF APPLIED METEOROLOGY  42  813--826  (2003)

A method is presented for computing global surface pressure fields from satellite scatterometer winds. Pressure gradients are estimated using a two-layer similarity planetary boundary layer model in the midlatitudes and a mixed-layer model in the Tropics. A global pressure field is then fit to the pressure gradients by least squares optimization. A series of surface pressure fields calculated from SeaWinds-on-QuikSCAT (Quick Scatterometer) measurements are compared with numerical weather analyses and buoy measurements. Surface pressure observations in the tropical oceans are scarce and come largely from ships of opportunity. At present no buoy in the Atlantic Ocean and only 10 buoys in the Pacific Ocean have pressure sensors. The method presented here suggests that 0.5degrees-resolution maps of sea surface pressure can be readily retrieved from available satellite remote sensing data every 12 h in near-real time. It is shown that these fields are at least of comparable quality to the ECMWF analyses.

Goswami2003

A note on the deficiency of NCEP/NCAR reanalysis surface winds over the equatorial Indian Ocean
B. N. Goswami and D. Sengupta
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  108    (2003)

The seasonal cycle and intraseasonal variability of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP) reanalysis surface winds over the Indian Ocean (IO) are assessed by comparing them with in situ surface observations from two moored buoys and winds from the SeaWinds scatterometer on the QuikSCAT satellite. The buoys are located in the central Bay of Bengal and eastern Arabian Sea. Both QuikSCAT and NCEP wind products reproduce closely the seasonal cycle and intraseasonal variability (10-60 day) in the in situ observations. In the equatorial IO, however, the seasonal mean NCEP wind speeds can be 2-3 m s(-1) smaller and the amplitude of intraseasonal variability only about half that of QuikSCAT winds. The systematic errors of NCEP zonal winds are comparable to the annual mean or amplitude of the seasonal cycle in the equatorial IO. It is suggested that the systematic error of mean and intraseasonal variability of reanalysis winds is related to systematic error in the NCEP analysis of precipitation.

Torres2003

Spatial patterns of wind and sea surface temperature in the Galician upwelling region
R. Torres and E. D. Barton and P. Miller and E. Fanjul
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  108    (2003)

[1] Cape Finisterre is the most northwest point in the Galician region and separates the meridional west and zonal north coasts of Galicia. The wind field there has high spatial and temporal variability throughout the year. No clear seasonal signal is evident; upwelling and nonupwelling patterns alternate in all seasons. Two outstanding features of the Galician region are persistent upwelling near Cape Finisterre even when not present farther south and alternation of strong upwelling on north and west coasts. Up to now, explanations have relied upon particular dynamics of oceanic flow past Finisterre. We find that major features of upwelling around Finisterre are related to strong spatial structure in the wind field. Analysis of QuikScat wind data for July 1999 to May 2001 shows strong repeatable patterns in the synoptic wind field. These wind patterns emerge as the combination of the two dominant modes in a complex empirical orthogonal function (CEOF) analysis representing over 85\% of the variance. Summer wind patterns give rise to characteristic distributions of upwelling along the coast and favor development of filaments in particular locations. The wind measured at Finisterre itself, often used as a general indicator of upwelling conditions around the Galician coast, is not always representative of the overall wind field. The relevant wind fields allowed a qualitative explanation of temperature structure seen in sea surface temperature images, and of differences in both the upwelling and downwelling regimes between the two years.

Draper2003

An advanced ambiguity selection algorithm for SeaWinds
D. W. Draper and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  41  538--547  (2003)

SeaWinds on QuikSCAT, a spaceborne Ku-band scatterometer, estimates ocean winds via the relationship between the normalized radar backscatter and the vector wind. Scatterometer wind retrieval generates several possible wind vector solutions or ambiguities at each resolution cell, requiring a separate ambiguity selection step to give a unique solution. In processing SeaWinds on QuikSCAT data, the ambiguity selection is ``nudged'' or initialized using numerical weather prediction winds. We describe a sophisticated new ambiguity selection approach developed at Brigham Young University (BYU) that does not require nudging. The BYU method utilizes a low-order data-driven Karhunen-Loeve (KL) wind field model to promote self-consistency. Ambiguity selected winds from the BYU method and standard SeaWinds processing are compared over a set of 102 revs. A manual examination of the data suggests that the nonnudging BYU method selects a more self-consistent wind field in the absence of cyclonic storms. Over a set of cyclonic storm regions, BYU performs better in 9\% of the cases and worse in 20\% of the cases. Overall, the BYU algorithm selects 93\% of the same ambiguities as the standard dataset. This comparison helps validate both nonnudging and nudging techniques and indicates that SeaWinds ambiguity selection can be generally accomplished without nudging.

Spencer2003

High-resolution measurements with a spaceborne pencil-beam scatterometer using combined range/doppler discrimination techniques
M. W. Spencer and W. Y. Tsai and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  41  567--581  (2003)

Conically scanning pencil-beam scatterometer systems, such as the recently launched SeaWinds radar, constitute an important class of instruments for spaceborne climate observation. In addition to ocean winds, scatterometer data are being applied to a wide range of land and cryospheric applications. A key issue for future scatterometer missions is improved spatial resolution. Pencil-beam scatterometers to date have been real-aperture systems where only range discrimination is used, resulting in a relatively coarse resolution of approximately 25 km. In this paper, the addition of Doppler discrimination techniques is proposed to meet the need for higher resolution. Here, the unique issues associated with the simultaneous application of range and Doppler processing to a conically scanning radar are addressed, and expressions for the theoretical measurement performance of such a system are derived. Important differences with side-looking imaging radars, which also may employ Doppler techniques, are highlighted. Conceptual design examples based on scatterometer missions of current interest are provided to illustrate this new high-resolution scatterometer approach. It is shown that spatial resolution of pencil-beam scatterometer systems can be improved by an order of magnitude by utilizing combined range/Doppler discrimination techniques, while maintaining the wide-swath and constant incidence angle neaaaaeded for many geophysical measurements.

Bourassa2003

SeaWinds validation with research vessels
M. A. Bourassa and D. M. Legler and J. J. O'Brien and S. R. Smith
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  108    (2003)

[1] The accuracy of the SeaWinds scatterometer's vector winds is assessed through comparison with research vessel observations. Factors that contribute to uncertainty in scatterometer winds are isolated and examined as functions of wind speed. For SeaWinds on QuikSCAT, ambiguity selection is found to be near perfect for surface wind speed (w) > 8 m s(-1); however, ambiguity selection errors cause directional uncertainty to exceed 20 for w < ?5 m s(-1). These average uncertainties for wind speed and direction are found to be 0.45 m s(-1) and 5$^\circ$ for the QSCAT-1 model function and 0.3 m s(-1) and 3$^\circ$ for the Ku-2000 model function. The QuikSCAT winds are examined as vectors through two new approaches. The first is a method for determining vector correlations that considers uncertainty in the comparison data set. The second approach is a wind speed-dependent model for the uncertainty in the magnitude of vector errors. For the QSCAT-1 (Ku-2000) model function this approach shows ambiguity selection dominates uncertainty for 2.5 < w < 5.5 m s(-1) (0.6 < w < 5.5 m s(-1)), uncertainty in wind speed dominates for w < 2.5 m s(-1) and 5.5 < w < 7.5 m s(-1) (w < 0.6 m s(-1) and 5.5 < w < 18 m s(-1)), and uncertainty in wind direction (for correctly selected ambiguities) dominates for w > 7.5 m s(-1) (w > 18 m s(-1)). This approach also shows that spatial variability in the wind direction, related to inexact spatial co-location, is likely to dominate rms differences between scatterometer wind vectors and in situ comparison measurements for w > 4.5 m s(-1). The techniques used herein are applicable to any validation effort with uncertainty in the comparison data set or with inexact co-location.

Lin2003

Satellite observations of modulation of surface winds by typhoon-induced upper ocean cooling
I. I. Lin and W. T. Liu and C. C. Wu and J. C. H. Chiang and C. H. Sui
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] Two remote sensing data sets, the Tropical Rainfall Measurement Mission Sea Surface Temperature (SST) and the NASA QuikSCAT ocean surface wind vectors, are analysed to study ocean-atmosphere interactions in cold SST regions formed in the trail of two typhoon events. Anomalously cold SST patches up to 6degreesC below the surrounding warm tropical ocean SST are found along the trail of typhoon tracks as cold, deep waters are entrained up to the mixed layer due to typhoon forcing. In both typhoon events, significant and systematic weakening of surface wind speed is found over cold SST patches relative to surface wind speed in surrounding regions. The wind speed anomalies disappear as the patches recover to the level of the surrounding SST. The results are consistent with the mechanism proposed by Wallace et al. that surface winds are modulated by SST via stability. As wind within the well-mixed boundary layer moves over the cold patch, boundary layer stability increases, vertical mixing is suppressed, and the vertical wind shear increases; reduction in surface wind speed is caused. In particular, our result shows that this mechanism can act on relatively small spatial (approximate to 100 km) and short (approximate to 1 day) time scales.

Gille2003

Measuring the sea breeze from QuikSCAT scatterometry
S. T. Gille and S. G. L. Smith and S. M. Lee
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] Differences between morning and evening winds from QuikSCAT scatterometer measurements are analyzed to diagnose the diurnal variability of the wind over the ocean. A statistically significant signal, associated with the sea breeze, is present along most of the world's coastlines. Significant diurnal variability is also present mid-ocean in the easterly trade wind belts.

Carr2003

CO2 exchange coefficients from remotely sensed wind speed measurements: SSM/I versus QuikSCAT in 2000 (vol 29, DOI no. 10.1029/2002GL015068, 2002)
M. E. Carr and W. Q. Tang and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

Wu2003

Design and calibration of the SeaWinds scatterometer
C. L. Wu and Y. Liu and K. H. Kellogg and K. S. Pak and R. L. Glenister
IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS  39  94--109  (2003)

The SeaWinds Scatterometer is a Ku-band Earth orbiting remote sensing radar. It has a! 1 m dish antenna shared by two beams with respective nadir look angles of 40 and 46 deg, scanning azimuthally to provide greater than 90\% daffy coverage of the Earth at an altitude of 800 km. The first sensor was launched in 1999 and produces sea surface wind field to 2 m/s accuracy at 25 km resolution. The design and calibration of the SeaWinds radar is described here.

Chao2003

A high-resolution surface vector wind product for coastal oceans: Blending satellite scatterometer measurements with regional mesoscale atmospheric model simulations
Y. Chao and Z. J. Li and J. C. Kindle and J. D. Paduan and F. P. Chavez
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

A 2-dimensional variational method is used to blend the satellite scatterometer measured (QuikSCAT) and regional mesoscale atmospheric model simulated (COAMPS) surface vector winds for coastal central California. The approach is distinct from existing methods in that it considers errors from both measurements and models. When compared with independent in situ observations, the blended wind product shows consistently higher correlation and smaller RMS errors than QuikSCAT or COAMPS winds. The proposed algorithm can be implemented over any part of the world ocean. It should be a valuable tool for describing small-scale atmospheric processes in coastal zones and for forcing high-resolution coastal ocean models.

Goswami2003

Indian Ocean surface winds from NCMRWF analysis as compared to QuikSCAT and moored buoy winds
B. N. Goswami and E. N. Rajagopal
PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-EARTH AND PLANETARY SCIENCES  112  61--77  (2003)

The quality of the surface wind analysis at the National Centre for Medium Range Weather Forecasts (NCMRWF), New Delhi over the tropical Indian Ocean and its improvement, in 2001 are examined by comparing it with in situ buoy measurements and satellite derived surface winds from NASA QuikSCAT satellite (QSCT) during 1999, 2000 and 2001. The NCMRWF surface winds suffered from easterly bias of 1.0-1.5 ms(-1) in the equatorial Indian Ocean (IO) and northerly bias of 2.0-3.0 ms(-1) in the south equatorial IO during 1999 and 2000 compared to QSCT winds. The amplitude of daily variability was also underestimated compared to that in QSCT. In particular, the amplitude of daily variability of NCMRWF winds in the eastern equatorial IO was only about 60\% of that of QSCT during 1999 and 2000. The NCMRWF surface winds during 2001 have significantly improved with the bias of the mean analyzed winds considerably reduced everywhere bringing it to within 0.5 ms(-1) of QSCT winds in the equatorial IO. The amplitude and phase of daily and intraseasonal variability are very close to that in QSCT almost everywhere during 2001. It is shown that the weakness in the surface wind analysis during 1999 and 2000 and its improvement in 2001 are related to the weakness in simulation of precipitation by the forecast model in the equatorial IO and its improvement in 2001.

Chen2002

A dual-frequency approach for retrieving sea surface wind speed from TOPEX altimetry
G. Chen and B. Chapron and R. Ezraty and D. Vandemark
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  107    (2002)

More than a dozen of wind speed (U) algorithms have been proposed during the past 2 decades, as a result of a continuing effort to improve altimeter wind measurement. The progress in terms of accuracy, however, is seen to be rather slow. The reported root mean square (RMS) error of prevailing algorithms varies mostly between 1.6 and 2.0 m/s for the dominant wind regime. As far as the TOPEX altimeter is concerned, three measured quantities, namely, the radar cross sections from Ku and C band (sigma(Ku) and sigma(C)), as well as the significant wave height (H-s), have been used in previous algorithm developments, resulting in a variety of single-, dual-, and three-parameter model functions. On the basis of the finding of a banded dependency of the U-sigma(Ku) relationship on sigma(C) a new approach for retrieving altimeter wind speed, termed linear composite method (LCM), is proposed in this study. The LCM model function appears as a set of sigma(C)-dependent linear relations between U and sigma(Ku). A unique advantage of this approach is that it allows the algorithm to be tuned or expanded for a given range of wind speed without affecting the rest. Over 1.7 million coincident TOPEX/NASA scatterometer (NSCAT) and TOPEX/QuikSCAT data covering a period of 2.5 years are used to adjust the model. Validation against extensive buoy measurements indicates that the LCM algorithm is almost unbiased and has an overall RMS error of 1.56 m/s, which is 12\% lower compared to the algorithm in operational use [Witter and Chelton, 1991]. In addition, a small (2.5-6\%, depending on the reference data set) but significant improvement is found for the LCM when compared to the most recent dual- parameter algorithm [Gourrion et al., 2002].

Boukabara2002

Physically based modeling of QuikSCAT SeaWinds passive microwave measurements for rain detection
S. A. Boukabara and R. N. Hoffman and C. Grassotti and S. M. Leidner
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES  107    (2002)

[1] We present a method for detecting rain-contaminated wind vector cells in QuikSCAT SeaWinds scatterometer observations. This rain detection method uses passive measurements of microwave brightness temperature obtained as a signal processing by-product from the standard SeaWinds active scatterometer measurements. The rain flag is developed theoretically first by calibrating the SeaWinds brightness temperatures using Special Sensor Microwave Imager (SSM/I) observations and then by using physically based simulations including the effects of both rain and ice precipitation. Rain retrievals are validated by comparison to SSM/I-observed rain rates and to other independently produced SeaWinds rain flags and produce rain maps that agree well with the SSM/I estimates. The rain detection method may be used to complement existing rain flags in the current operational QuikSCAT data product. In addition, an atmospheric correction algorithm was developed to dynamically adjust the backscatter coefficient measurements for variations in water vapor and cloud liquid water; results are not significantly different from the climatological correction currently implemented.

Draper2002

An assessment of SeaWinds on QuikSCAT wind retrieval
D. W. Draper and D. G. Long
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  107    (2002)

[1] The scatterometer ocean wind retrieval process produces several possible solutions or ambiguities at each point, requiring a separate ambiguity selection step to infer a unique wind vector field. An ambiguity selection error occurs when the selected wind vector is not the closest ambiguity to the true wind. The current ambiguity selection routine for SeaWinds is ad hoc, but performs well under most circumstances. Factors such as instrument noise and rain can also cause the estimated wind flow to deviate from the true wind. A quality assurance (QA) analysis is performed to assess the ambiguity selection effectiveness and noise level of the retrieved wind using a low-order wind field model. The wind field model is data-driven and shown to be rather insensitive to the training data set. The QA analysis demonstrates that the SeaWinds ambiguity selection process is at least 95\% effective. Ambiguity selection errors are correlated with storms and rain corruption. A subjective analysis on a set of cyclonic storm passes confirms that the wind retrieval is somewhat less effective in storm regions.

Grodsky2003

The intertropical convergence zone in the South Atlantic and the equatorial cold tongue
S. A. Grodsky and J. A. Carton
JOURNAL OF CLIMATE  16  723--733  (2003)

Recent observations from the QuikSCAT and Tropical Rainfall Measuring Mission satellites, as well as a longer record of Special Sensor Microwave Imager winds are used to investigate the existence and dynamics of a Southern Hemisphere partner to the intertropical convergence zone in the tropical Atlantic Ocean. The southern intertropical convergence zone extends eastward from the coast of Brazil in the latitude band 10degrees-3degreesS and is associated with seasonal precipitation exceeding 6 cm month(-1) during peak months over a part of the ocean characterized by high surface salinity. It appears in austral winter when cool equatorial upwelling causes an anomalous northeastward pressure gradient to develop in the planetary boundary layer close to the equator. The result is a zonal band of surface wind convergence that exceeds 10(-6) s(-1), with rainfall stronger than 2 mm day(-1), and an associated decrease in ocean surface salinity of 0.2 parts per thousand.

Kelly2002

Scatterometer winds explain damped Rossby waves
K. A. Kelly and L. A. Thompson
GEOPHYSICAL RESEARCH LETTERS  29    (2002)

[1] Westward propagating waves in the North Pacific Ocean from 10-16degreesN are overwhelmed by a zonally coherent response at the annual period, as observed in sea surface height (SSH) anomalies from the TOPEX/ POSEIDON altimeter. SSH from a simple model of wind- forced Rossby waves and from seasonal heating are compared with observed SSH to understand the processes responsible for the observed signal. The seasonal heating cycle is out- of- phase and too weak to explain the SSH. The oceanic response to wind stress curl forcing more closely resembles the observations, but the response to NCEP Reanalysis winds does not show a strong annual cycle. Wind stress curl from the QuikSCAT/ SeaWinds scatterometer has a strong and zonally coherent annual cycle that produces a corresonding strong annual signal in SSH. The model forced by scatterometer winds demonstrates that the response to Ekman pumping is the source of the strong annual cycle in the SSH.

Sura2003

Stochastic analysis of Southern and Pacific Ocean sea surface winds
P. Sura
JOURNAL OF THE ATMOSPHERIC SCIENCES  60  654--666  (2003)

This paper shows that the synoptic variability of zonal and meridional midlatitude Pacific and Southern Ocean sea surface winds can be well described by a univariate stochastic dynamical system directly derived from data. The method used to analyze blended Quick Scatterometer (QuikSCAT)-NCEP winds is a general method to estimate drift and diffusion coefficients of a continuous stationary Markovian system. Almost trivially, the deterministic part consists of a simple, nearly linear damping term. More importantly, the stochastic part appears to be a state-dependent white noise term, that is, multiplicative noise. The need for a multiplicative noise term to describe the variability of midlatitude winds can be interpreted by the fact that the variability of midlatitude winds increases with increasing wind speed. The results indicate that a complete stochastic description of midlatitude winds requires a state-dependent white noise term, that is, multiplicative noise. A simple Ornstein-Uhlenbeck process is not sufficient to describe the wind data within a stochastic framework. The method used fails for tropical regions, suggesting that tropical variability might be non-Markovian.

Portabella2002

Characterization of residual information for SeaWinds quality control
M. Portabella and A. Stoffelen
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  2747--2759  (2002)

Recent work has shown the important properties of the wind inversion residual or maximum-likelihood estimator (MLE) for quality Control (QC) of QuikSCAT Hierarchichal Data Format (HDF) observations. Since March 2000, the QuikSCAT near-real-time (NRT) Binary Universal Format Representation (BUFR) product is available. As this product is used for numerical weather prediction (NWP) assimilation purposes, a QC procedure for the BUFR product is needed. We study the behavior of the MLE in order to determine whether the HDF QC procedure is appropriate for BUFR data. A comparison using real HDF and BUFR data reveals that the MLE distributions of HDF and BUFR differ and are actually poorly correlated. One important difference between BUFR and HDF is the amount of signal averaging prior to wind inversion. The averaging reduces the number of observations used in the wind retrieval for the BUFR product as compared to HDF. We show with a simple example that different MLE distributions are indeed expected due to this averaging. We also run a simulation in order to link theory and reality and better understand the behavior of the MLE. Despite the different MLE behavior in BUFR and HDF, the quality of the retrieved winds, as compared with the European Centre for Medium-Range Weather Forecasts winds, is very similar. We develop an MLE-based QC procedure for BUFR, similarly to the one in HDF, and we compare both. The skill of the QC in BUFR is again very similar to the one in HDF, showing that despite the different MLE behavior in both formats, the properties of the MLE as a QC indicator remain very similar.

Liu2002

Double intertropical convergence zones - a new look using scatterometer
W. T. Liu and X. S. Xie
GEOPHYSICAL RESEARCH LETTERS  29    (2002)

The high-resolution wind vectors observed by the space-based scatterometer QuikSCAT, from 1999 to 2002, show that the double intertropical convergence zones (ITCZ) exist in the Atlantic and the eastern Pacific oceans for most of the annual cycle, and are far more extensive than previously recognized. For most of the time, the southern ITCZ is weaker than the northern one. The stronger ITCZ occurs when the northerly trade winds meet the southerly trade winds over warm water, resulting in deep convection. The weaker ITCZ over cooler water is caused by the deceleration of the surface winds as they approach the cold upwelling water near the equator. Decreases in vertical mixing and increases in vertical wind shear in the atmospheric boundary layer are suggested to be the causes of the deceleration of the trade winds as they move from warmer to colder water.

Hu2002

QuikSCAT reveals the surface circulation of the Catalina Eddy
H. Hu and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  29    (2002)

[1] The Catalina Eddy, a small, recurring cyclonic vortex in the ocean off Los Angeles, is of keen interest to local weather forecasters because of the moderating oceanic effect it brings to the city. Its small size and shallow vertical extent have made it difficult to monitor and predict using conventional data. The microwave scatterometer on the QuikSCAT spacecraft has generated high-resolution surface wind vectors that provide the first visualization of the complete cyclonic flow of the eddy. Moreover, the superior performance of the QuikSCAT scatterometer demonstrates the relative inaccuracy and inconsistency of predictions of the eddy based upon numerical weather prediction models.

Pan2002

Observation of western boundary current atmospheric convergence zones using scatterometer winds
J. Y. Pan and X. H. Yan and Q. N. Zheng and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  29    (2002)

[1] A merged scatterometer wind data set from ERS-1/2, NSCAT, and QuikSCAT missions was used to observe the atmospheric convergence zones (ACZs) caused by the western boundary currents, the Gulf Stream and Kuroshio. The long-term means of the atmospheric convergence show the ACZs' spatial features, which are related to the precipitation patterns in these regions. Seasonal images of the ACZs were produced to show annual cycles of the ACZs, indicating that the intensities of the ACZs over these two regions strengthen in winter and weaken in summer. Furthermore, we calculated the total convergence over the Gulf Stream and Kuroshio ACZs, which is defined as the integral of the convergence over the ACZs. The interannual variability of the total convergence was extracted by using a multi-stage filter, revealing that in response to strong El Nino events in 1991-92 and 1997-98, the total convergence reached maxima.

Thompson2002

Orbit determination for the QuikSCAT spacecraft
B. F. Thompson and M. C. Meek and K. L. Gold and P. Axelrad and G. H. Born and D. G. Kubitschek
JOURNAL OF SPACECRAFT AND ROCKETS  39  852--858  (2002)

An operational orbit determination system for QuikSCAT has been developed to meet the requirement for 100-m (3sigma) positioning knowledge. This is nominally accomplished by processing global positioning system (GPS) position solutions in a dynamic filter. The operational orbit determination system produced 24-h overlapping arc position errors between 15 and 25 m (root-sum-square) and 3-h arc overlaps between 5 and 6 m (root-sum-square) for seven-day and one-day arcs, respectively. We also investigated the use of short segments of GPS pseudorange and carrier phase data and obtained results that differ by less than 10 m from the nominal orbit solutions. A third investigation considered the feasibility of a backup orbit determination system using antenna azimuth and elevation angles from three ground tracking stations. The methods and results of processing these three data types are presented.

Pan2003

Interpretation of scatterometer ocean surface wind vector EOFs over the Northwestern Pacific
J. Y. Pan and X. H. Yan and Q. N. Zheng and W. T. Liu and V. V. Klemas
REMOTE SENSING OF ENVIRONMENT  84  53--68  (2003)

Satellite scatterometer winds over the northwestern Pacific were analyzed with the vector empirical orthogonal function (VEOF) method. The Hilbert-Huang transform (HHT), a newly developed non-linear and non-stationary time series data processing method, was also employed in the analysis. A combination of European Remote Sensing Satellite (ERS) - 1/2 scatterometer, NASA Scatterometer (NSCAT) and NASA's Quick Scatterometer (QuikSCAT) winds covering the period from January 1992 to April 2000 and the area of 0-50degreesN, 100-148degreesE constitutes the baseline for this study. The results indicate that annual cycles dominate the two leading VEOF modes. The first VEOF shows the East Asian monsoon features and the second represents a spring-autumn oscillation. We removed the annual signal from the data set and calculated the interannual VEOFs. The first interannual VEOF represents the interannual variability existing in the spring-autumn oscillation. The temporal mode is correlated with the Southern Oscillation Index (Sol), but has a half-year lag with respect to the SOI. The spatial mode of the first interannual VEOF reflects the response of the tropical and extratropical winds to ENSO events. The second interannual VEOF is another ENSO related mode, and the temporal VEOF mode is correlated with the SOI with a correlation coefficient of 0.78, revealing the wind variability over mid-latitudes, which is associated with ENSO events. Further analysis indicated that the wind variability over the coast of East Asia represents anomalies of a Hadley cell. The quasi-biennial oscillation (QBO) was found in the temporal mode, indicating and verifying that the QBO in the wind fields is related to ENSO events. The third VEOF shows the interannaul variability in the winter-summer mode and displays the interannual variability of the East Asian monsoon. The three leading interannual VEOFs are statistically meaningful as confirmed by a significance test. (C) 2002 Elsevier Science Inc. All rights reserved.

Gourrion2002

A two-parameter wind speed algorithm for Ku-band altimeters
J. Gourrion and D. Vandemark and S. Bailey and B. Chapron and G. P. Gommenginger and P. G. Challenor and M. A. Srokosz
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  19  2030--2048  (2002)

Globally distributed crossovers of altimeter and scatterometer observations clearly demonstrate that ocean altimeter backscatter correlates with both the near-surface wind speed and the sea state. Satellite data from TOPEX/Poseidon and NSCAT are used to develop an empirical altimeter wind speed model that attenuates the sea-state signature and improves upon the present operational altimeter wind model. The inversion is defined using a multilayer perceptron neural network with altimeter-derived backscatter and significant wave height as inputs. Comparisons between this new model and past single input routines indicates that the rms wind error is reduced by 10\%-15\% in tandem with the lowering of wind error residuals dependent on the sea state. Both model intercomparison and validation of the new routine are detailed, including the use of large independent data compilations that include the SeaWinds and ERS scatterometers, ECMWF wind fields, and buoy measurements. The model provides consistent improvement against these varied sources with a wind-independent bias below 0.3 m s(-1). The continuous form of the defined function, along with the global data used in its derivation, suggest an algorithm suitable for operational application to Ku-band altimeters. Further model improvement through wave height inclusion is limited due to an inherent multivaluedness between any single realization of the altimeter measurement pair [sigma(o), H-s] and observed near-surface winds. This ambiguity indicates that H-s is a limited proxy for variable gravity wave properties that impact upon altimeter backscatter.

Ebuchi2002

Evaluation of wind vectors observed by QuikSCAT/SeaWinds using ocean buoy data
N. Ebuchi and H. C. Graber and M. J. Caruso
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  19  2049--2062  (2002)

Wind vectors observed by the QuikSCAT/SeaWinds satellite mission are validated by comparing with wind and wave data from ocean buoys. Effects of oceanographic and atmospheric environment on scatterometer measurements are also assessed using the buoy data. Three versions of QuikSCAT/SeaWinds wind data were collocated with buoy observations operated by the National Data Buoy Center (NDBC), Tropical Atmosphere Ocean (TAO), and Pilot Research Moored Array in the Tropical Atlantic (PIRATA) projects, and the Japan Meteorological Agency (JMA). Only buoys located offshore and in deep water were analyzed. The temporal and spatial differences between the QuikSCAT/SeaWinds and buoy observations were limited to less than 30 min and 25 km. The buoy wind speeds were converted to equivalent neutral winds at a height of 10 m above the sea surface. The comparisons show that the wind speeds and directions observed by QuikSCAT/SeaWinds agree well with the buoy data. The root-mean-squared differences of the wind speed and direction for the standard wind data products are 1.01 m s(-1) and 23degrees, respectively, while no significant dependencies on the wind speed or cross-track cell location are discernible. In addition, the dependencies of wind speed residuals on oceanographic and atmospheric parameters observed by buoys are examined using the collocated data. A weak positive correlation of the wind speed residuals with the significant wave height is found, while dependencies on the sea surface temperature or atmospheric stability are not physically significant.

Stiles2002

Impact of rain on spaceborne Ku-band wind scatterometer data
B. W. Stiles and S. H. Yueh
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  1973--1983  (2002)

The accuracy of Ku-band ocean wind scatterometers (i.e., NSCAT and SeaWinds) is impacted to varying degrees by rain. In order to determine how to best flag rain-contaminated wind vector cells and ultimately to calibrate out the effects of rain as much as possible, we must understand the impact of rain on the backscatter measurements that are used to retrieve wind vectors. This study uses collocated SSM/I rain rate measurements, NCEP wind fields, and SeaWinds on QuikSCAT backscatter measurements to empirically fit a simple theoretical model of the effect of rain on sigma(o), and to check the validity of that model. The chief findings of the study are 1) horizontal polarization measurements are more sensitive to rain than vertical polarization, 2) sensitivity to rain varies dramatically with wind speed, and 3) the additional backscatter due to rain overshadows the rain-related attenuation.

Carr2002

CO2 exchange coefficients from remotely sensed wind speed measurements: SSM/I versus QuikSCAT in 2000
M. E. Carr and W. Q. Tang and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  29    (2002)

We compare here the air-sea exchange coefficient for CO2 estimated with monthly mean wind speed measured by the Special Sensing Microwave Imager (SSM/I), K-S, and by the scatterometer QuikSCAT, K-Q, for the year 2000. K-S and K-Q present the same patterns, although are larger than in similar to65\% of the world ocean. Zonal mean K-S are consistently larger, except similar to50degreesS and north of 10degreesS in the Indian Ocean. Global oceanic uptake, F-Q, estimated using K-Q and climatological Deltap(CO2) ranges from 0.43 (July) to 2.6 Gt C y(-1) (December). The global sink estimated from SSM/I is similar to10\% larger than F-Q for most of the year. This comparison supports the use of SSM/I to quantify interannual variability of the global exchange coefficient of CO2.

Meissner2002

An updated analysis of the ocean surface wind direction signal in passive microwave brightness temperatures
T. Meissner and F. Wentz
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  1230--1240  (2002)

We analyze the wind direction signal for vertically (v) and horizontally (h) polarized microwave radiation at 37 GHz, 19 GHz, and 11 GHz and an Earth incidence angle of 53degrees. We use brightness temperatures from SSM/I and TMI and wind vectors from buoys and the QUIKSCAT scatterometer. The wind vectors are space and time collocated with the radiometer measurements. Water vapor, cloud water and sea surface temperature are obtained from independent measurements and are uncorrelated with the wind direction. We find a wind direction signal that is noticeably smaller at low and moderate wind speeds than a previous analysis had indicated. We attribute the discrepancy to errors in the atmospheric parameters that were present in data set of the earlier study. We show that the polarization combination 2v - h is almost insensitive to atmospheric changes and agrees with the earlier results. The strength of our new signals agrees well with recent JPL aircraft radiometer measurements. It is significantly smaller than the prediction of the two-scale sea surface emission model for low and intermediate wind speeds.

Zhao2002

Validation of sea ice motion from QuikSCAT with those from SSM/I and buoy
Y. H. Zhao and A. K. Liu and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  1241--1246  (2002)

Arctic sea ice motion for the period from October 1999 to March 2000 derived from QuikSCAT and special sensor microwave/imager (SSM/I) data using the wavelet analysis method agrees well with ocean buoy observations. Results from QuikSCAT and SSM/I are compatible when compared with buoy observations and complement each other. Sea ice drift merged from daily results from QuikSCAT, SSM/I, and buoy data gives more complete coverage of sea ice motion. Based on observations of six months of sea ice motion maps, the sea ice motion maps in the Arctic derived from QuikSCAT data appear to have smoother (less noisy) patterns than those from NSCAT, especially in boundary areas, possibly due to constant radar scanning incidence angle. For late summer, QuikSCAT data can provide good sea ice motion information in the Arctic as early as the beginning of September. For early summer, QuikSCAT can provide at least partial sea ice motion information until mid-June. In the Antarctic, a case study shows that sea ice motion derived from QuikSCAT data is consistent with pressure field contours.

Portabella2002

A comparison of KNMI quality control and JPL rain flag for SeaWinds
M. Portabella and A. Stoffelen
CANADIAN JOURNAL OF REMOTE SENSING  28  424--430  (2002)

In the past few years, scatterometer winds have been successfully assimilated in weather analysis. A good assessment of the information content of these winds is particularly important for such activities. Besides retrieval problems in cases of a confused sea state, a particularly acute problem of Ku-band scatterometry is the sensitivity to rain. Elimination of poor-quality data is therefore a prerequisite for the successful use of the new National Aeronautics and Space Administration (NASA) scatterometer, QuikSCAT. This issue has been the topic of recent work. On the one hand, the Royal Dutch Meteorological Institute (KNMI) has developed a quality-control (QC) procedure that detects and rejects the poor-quality QuikSCAT data (including rain contamination). On the other hand, the Jet Propulsion Laboratory (JPL) has developed a ``rain flag'' for QuikSCAT. In this paper, we test the KNMI QC against the JPL rain flag to improve QC for QuikSCAT. Collocations with the European Centre for Medium-range Weather Forecasts (ECMWF) winds and special sensor microwave imager (SSM/I) rain data are used for validation purposes. The results show that the KNMI QC is more efficient in rejecting poor-quality data than the JPL rain flag, whereas the latter is more efficient in rejecting rain-contaminated data than the former. The JPL rain flag, however, rejects too much of the consistent wind data in dynamically active areas. The KNMI QC is a good QC procedure in the parts of the swath where the wind retrieval ability of QuikSCAT is high. In the nadir region, however, the KNMI QC efficiency and the wind retrieval skill are relatively low. In the nadir region, the KNMI QC needs additional information from the JPL rain flag to reject rain-contaminated data.

Sharp2002

Early detection of tropical cyclones using seawinds-derived vorticity
R. J. Sharp and M. A. Bourassa and J. J. O'Brien
BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY  83  879--889  (2002)

Yueh2002

Polarimetric radar remote sensing of ocean surface wind
S. H. Yueh and W. J. Wilson and S. Dinardo
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  793--800  (2002)

Experimental data are presented to support the development of a new concept for ocean wind velocity measurement (speed and direction) with the polarimetric microwave radar technology. This new concept has strong potential for improving the wind direction accuracy and extending the useful swath width by up to 30\% for follow-on NASA spaceborne scatterometer mission to SeaWinds series. The key issue is whether there is a relationship between the polarization state of ocean backscatter and surface wind velocity at NASA scatterometer frequencies (13 GHz). An airborne Ku-band polarimetric scatterometer (POLSCAT) was developed for proof-of-concept measurements. A set of aircraft flights indicated repeatable wind direction signals in the POLSCAT observations of sea surfaces at 9-11 m/s wind speed. The correlation coefficients between co- and cross-polarized radar response of ocean surfaces have a peak-to-peak amplitude of about 0.4 and are shown to have an odd-symmetry with respect to the wind direction, unlike the normalized radar cross sections.

Katsaros2002

Microwave remote sensing of tropical cyclones from space
K. B. Katsaros and P. W. Vachon and W. T. Liu and P. G. Black
JOURNAL OF OCEANOGRAPHY  58  137--151  (2002)

This article reviews several microwave instruments employed in research and analysis of tropical cyclones (TCs), typhoons, and hurricanes. The instruments discussed include scatterometers, microwave radiometers, synthetic aperture radars (SARs), and rain radar from space. Examples of the particular contribution by one or more of these instruments in analysis of several storms illustrate the comprehensive new views provided by the SeaWinds scatterometers, the detailed high-resolution wind field provided by RADARSAT-1 SAR, particularly inside and in the vicinity of hurricane ``eyes,'' and the presence of secondary flows in the region between rainbands in TCs. The high spatial resolution of precipitation data from the Tropical Rainfall Measuring Mission's rain radar, combined with scatterometer or SAR data, give a significant improvement in the details that can be seen from space, at the surface, and in the precipitating areas of TCs. The microwave instruments provide a penetrating view below the upper level cirrus clouds.

Nghiem2001

Detection of snowmelt regions on the Greenland ice sheet using diurnal backscatter change
S. V. Nghiem and K. Steffen and R. Kwok and W. Y. Tsai
JOURNAL OF GLACIOLOGY  47  539--547  (2001)

Snowmelt regions on Greenland ice are mapped daily with the SeaWinds wideswath Ku-band (13.4 GHz) scatterometer on the QuikSCAT satellite. The approach exploits the high temporal resolution of SeaWinds/QuikSCAT data for the melt mapping using diurnal backscatter change independent of the absolute calibration. The results reveal several pronounced melting and refreezing events, and effects of topography are evident in the melt patterns. The spatial resolution is sufficient to identify melt features on the Sukkertoppen Iskappe west of the main ice sheet. An anomalous warming event, caused by downward mixing of warm air, is detected in late September 1999 over the west flank of the southern Greenland ice sheet. Time-series images of melt regions are presented over the period from summer to the fall freeze-up. The satellite observations are verified with in situ measurements from the Greenland Climate Network stations.

Weissman2002

Effects of rain rate and wind magnitude on SeaWinds scatterometer wind speed errors
D. E. Weissman and M. A. Bourassa and J. Tongue
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  19  738--746  (2002)

Rain within the footprint of the SeaWinds scatterometer on the QuikSCAT satellite causes more significant errors than existed with its predecessor, the NASA scatterometer (NSCAT) on Advanced Earth Observing Satellite-I (ADEOS-I). Empirical relations are developed that show how the rain-induced errors in the scatterometer wind magnitude depend on both the rain rate and on the wind magnitude. These relations are developed with collocated National Data Buoy Center (NDBC) buoy measurements (to provide accurate sea surface winds) and simultaneous Next Generation Weather Radar (NEXRAD) observations of rain reflectivity. An analysis, based on electromagnetic scattering theory, interprets the dependence of the scatterometer wind errors on volumetric rain rate over a range of wind and rain conditions. These results demonstrate that the satellite scatterometer responds to rain in a manner similar to that of meteorological radars, with a Z-R relationship. These observations and results indicate that the combined (wind and rain) normalized radar cross section will lead to erroneously large wind estimates when the rain-related radar cross section exceeds a particular level that depends on the rain rate and surface wind speed.

Tran2002

The dependence of Nadir ocean surface emissivity on wind vector as measured with microwave radiometer
N. Tran and D. Vandemark and C. S. Ruf and B. Chapron
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  515--523  (2002)

Global brightness temperature observations of TOPEX/Poseidon microwave radiometer (TMR) at 18, 21, and 37 GHz have been collocated with near-simultaneous SeaWinds wind vector data as well as with monthly sea surface temperature and salinity products. The combined data allow us to study the dependence of zenith-directed ocean surface emissivity, at each frequency, upon both wind speed and direction. Results show a clear two-branch wind speed dependence; weak and linear below 7 m . s(-1) with an increase in sensitivity above that point. The observed emissivity also depends on the angle between the wind direction and TMR's antenna polarization orientation, providing satellite confirmation of aircraft-derived results. There is little change in these wind vector dependencies with frequency.

Yeh2002

Kinematic characteristics of a mei-yu front detected by the QuikSCAT oceanic winds
H. C. Yeh and G. T. J. Chen and W. T. Liu
MONTHLY WEATHER REVIEW  130  700--711  (2002)

Based on conventional surface observations and NASA Quick Scatterometer (QuikSCAT) data, a heavy rainfall event that occurred in the Taiwan mei-yu season was chosen to further study the kinematic characteristics of the accompanying surface front. With the help of the QuikSCAT oceanic surface winds, it was found that the location and propagation of a mei-yu front over the ocean to the east of Taiwan during 10-12 June 2000 are better represented by the frontal wind shift line, which was located approximately on the leading edge of the baroclinic zone. The mesoscale system with cyclonic circulation embedded within the frontal zone was clearly shown in the wind field and kinematic parameters (horizontal divergence and vorticity) as well as satellite clouds and rainfall estimations. The conditional instability of the second kind (CISK) process was suggested to be responsible for the intensification of the mei-yu front and the frontal disturbance over the ocean. Under the influence of island topography over Taiwan and the mountains over southern China, the frontal wind shift line distorted and receded within the Taiwan Strait after the mei-yu front reached northern Taiwan. In the later period, the front moved southward faster to the area immediately east of Taiwan as compared to that over southwestern Taiwan and the Taiwan Strait due to earlier arrival of strong postfrontal northerly winds. The localized southwesterly jetlike feature was observed to the southeast of Taiwan and persisted until the arrival of the strong northerly winds. These localized phenomena were detected by the QuikSCAT oceanic winds.

Drinkwater2001

Greenland snow accumulation estimates from satellite radar scatterometer data
M. R. Drinkwater and D. G. Long and A. W. Bingham
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES  106  33935--33950  (2001)

Data collected by the C band ERS-2 wind scatterometer (EScat), the Ku band ADEOS-1 NASA scatterometer (NSCAT), and the Ku band SeaWinds on QuikScat (QSCAT) satellite instruments are used to illustrate spatiotemporal variability in snow accumulation on the Greenland ice sheet. Microwave radar backscatter images of Greenland are derived using the scatterometer image reconstruction (SIR) method at 3-day intervals over the periods 1991-1998 and 1996-1997 for EScat and NSCAT, respectively. The backscatter coefficient sigmadegrees normalized to 40degrees incidence, A, and gradient in backscatter, B, in the range 20degrees-60degrees are compared with historical snow accumulation data and recent measurements made in the Program for Arctic Regional Climate Assessment (PARCA) shallow snow pits. Empirical relationships derived from these comparisons reveal different exponential relationships between C and Ku band A values and dry snow zone mean annual accumulation, Q. Frequency difference images between overlapping scatterometer images suggest that C band data are more sensitive to snow layering and buried inhomogeneities, whereas Ku band data are more sensitive to volume scattering from recently accumulated snow. Direct comparisons between NSCAT B values and in situ Q measurements show a linear relationship between In (Q) and B, with a negative rank correlation of R = -0.8. The root-mean-square residual in fitting regression line equation In (Q) = 3.08 - 17.83B to the data is 0.05-m snow water equivalent. This simple Ku band empirical relationship is exploited to investigate decadal changes in dry snow zone accumulation between Seasat (1978) and NSCAT (1996). Additional comparisons between NSCAT and recent QSCAT (1999) data reveal significant upslope shifts in the dry snow line along the southwestern flank of the ice sheet. Recent acceleration in the increase in intensity of scattering is observed in the percolation zone, suggesting increased melting between 2000- and 3000-m elevation in the southern half of the ice sheet.

Stiles2002

Direction interval retrieval with thresholded nudging: A method for improving the accuracy of QuikSCAT winds
B. W. Stiles and B. D. Pollard and R. S. Dunbar
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  79--89  (2002)

The SeaWinds scatterometer was developed by NASA JPL, Pasadena, CA, to measure the speed and direction of ocean surface winds. It was then launched onboard the QuikSCAT spacecraft. The accuracy of the majority of the swath and the size of the swath are such that the SeaWinds on QuikSCAT Mission (QSCAT) meets its science requirements despite shortcomings at certain cross-track positions. Nonetheless, it is desirable to modify the baseline processing in order to improve the quality of the less accurate portions of the swath, in particular near the far swath and nadir. Two disparate problems have been identified for these regions. At far swath, ambiguity removal skill is degraded due to the absence of inner beam measurements, limited azimuth diversity and boundary effects. Near nadir, due to nonoptimal measurement geometry, (measurement azimuths approximately 180degrees apart) there is a marked decrease in directional accuracy even when ambiguity removal works correctly. Two algorithms have been developed: direction interval retrieval (DIR) to address the nadir performance issue and thresholded nudging (TN) to improve ambiguity removal at far swath. We illustrate the impact of the two techniques by exhibiting prelaunch simulation results and postlaunch statistical performance metrics with respect to ECMWF wind fields and buoy data.

Patoux2002

A gradient wind correction for surface pressure fields retrieved from scatterometer winds
J. Patoux and R. A. Brown
JOURNAL OF APPLIED METEOROLOGY  41  133--143  (2002)

Given a field of geostrophic winds and at least one pressure observation, a pressure field can be computed. If the winds are in reality gradient winds, then a correction must be applied to calculate the actual geostrophic winds. Here a method is proposed for including a gradient wind correction in the retrieval of geostrophic winds from Quick Scatterometer (QuikSCAT) surface measurements with a planetary boundary layer model. This correction translates into a better estimate of the corresponding surface pressure fields. The scheme is assessed by comparing these pressure fields to buoy measurements in the Gulf of Alaska and to radiosonde measurements in Hurricane Floyd. The gradient wind correction has a curvature component and a time-dependent component. Their relative magnitude is evaluated.

Yueh2001

QuikSCAT geophysical model function for tropical cyclones and application to hurricane floyd
S. H. Yueh and B. W. Stiles and W. Y. Tsai and H. Hu and W. T. Liu
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  39  2601--2612  (2001)

The QuikSCAT radar measurements of several tropical cyclones in 1999 have been studied to develop the geophysical model function (GMF) of Ku-band radar sigma(0)s for extreme high wind conditions. To account for the effects of precipitation, we analyze the co-located rain rates from the Special Sensor Microwave/Imager (SSM/I) and propose the rain rate as a parameter of the GMF The analysis indicates the deficiency of the NSCAT2 GMF developed for the NASA scatterometer, which overestimates the ocean sigma(0) for tropical cyclones and ignores the influence of rain. It is suggested that the QuikSCAT sigma(0) is sensitive to the wind speed of up to about 40-50 m.s(-1). We introduce modifications to the NSCAT2 GMF and apply the modified GMF to the QuikSCAT observations of Hurricane Floyd. The QuikSCAT wind estimates for Hurricane Floyd in 1999 was improved with the maximum wind speed reaching above 60 m/s. We perform an error analysis by comparing the QuikSCAT winds with the analyses fields from the National Oceanic and Atmospheric Administration (NOAA) Hurricane Research Division (HRD). The reasonable agreement between the improved QuikSCAT winds and the HRD analyses supports the applications of scatterometer wind retrievals for hurricanes.

Born2001

An analytical theory for orbit determination
G. H. Born and D. B. Goldstein and B. Thompson
JOURNAL OF THE ASTRONAUTICAL SCIENCES  49  345--361  (2001)

The QuikScat spacecraft was launched in June 1999 and is currently measuring vector winds over the global oceans with a spatial resolution of 25 km and a temporal resolution of one day. The University of Colorado is responsible for mission operations, including production of the ephemeris used to produce the science data records. The navigation solutions from the onboard Motorola Vice Roy receiver are smoothed in a post-processing mode to produce the operational ephemeris. The objective of this study is to develop a simple yet effective method of smoothing the navigation solutions from the onboard GPS receiver in a post-processing mode to provide a medium accuracy continuous ephemeris. The navigation solutions prior to Selective Availability being turned off in May, 2000 had noise at the 100 ra level. Currently the noise level is around 10 in. However, this noise is high frequency relative to the dominant orbit perturbations periods. Hence, orbit determination accuracy with SA off is not significantly different from that resented here. The approach used is to augment a J(2) analytical theory for near-circular orbits to include additional frequencies due primarily to tesseral harmonics. Accuracy of the theory was tested through fits to simulated and actual QuikScat GPS receiver data.

Patoux2001

A scheme for improving scatterometer surface wind fields
J. Patoux and R. A. Brown
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES  106  23985--23994  (2001)

A method is presented for improving QuikSCAT surface wind fields. The University of Washington Planetary Boundary Layer model is used to retrieve a surface pressure field from any swath of QuikSCAT surface wind vectors. An alternate set of surface wind vectors is computed from the newly calculated pressure field. The latter can be smoothed and the process can be iterated. New surface wind vectors can be calculated where ambiguity removal fails and where measurements are missing. The present methodology preserves boundary layer dynamics and is an improvement over a statistical filter.

Patoux2001

Spectral analysis of QuikSCAT surface winds and two-dimensional turbulence
J. Patoux and R. A. Brown
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES  106  23995--24005  (2001)

A spectral decomposition of QuikSCAT surface wind vectors reveals different levels of variance and different values of the spectral slope in various regions of the world ocean for the 12 months investigated. The traditional considerations on the factors affecting the shape of the spectra are reviewed and compared to the results. In particular, the influence of large-scale synoptic systems is shown by comparing the steeper and more energetic spectra of the midlatitudes to the shallower spectra of the tropics. Similarly, the signature of convection is investigated by comparing spectra in the tropical convectively active and dry zones of the Pacific Ocean. Spectra of vorticity and divergence are calculated, along with spectral vorticity-to-divergence ratios. Their spatial and temporal variations are discussed. It is hypothesized that when convection is enhanced in the tropics, the spectral analysis captures the mesoscale/synoptic structures in which convection is embedded and that the spectra exhibit some of the characteristics of their midlatitude counterparts (i.e., steeper and more energetic).

Pan2001

Vector empirical orthogonal function modes of the ocean surface wind variability derived from satellite scatterometer data
J. Y. Pan and X. H. Yan and Q. Zheng and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  28  3951--3954  (2001)

Ocean surface winds derived from NSCAT, QuikSCAT and ERS-1/2 scatterometer observations during a period from January 1992 to April 2000 were analyzed using the vector empirical orthogonal function (VEOF) method. With the boreal winter and summer oscillation, the first VEOF is dominated by the Indian and East Asian monsoons and also shows an annual cycle of the trade winds. The second VEOF represents the boreal autumn and spring oscillation, and reveals a transition state between winter and summer. The third VEOF indicates the wind variability associated with El Nino Southern Oscillation (ENSO) events, because the temporal mode has a high correlation coefficient of 0.8 with the Southern Oscillation Index (SOI). Further more, the third mode reveals the teleconnection of the Indian monsoon and wind variability over high latitude oceans', such as the Aleutian Low system, with ENSO events.

Chen2001

Applying satellite remote sensing to predicting 1999-2000 La Nina
D. Chen
REMOTE SENSING OF ENVIRONMENT  77  275--278  (2001)

The usability of altimeter sea level data (TOPEX/POSEIDON) and scatterometer wind data (QuikSCAT) in El Nino and the Southern Oscillation (ENSO) prediction is investigated with the latest version of the Lamont forecast model. The emphasis of this study is on the effectiveness of these data sets in initializing the model to forecast the 1999-2000 La Nina conditions. Both the altimeter and scatterometer observations helped to improve the model, with the former being more effective for this period. It is possible and extremely useful to apply these data to real-time ENSO forecasting. In principle, it is advisable to assimilate multiple data sets so that they can complement one another in providing the correct initial conditions for the model. (C) 2001 Elsevier Science Inc. All rights reserved.

Portabella2001

Rain detection and quality control of SeaWinds
M. Portabella and A. Stoffelen
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  18  1171--1183  (2001)

A good assessment of the information content of scatterometer winds is particularly important in order to assimilate them in weather analysis. Besides retrieval problems in cases of a confused sea state, a particularly acute problem of Ku-band scatterometry is the sensitivity to rain. Elimination of poor quality data is therefore a prerequisite for the successful use of the National Aeronautics and Space Administration (NASA) Scatterometer (NSCAT) or QuikSCAT winds. Following the quality control for the European Remote-Sensing Satellite and NSCAT scatterometers performed at the Royal Netherlands Meteorological Institute, the authors further develop this methodology for QuikSCAT and define a quality indicator called the normalized residual (Rn). In order to characterize and validate the normalized residual, the authors use collocated Special Sensor Microwave Imager rain and European Centre for Medium-Range Weather Forecasts wind data. The results show indeed correlation between Rn and data quality. A wind speed dependent Rn threshold is shown to be adequate in terms of rejecting poor quality data (particularly rain) and keeping fair quality data. This opens the way to a quantitative use of SeaWinds measurements in weather analysis.

Schlax2001

Sampling errors in wind fields constructed from single and tandem scatterometer datasets
M. G. Schlax and D. B. Chelton and M. H. Freilich
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  18  1014--1036  (2001)

Sampling patterns and sampling errors from various scatterometer datasets are examined. Four single and two tandem scatterometer mission scenarios are considered. The single scatterometer missions are ERS (with a single, narrow swath), NSCAT and ASCAT (dual swaths), and QuikSCAT (a single, broad swath obtained from the SeaWinds instrument). The two tandem scenarios are combinations of the broad-swath SeaWinds scatterometer with ASCAT and QuikSCAT. The dense, nearly uniform distribution of measurements within swaths, combined with the relatively sparse, nonuniform placement of the swaths themselves create complicated space-time sampling patterns. The temporal sampling of all of the missions is characterized by bursts of closely spaced samples separated by longer gaps and is highly variable in both latitude and longitude. Sampling errors are quantified by the expected squared bias of particular linear estimates of component winds. Modifications to a previous method that allow more efficient expected squared bias calculations are presented and applied. Sampling errors depend strongly on both the details of the temporal sampling of each mission and the assumed temporal scales of variability in the wind field but are relatively insensitive to different spatial scales of variability. With the exception of ERS, all of the scatterometer scenarios can be used to make low-resolution (3 degrees and 12 days) wind component maps with errors at or below the 1 m s(-1) level. Only datasets from the broad-swath and tandem mission scenarios can be used for higher-resolution maps with similar levels of error, emphasizing the importance of the improved spatial and temporal coverage of those missions. A brief discussion of measurement errors concludes that sampling error is generally the dominant term in the overall error budget for maps constructed from scatterometer datasets.

Polito2001

Oceanic and atmospheric anomalies of tropical instability waves
P. S. Polito and J. P. Ryan and W. T. Liu and F. P. Chavez
GEOPHYSICAL RESEARCH LETTERS  28  2233--2236  (2001)

Tropical instability waves (TIWs) are detected in remotely-sensed sea surface height (SSH), temperature (SST), and wind records of the eastern equatorial Pacific. Analyses of TIW anomaly relationships reveal strong dynamical influence of TIWs within approximately 5 degrees of the equator. The first influence is advective heat flux. The primary forcing of TIW SST anomalies is advection of the meridional temperature gradient by TIW currents. The second influence is modification of the wind stress and Ekman pumping fields by TIW surface ocean currents. By affecting surface stress and hence roughness, TIW currents in this low-wind region introduce a significant bias in scatterometer vector wind measurement. This bias is evident in both NSCAT and QuikSCAT winds. The difference between wind measurements from TAO moorings and scatterometers is phase-locked with TIW SST oscillations. These results have important implications for scatterometry and for understanding tropical dynamics, thermodynamics and biogeochemistry.

Chelton2001

Observations of coupling between surface wind stress and sea surface temperature in the eastern tropical Pacific
D. B. Chelton and S. K. Esbensen and G. Schlax and N. Thum and M. H. Freilich and F. J. Wentz and C. L. Gentemann and M. J. McPhaden and P. S. Schopf
JOURNAL OF CLIMATE  14  1479--1498  (2001)

Satellite measurements of surface wind stress from the QuikSCAT scatterometer and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission Microwave Imager are analyzed for the three-month period 21 July-20 October 1999 to investigate ocean-atmosphere coupling in the eastern tropical Pacific. Oceanic tropical instability waves (TIWs) with periods of 20-40 days and wavelengths of 1000-2000 km perturb the SST fronts that bracket both sides of the equatorial cold tongue, which is centered near 1 degreesS to the east of 130 degreesW. These perturbations are characterized by cusp-shaped features that propagate systematically westward on both sides of the equator. The space-time structures of these SST perturbations are reproduced with remarkable detail in the surface wind stress field. The wind stress divergence is shown to be linearly related to the downwind component of the SST gradient with a response on the south side of the cold tongue that is about twice that on the north side. The wind stress curl is linearly related to the crosswind component of the SST gradient with a response that is approximately half that of the wind stress divergence response to the downwind SST gradient. The perturbed SST and wind stress fields propagate synchronously westward with the TIWs. This close coupling between SST and wind stress supports the Wallace et al. hypothesis that surface winds vary in response to SST modification of atmospheric boundary layer stability.

Katsaros2001

QuikSCAT's SeaWinds facilitates early identification of tropical depressions in 1999 hurricane season
K. B. Katsaros and E. B. Forde and P. Chang and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  28  1043--1046  (2001)

Far from land and surface ship observations, most tropical depressions are identified by examining images from geostationary satellites for the presence of rotation of the convective cloud masses. During the 1999 hurricane season, surface wind vectors obtained by the SeaWinds scatterometer on the QuikSCAT satellite for the tropical Atlantic and Caribbean Sea were examined to test the hypothesis that developing tropical depressions (TDs) could be observed with this satellite sensor, before identification by the traditional means. QuikSCAT was able to detect the presence of closed circulation in the surface winds before the systems were designated as depressions. The satellite's unprecedented large swath width of 1800 km allows twice a day observation of most of the tropical oceans. SeaWinds data can, therefore, provide valuable guidance that are an important addition to the tools available to the tropical cyclone forecasting community.

Early2001

Image reconstruction and enhanced resolution imaging from irregular samples
D. S. Early and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  39  291--302  (2001)

While high resolution, regularly gridded observations are generally preferred in remote sensing, actual observations are often not evenly sampled and have lower-than-desired resolution, Hence, there is an interest in resolution enhancement and image reconstruction. This paper discusses a general theory and techniques for image reconstruction and creating enhanced resolution images from irregularly sampled data. Using irregular sampling theory, we consider how the frequency content in aperture function-attenuated sidelobes can be recovered from oversampled data using reconstruction techniques, thus taking advantage of the high frequency content of measurements made with nonideal aperture filters, We show that with minor modification, the algebraic reconstruction technique (ART) is functionality equivalent to Grochenig's irregular sampling reconstruction algorithm. Using simple Monte Carlo simulations, we compare and contrast the performance of additive ART, multiplicative ART, and the scatterometer image reconstruction (SIR) (a derivative of multiplicative ART) algorithms with and without noise, The reconstruction theory and techniques have applications with a variety of sensors and can enable enhanced resolution image production from many nonimaging sensors. The technique is illustrated with ERS-2 and SeaWinds scatterometer data.

Lin2000

An analysis of a rotating, range-gated, fanbeam spaceborne scatterometer concept
C. C. Lin and B. Romen and J. J. W. Wilson and F. Impagnatiello and P. S. Park
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  38  2114--2121  (2000)

A new simple scatterometer concept combines the advantages of both the fixed, multiple beam, sidelooking radar such as AMI-Wind (ERS-1/2) and NSCAT (ADEOS), and the conically scanning pencil-beam radar such as SeaWinds, A wide, fanbeam antenna is rotated around a vertical axis with a slow rotation rate. For a satellite at an altitude of 725 km, the antenna footprint sweeps a circular donut of 1500 km diameter. Such a slow conical scan combined with the motion of the satellite at approximate to 7 km/s ground speed results in highly overlapping successive sweeps such that an image pixel is revisited up to 10 similar to 11 times during an overpass. The pixels in the radial direction are resolved by range-gating the radar echo. Depending on the across-track position of the imaged pixel, the measurement acquisitions during an overpass consist of a set of sigma degrees at different combinations of the azimuth and incident angles. A preliminary optimization of the system resulted in a C-band radar concept with a 15 km multiple-look spatial resolution and global coverage in two days. A sketch of the developed concept, preliminary system design, and predicted performance are described.

Kim2000

A physical-model-based, field-wise and self-contained algorithm for removing directional ambiguities of ocean surface winds retrieved from scatterometer measurements
Y. J. Kim
GEOPHYSICAL RESEARCH LETTERS  27  2665--2668  (2000)

An algorithm is introduced to remove the directional ambiguities in ocean surface winds measured by scatterometers, which requires scatterometer data only. It is based on two Versions of PBL (planetary boundary layer) models and a lowpass filter. A pressure field is first derived from the median-filtered scatterometer winds, is then noise-filtered, and is finally converted back to the winds, respectively, by an inverted PBL model, a smoothing algorithm, and a PBL model. The derived wind field is used to remove the directional ambiguities in the scatterometer data. This new algorithm is applied to Hurricane Eugene and produces results comparable to those from the current standard ambiguity removal algorithm for NASA/JPL SeaWinds project, which requires external numerical weather forecast/analyses data.

Liu2000

Atmospheric manifestation of tropical instability wave observed by QuikSCAT and tropical rain measuring mission
W. T. Liu and X. S. Xie and P. S. Polito and S. P. Xie and H. Hashizume
GEOPHYSICAL RESEARCH LETTERS  27  2545--2548  (2000)

Observations from two new spaceborne microwave instruments in 1999 clearly reveal the atmospheric manifestation of tropical instability waves north of the Pacific equatorial cold tongue. A unique zonal-temporal bandpass filter enables the isolation of the propagating signals and the determination of their phase differences. The phase differences between the propagation of wind and sea surface temperature (SST) signals observed from space and the vertical wind profiles measured from a research ship are consistent with the hypothesis that the coupling between wind and SST is caused by buoyancy instability and mixing, which reduces the wind shear in the atmospheric boundary layer. The coupling causes higher evaporative cooling over the warm phase and infers a negative thermal feedback.

Figa2000

On the assimilation of Ku-band scatterometer winds for weather analysis and forecasting
J. Figa and A. Stoffelen
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  38  1893--1902  (2000)

Following the successful assimilation of European remote sensing satellite (ERS) scatterometer winds for weather analysis and forecasting, we further develop this methodology for the assimilation of the NASA scatterometer (NSCAT) and QuikSCAT Ku-band scatterometer data. Besides retrieval problems in cases of a confused sea state, the quality control (QC) developed here identifies cases with rain on a mind vector cell (WVC) by WVC basis. The elimination of such geophysical conditions is a prerequisite to arrive at a successful assimilation of Ku band scatterometer data. Moreover, me propose to assimilate ambiguous winds rather than radar backscatter measurements, as is being done at most meteorological centers assimilating ERS scatterometer data. After our quality assessment, NSCAT winds still have more difficult ambiguity removal properties than ERS winds, A further testing of the data assimilation method proposed is being carried out at the European Center for Medium-range Weather Forecasts in NSCAT impact experiments. A normalized wind inversion residual is used for QC, In order to determine a threshold for the rejection of poor quality wind solutions, the inversion residual and the wind vector departure from the ECMWF model, are correlated. We end up rejecting around 7.4\% of wind vector solutions and 4.2\% of the NSCAT WVC's, In order to perform a qualitative assessment of this rejection, comparisons to collocated SSM/I rain and ECMWF minds are used. Confused sea state and presence of rain seem to be the most likely causes for the rejection of WVC's, As expected, the remaining number of ambiguities is larger than in the case of the ERS scatterometer but dependent on wind direction. The proposed cost function for the assimilation of NSCAT winds in weather analysis contains up to four ambiguous minds, the derived QC information, and the probability of each wind solution, We believe that the results of our study can be successfully extended for the interpretation and retrieval of good quality winds from QuikSCAT and their assimilation in weather analysis.

Tsai2000

Polarimetric scatterometry: A promising technique for improving ocean surface wind measurements from space
W. Y. Tsai and S. V. Nghiem and J. N. Huddleston and M. W. Spencer and B. W. Stiles and R. D. West
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  38  1903--1921  (2000)

Spaceborne wind scatterometers provide useful measurements of ocean surface winds and are important to climatological studies and operational weather forecasting. Past and currently planned scatterometers use measurements of the copolarized backscatter cross-section at different azimuth angles to infer ocean surface wind speed and direction. Although successful, current scatterometer designs have limitations such as degraded wind performance in the near-nadir and outer regions of the measurement swath and a reliance on external wind information for vector ambiguity, removal, Theoretical studies of scattering from the mind-induced ocean surface indicate that polarimetric measurements provide orthogonal and complementary directional information to aid the wind retrieval process. In this paper, potential benefits of making polarimetric backscatter measurements to improve wind retrieval performance are addressed, To investigate the performance of a polarimetric scatterometer, a modified version of the SeaWinds end-to-end simulator at the Jet Propulsion Laboratory (JPL), Pasadena, CA, is employed, To model the effect of realistic measurement errors, expressions for polarimetric measurement variance and bias are derived. It is shown that a polarimetric scatterometer can be realized with straightforward and inexpensive modifications to a current scanning pencil-beam scatterometer system such as SeaWinds, Simulation results show that such a system can improve wind performance in the nadir region and eliminate the reliance on external wind information. The mechanism by which the addition of polarimetric measurements improves wind vector retrieval is discussed in detail. Field experiments are suggested to better characterize the polarimetric scattering properties of the wind-modulated ocean surface for future applications to wind scatterometry.

Spencer2000

Improved resolution backscatter measurements with the SeaWinds pencil-beam scatterometer
M. W. Spencer and C. L. Wu and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  38  89--104  (2000)

The SeaWinds scatterometer was launched on the NASA QuikSCAT spacecraft in June 1999 and is planned for the Japanese ADEOS-II mission in 2000. In addition to generating a global Ku-band backscatter data set useful for a variety of climate studies, these flights will provide ocean-surface wind estimates for use in operational weather forecasting. SeaWinds employs a compact `` pencil-beam '' design rather than the ``fan-beam'' approach previously used with SASS on Seasat, NSCAT on ADEOS-I, and the AMI scatterometer on ERS-1, 2, As originally envisioned and reported, the resolution of the SeaWinds backscatter measurements were to be antenna-beamwidth limited. In order to satisfy an emerging demand for higher resolution backscatter data, however, the SeaWinds signal-processing design has been significantly modified. Here, the various options considered for improving the resolution of the SeaWinds measurements are discussed, and the selected:hardware modification (the addition of deramp processing for range discrimination) is described. The radar equation specific to a rotating pencil-beam scatterometer with digital range filtering is developed, and the new challenges associated with calibrating the resulting improved resolution measurements are discussed. A formulation for assessing the variance of the measurements due to fading and thermal noise is presented. Finally, the utility of improved resolution SeaWinds measurements for land and ice studies is demonstrated by simulated enhanced-resolution imaging of a synthetic Earth backscatter scene.

Oliphant1999

Accuracy of scatterometer-derived winds using the Cramer-Rao bound
T. E. Oliphant and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  37  2642--2652  (1999)

A wind scatterometer makes measurements of the normalized radar-backscatter coefficient sigma degrees of the ocean surface. To retrieve the wind, a geophysical model function (GMF), which relates sigma degrees to the near-surface wind, is used. The wind vector can be estimated using maximum-likelihood techniques from several sigma degrees measurements made at different azimuth angles. The probability density of the measured sigma degrees is assumed to be Gaussian with a variance that depends on the true sigma degrees and therefore, depends on the wind through the GMF, With this model for wind estimation, the Cramer-Rao (C-R) bound is derived for wind estimation, and its implications for wind retrieval are discussed, As part of this discussion, the role of geophysical modeling error is considered and shown to play a significant role in the performance of near-surface wind estimates, The C-R bound is illustrated using parameters from the ERS AMI, NSCAT, and SeaWinds scatterometers.

Scharton1999

Combined loads, vibration, and modal testing on the QuikSCAT spacecraft
T. Scharton and M. Vujcich
JOURNAL OF THE IEST  42  36--42  (1999)

Combining the quasi-static loads, workmanship verification, and model validation tests of aerospace hardware into a single vibration test sequence can reduce schedule and cost considerably. The enabling factor in the implementation of the combined dynamic testing approach is the measurement of the dynamic forces exerted on the test item by the shaker. The dynamic testing of the QuikSCAT spacecraft is discussed as an example of a successful combined loads, workmanship, and model validation test program.

Hussein1999

Application of cylindrical near-field measurement technique to the calibration of spaceborne radar antennas: NASA scatterometer and SeaWinds
Z. A. Hussein and Y. Rahmat-Samii
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  37  360--373  (1999)

Modern spaceborne radar scatterometers, such as the NASA scatterometer (NSCAT) and SeaWinds radar instruments, require precise determination of the normalized backscattered radar cross section within a few tenth of a decibel, This is needed to achieve the desired wind velocity and direction measurement accuracy of 2 mis and 20 degrees, respectively. This high level of precision demands a priori prelaunch accurate knowledge and determination of the radar antenna's absolute gain and relative radiation patterns characteristics over wide angular range. such characterizations may be performed on a far-field range, compact range, or in an indoor near-field measurement Facility, Among the unique advantage of the near-held measurement is that most of the information of the radar antenna radiation properties fan be obtained anywhere outside the near-field measurement surface. Two recently designed radar scatterometers are considered in this paper, NASA scatterometer (NSCAT) and SeaWinds, to demonstrate the utility of a newly completed cylindrical near-field measurement range. As an example of an advanced calibration methodology, the data based on a recently measured JPL/NASA scatterometer (NSCAT) radar antenna are used to experimentally demonstrate the role of the probe pattern compensation, probe multiple reflection effects, probe mispositioning effects, scan area truncation effects, etc, A measurement lest on a standard gain horn (SGH) has been performed to achieve and verify the absolute gain calibration accuracy. A comparison between direct far-field measured data and those obtained from cylindrical near-field measurements for the SeaWinds radar antenna was found in excellent agreement. Tt is demonstrated that the near-field measurement technique is a viable approach in accurately characterizing the performance of spaceborne radar antennas.

Risien2004

Variability in satellite winds over the Benguela upwelling system during 1999-2000
C. M. Risien and C. J. C. Reason and F. A. Shillington and D. B. Chelton
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  109    (2004)

[ 1] Wind stress variability over the Benguela upwelling system is considered using 16 months ( 01 August 1999 to 29 November 2000) of satellite-derived QuikSCAT wind data. Variability is investigated using a type of artificial neural network, the self-organizing map (SOM), and a wavelet analysis. The SOM and wavelet analysis are applied to an extracted data set to find that the system may be divided into six discrete wind regimes. The wavelet power spectra for these wind regions span a range of frequencies from 4 to 64 days, with each region appearing to contain distinct periodicities. To the north, 10degrees-23.5degreesS, the majority of the power occurs during austral winter, with a 4-16 day periodicity. Further investigation of National Centers for Environmental Prediction reanalysis outgoing longwave radiation data indicates that the winter intensification of wind stress off the Angolan coast is linked with convective activity over equatorial West Africa. The summer activity appears to be linked with the intensification of the Angolan heat low. Convective activity over the Congo basin appears to impact upon wind stress variability, off the Angolan coast, throughout the year. Farther south, 24degrees-35degrees S, the majority of the power occurs in the summer. Here a bimodal distribution occurs, with peaks of 4-12 and 25-50 days. The southernmost regions appear to be forced at higher frequencies by both midlatitude cyclones ( austral winter) and mesoscale coastal lows ( austral summer). At lower frequencies, eastward propagating periodic wind events that originate over eastern South America appear to be important to the forcing of wind stress over the southern Benguela.

Tang2004

Remote sensing oceanography of a harmful algal bloom off the coast of southeastern Vietnam
D. L. Tang and H. Kawamura and H. Doan-Nhu and W. Takahashi
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  109    (2004)

[ 1] Harmful algal blooms (HABs) in the southeastern Vietnamese coastal waters have caused large economic losses in aquacultured and wild fisheries in recent years; however, there have been few oceanographic studies on these HAB events. The present study reports an extensive HAB off southeastern Vietnamese waters during late June to July 2002 with in situ observations and analyzes the oceanographic conditions using satellite remote sensing data. The HAB had high chlorophyll a ( Chl a) concentrations ( up to 4.5 mg m-3) occurring -200 km off the coast and -200 km northeast of the Mekong River mouth for a period of -6 weeks. The bloom was dominated by the harmful algae haptophyte Phaeocystis cf. globosa and caused a very significant mortality of aquacultured fish and other marine life. In the same period, sea surface temperature (SST) imagery showed a cold water plume extending from the coast to the open sea, and QuikScat data showed strong southwesterly winds blowing parallel to the coastline. This study indicated that the HAB was induced and supported by offshore upwelling that brings nutrients from the deep ocean to the surface and from coastal water to offshore water and that the upwelling was driven by strong wind through Ekman transport when winds were parallel to the coastline. This study demonstrated the possibility of utilizing a combination of satellite data of Chl a, SST, and wind velocity together with coastal bathymetric information and in situ observations to give a better understanding of the biological oceanography of HABs.

Draper2004

Evaluating the effect of rain on SeaWinds scatterometer measurements
D. W. Draper and D. G. Long
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  109    (2004)

A simple wind/rain backscatter model is used with co-located precipitation radar ( PR) data from the Tropical Rainfall Measuring Mission (TRMM) satellite to evaluate the effect of rain on SeaWinds on QuikSCAT sigmadegrees observations. The model incorporates wind-induced surface scattering, the surface rain perturbation, and atmospheric rain attenuation and scattering. The co-located PR measurements afford direct computation of SeaWinds-scale averaged rain rate and atmospheric rain attenuation and scattering. An estimate of the wind-induced surface backscatter is computed via numerical weather prediction (NWP) winds. By synergistically combining the SeaWinds, NWP, and PR data, estimates of surface rain perturbation and combined surface/atmospheric scattering are made as a function of PR-derived rain rate. The scattering from rain is dominated mainly by the surface perturbation low rain rates, and by atmospheric scattering at high rain rates. The backscatter model estimates 94\% of the observed rain-contaminated SeaWinds on QuikSCAT sigmadegrees values to within 3 dB. Using the model, the conditions are determined for which it is possible to estimate rain from scatterometer measurements and where wind retrieval is not possible.

Monaldo2004

A systematic comparison of QuikSCAT and SAR ocean surface wind speeds
F. M. Monaldo and D. R. Thompson and W. G. Pichel and P. Clemente-Colon
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  42  283--291  (2004)

We performed a systematic comparison of wind speed measurements from the SeaWinds QuikSCAT scatterometer and wind speeds computed from RADARSAT-1 synthetic aperture radar (SAR) normalized radar cross section measurements. These comparisons were made over in the Gulf of Alaska and extended over a two-year period, 2000 and 2001. The SAR wind speed estimates require a wind direction to initialize the retrieval. Here, we first used wind directions from the Navy Operational Global Atmospheric Prediction System (NOGAPS) model. For these retrievals, the standard deviation between the resulting SAR and QuikSCAT wind speed measurements was 1.78 m/s. When we used the QuikSCAT-measured wind directions to initialize the inversion, comparisons improve to a standard deviation of 1.36 m/s. We used these SAR-scatterometer comparisons to generate a new C-band horizontal polarization model function. With this new model function, the wind speed inversion improves to a standard deviation of 1.24 m/s with no mean bias. These results strongly suggest that SAR and QuikSCAT measurements can be combined to make better high-resolution wind measurements than either instrument could alone in coastal areas.

Portabella2004

A probabilistic approach for SeaWinds data assimilation
M. Portabella and A. Stoffelen
QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY  130  127--152  (2004)

Scatterometer sea surface wind observations are being successfully assimilated into numerical weather prediction models. The quality of the winds retrieved from the new SeaWinds scatterometer (onboard the QuikSCAT satellite) depends on the subsatellite cross-track location. In particular, the poor azimuth separation or diversity between views in the nadir region results in poor quality winds. In the QuikSCAT nadir region, where the local cost-function minima are broad. the use of the standard procedure results in arbitrary and inaccurate winds. A new scheme. Which accounts for broad cost-function minima by allowing more ambiguous wind solutions, i.e. a multiple solution scheme (MSS), is proposed as an alternative to the standard procedure. The probability of every ambiguous solution being the 'true' wind is empirically derived, and used in the ambiguity-removal procedure to make the scheme flexible enough to accept many wind solutions. A comparison between the standard wind retrieval and the MSS procedures at 100 km resolution is then performed, using independent model winds for validation. The MSS turns out to be more in agreement with the model reference than the standard procedure, especially at nadir. Moreover, it shows more spatially consistent and realistic winds by more effectively exploiting the information content of the observations.

Chelton2004

Satellite measurements reveal persistent small-scale features in ocean winds
D. B. Chelton and M. G. Schlax and M. H. Freilich and R. F. Milliff
SCIENCE  303  978--983  (2004)

Four-year averages of 25-kilometer-resolution measurements of near-surface wind speed and direction over the global ocean from the QuikSCAT satellite radar scatterometer reveal the existence of surprisingly persistent small-scale features in the dynamically and thermodynamically important curl and divergence of the wind stress. Air-sea interaction over sea surface temperature fronts throughout the world ocean is evident in both the curl and divergence fields, as are the influences of islands and coastal mountains. Ocean currents such as the Gulf Stream generate distinctive patterns in the curl field. These previously unresolved features have important implications for oceanographic and air-sea interaction research.

Olsen2004

Sea-air flux of CO2 in the Caribbean Sea estimated using in situ and remote sensing data
A. Olsen and J. A. Trinanes and R. Wannitilchof
REMOTE SENSING OF ENVIRONMENT  89  309--325  (2004)

Empirical relationships between sea surface carbon dioxide fugacity (fCO(2)(sw)) and sea surface temperature (SST) were applied to datasets of remotely sensed SST to create fCO(2)(sw) fields in the Caribbean Sea. SST datasets from different sensors were used, as well as the SST fields created by optimum interpolation of bias corrected AVHRR data. Empirical relationships were derived using shipboard fCO(2)(sw) data, in situ SST data, and SST data from the remote sensing platforms. The results show that the application of a relationship based on shipboard SST data, on fields of remotely sensed SST yields biased fCO(2)(sw) values. This bias is reduced if the fCO(2)(sw)-SST relationships are derived using the same SST data that are used to create the SST fields. The fCO(2)(sw) fields found to best reproduce observed fCO(2)(sw) are used in combination with wind speed data from QuikSCAT to create weekly maps of the sea-air CO2 flux in the Caribbean Sea in 2002. The region to the SW of Cuba was a source Of CO2 to the atmosphere throughout 2002, and the region to the NE was a sink during winter and spring and a source during summer and fall. The net uptake of CO2 in the region was doubled when potential skin layer effects on fCO(2)(sw) were taken into account. (C) 2003 Elsevier Inc. All rights reserved.

Weissman2003

Calibrating the quikscat/seawinds radar for measuring rainrate over the oceans
D. E. Weissman and M. A. Bourassa and J. J. O'Brien and J. S. Tongue
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  41  2814--2820  (2003)

This effort continues a study of the effects of rain, over the oceans, on the signal retrieved by the SeaWinds scatterometer. It is determined that the backscatter radar cross section can be used to estimate the volumetric rain rate, averaged horizontally, across the surface resolution cells of the scatterometer. The dual polarization of the radar has a key role in developing this capability. The relative magnitudes of the radar backscatter depends on the volumetric rain rate, the rain column height and surface wind velocity, the viewing angle, as well as the polarization (due to the oblateness of raindrops at the higher rain rates). The approach to calibrating the SeaWinds normalized radar cross section (NRCS) is to collect National Weather Service Next Generation Weather Radar (NEXRAD) radar-derived rain rate measurements (4-km spatial resolution and 6-min rotating cycles) colocated in space (offshore) and time with scatterometer observations. These calibration functions lead to a Z-R relationship, which is then used at mid-ocean locations to estimate the rain rate in 0.25degrees or larger resolution cells, which are compared with Tropical Rainfall Mapping Mission (TRMM) Microwave Imager (TMI) rain estimates. Experimental results to date are in general agreement with simplified theoretical models of backscatter from rain, for this frequency, 14 GHz. These comparisons show very good agreement on a cell-by-cell basis with the TMI estimates for both wide areas (1000 km) and smaller area rain events.

Pickett2003

QuikSCAT satellite comparisons with nearshore buoy wind data off the US West Coast
M. H. Pickett and W. Q. Tang and L. K. Rosenfeld and C. H. Wash
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  20  1869--1879  (2003)

To determine the accuracy of nearshore winds from the QuikSCAT satellite, winds from three satellite datasets ( scientifically processed swath, gridded near-real-time, and gridded science datasets) were compared to those from 12 nearshore and 3 offshore U. S. West Coast buoys. Satellite observations from August 1999 to December 2000 that were within 25 km and 30 min of each buoy were used. Comparisons showed that satellite - buoy wind differences near shore were larger than those offshore. Editing the satellite data by discarding observations recorded in rain and those recorded in light winds improved the accuracy of all three datasets. After removing rain-flagged data and wind speeds less than 3 m s(-1), root-mean-squared differences (satellite minus buoy) for swath data, the best of the three datasets, were 1.4 m s(-1) and 37degrees based on 5741 nearshore comparisons. By removing winds less than 6 m s(-1), these differences were reduced to 1.3 m s(-1) and 26degrees. At the three offshore buoys, the root-mean-squared differences for the swath data, with both rain and winds less than 6 m s(-1) removed, were 1.0 m s(-1) and 15degrees based on 1920 comparisons. Although the satellite's scientifically processed swath data near shore do not match buoy observations as closely as those offshore, they are sufficiently accurate for many coastal studies.

Chan2003

Interannual variations of tropical cyclone size over the western {North Pacific}
J. C. L. Chan and C. K. M. Yip
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

This paper presents the results of an investigation on the interannual variations of tropical cyclone (TC) size, defined as the average radius of 15 m s(-1) surface winds, over the western North Pacific from 1999 to 2002. The wind data are from QuikSCAT. The average TC size is found to increase from 1999 to 2002, with the TC sizes in 1999 and 2000 being significantly smaller than those in both 2001 and 2002. Differences in the environmental flow patterns associated with the tracks and formation positions of the TCs among these years apparently explain such size changes. Since 1999 and 2000 were La Nina years, TCs in these two years formed more westward with fewer recurvers so that they tended to be smaller. On the other hand, with 2002 being an El Nino year, TCs formed further eastward with more recurvers, and thus tended to be larger.

Cardellach2003

Mediterranean Balloon Experiment: ocean wind speed sensing from the stratosphere, using GPS reflections
E. Cardellach and G. Ruffini and D. Pino and A. Rius and A. Komjathy and J. L. Garrison
REMOTE SENSING OF ENVIRONMENT  88  351--362  (2003)

The MEditerranean Balloon EXperiment (MEBEX), conducted in August 99 from the middle-up stratosphere, was designed to assess the wind retrieval sensitivity of Global Navigation Satellite Systems Reflections (GNSSR) technology from high altitudes. Global Positioning System reflected signals (GPSR) collected at altitudes around 37 km with a dedicated receiver have been inverted to mean square slopes (MSS) of the sea surface and wind speeds. The theoretical tool to interpret the geophysical parameters was a bistatic model, which also depends on geometrical parameters. The results have been analyzed in terms of internal consistency, repeatability and geometry-dependent performance. In addition, wind velocities have been compared to independent measurements by QuikSCAT, TOPEX, ERS/RA and a Radio Sonde, with an agreement better than 2 m/s. A Numerical Weather Prediction Model (NWPM, the MM5 mesoscale forecast model) has also been used for comparison with varying results during the experiment. The conclusion of-this study confirms the capability of high altitude GPSR/Delay-map receivers with low gain antennas to infer surface winds. (C) 2003 Elsevier Inc. All rights reserved.

Yueh2003

QuikSCAT wind retrievals for tropical cyclones
S. H. Yueh and B. W. Stiles and W. T. Liu
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  41  2616--2628  (2003)

The use of QuikSCAT data for wind retrievals of tropical cyclones is described. The evidence of QuikSCAT sigma(0) dependence on wind direction for >30-m/s wind speeds is presented. The QuikSCAT sigma(0) s show a peak-to-peak wind direction modulation of similar to1 dB at 35-m/s wind speed, and the amplitude of modulation decreases with increasing wind speed. The decreasing directional sensitivity to wind speed agrees well with the trend of QSCAT1 model function at near 20 m/s. A correction of the QSCAT1 model function for above 23-m/s wind speed is proposed. We explored two microwave radiative transfer models to correct the attenuation and scattering effects of rain for wind retrievals. One is derived from the collocated QuikSCAT and Special Sensor Microwave/Imager (SSM/I) dataset, and the other one is a published parametric model developed for rain radars. These two radiative transfer models account for the effects of volume scattering, scattering from rain-roughened surfaces and rain attenuation. The models suggest that the sigma(0) s of wind-roughened sea surfaces for 40-50-m/s winds are comparable to the sigma(0) s of rain contributions for up to about 10-15 mm/h. Both radiative transfer models have been used to retrieve the ocean wind vectors from the collocated QuikSCAT and SSM/I rain rate data for several tropical cyclones. The resulting wind speed estimates of these tropical cyclones show improved agreement with the wind fields derived from the best track analysis and Holland's model for up to about 15-mm/h SSM/I rain rate. A comparative analysis of maximum wind speed estimates suggests that other rain parameters likely have to be considered for further improvements.

Li2003

Satellite data analysis and numerical simulation of tropical cyclone formation
T. Li and B. Fu and X. Y. Ge and B. Wang and M. Peng
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] Forecast of tropical cyclone ( TC) formation has long been a great challenge owing to lack of reliable observations over vast open oceans. Recent satellite products provide a unique opportunity to reveal detailed atmospheric wave structures prior to TC formation. Using the QuikSCAT surface wind and the Tropical Rainfall Measurement Mission Microwave Image data, we document temporal and spatial structures of Rossby wave trains induced by energy dispersion from a pre- existing TC and easterly wave propagation characteristics prior to cyclogenesis in the western North Pacific. Using a baroclinic model, we further simulate cyclogenesis processes associated with the TC energy dispersion and easterly waves.

Pasch2003

Comments on ``Early detection of tropical cyclones using SeaWinds-derived vorticity''
R. J. Pasch and S. R. Stewart and D. P. Brown
BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY  84  1415--1416  (2003)

Sharp2003

Comments on ``Early detection of tropical cyclones using SeaWinds-derived vorticity'' - Reply
R. J. Sharp and M. A. Bourassa and J. J. O'Brien
BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY  84  1417--1417  (2003)

Castro2003

Evolution and extension of the Santa Ana winds of February 2002 over the ocean, off California and the Baja California Peninsula
R. Castro and A. Pares-Sierra and S. G. Marinone
CIENCIAS MARINAS  29  275--281  (2003)

The evolution and extension of Santa Ana winds over the Pacific Ocean, off the coasts of California and the Baja California Peninsula, is documented from QuikScat satellite observations. The typical wind pattern in this region is towards the southeast, but between 9 and 12 February 2002 it was modified by Santa Ana winds, changing in direction towards the south, southwest and west. The changes were notable at first in southern California, but the largest indices of variation occurred between 26degreesN and 28degreesN off the Baja California Peninsula, with maxima in the Gulf of California, mainly in the northern part. The maximum extension of the winds to the west of the coast was on I I February, reaching 32degreesN and 125degreesW, or similar to700 km; however, towards the southwest the winds extended to 25degreesN and 124degreesW, or similar to1000 km. The topography of the peninsula plays an important role in controlling the winds passing from the Gulf of California to the Pacific, causing plumes of sand and dust in several locations over the ocean.

Hilburn2003

Development of scatterometer-derived surface pressures for the Southern Ocean
K. A. Hilburn and M. A. Bourassa and J. J. O'Brien
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  108    (2003)

[1] High-resolution, research-quality surface pressures are objectively calculated over the Southern Ocean using winds derived from the SeaWinds scatterometer on the QuikSCAT satellite. The pressure fields are validated in comparison to in situ observations. Overall, the scatterometer-derived surface pressures are a small improvement over the National Centers for Environmental Prediction/National Center for Atmospheric Research ( NCEP/NCAR) reanalysis, which is used as the objective technique's background field. This improvement is understated primarily because the comparison data undersample storms. Instances are found where the NCEP/NCAR reanalysis misses storms entirely and the scatterometer-derived pressures are a large improvement ( as much as 20 hPa).

Tournadre2003

Impact of rain cell on scatterometer data: 1. Theory and modeling
J. Tournadre and Y. Quilfen
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  108    (2003)

[1] The two scatterometers currently in operation, the Ku-band NASA Seawinds on the QuikScat satellite and the C-band AMI-Wind on the ERS-2 satellite, are designed to infer the ocean wind vectors from sea surface radar backscatter measurements. They provide excellent coverage of the ocean, and their wind products are of great value for ocean and meteorological communities. However, the presence of rain within scatterometer cells can significantly modify the sea surface backscatter coefficient and hence alter the wind vector retrieval. These perturbations can hamper the analysis of wind fields within atmospheric low-pressure systems or tropical cyclones. Rain perturbations result from volume scattering and attenuation by raindrops in the atmosphere as well as changes of sea surface roughness by impinging drops. For scatterometers operating at Ku-Band, attenuation and volume scattering are strong and one order of magnitude larger than at C-band. The wind retrieval will thus be less affected for the C-band AMI-Wind instrument than for the Ku-band Seawinds. A theoretical model, based on radiative transfer formulation including rain attenuation and scattering, has been developed to quantify the modification by rain of the measured backscatter and of the retrieved wind vectors. Changes in surface roughness, a complex phenomenon not yet fully understood and parameterized, is not considered here although it could be of importance for high rain rates. As a scatterometer cell covers several hundred square kilometers, inhomogeneities of rain within the cell will further modify the measured backscatter, particularly in case of small, intense precipitating rain cells. Using analytical rain cell models and constant wind fields, the effects of partial beam filling by rain is investigated. The model results show that Ku-band scatterometer data are greatly affected by rain and are extremely sensitive to the distribution of rain within scatterometer cells, i.e., to the distance between the rain cell center and the scatterometer resolution cell center. When the scatter from the sea surface is low, the additional volume scattering from rain will have a marked effect leading to an overestimation of the low wind speed actually present. Conversely, when the backscatter is already high (at high winds), attenuation by rain will reduce the signal causing an underestimation of the wind speed. The wind direction is modified in a complex manner and mainly depends on the rain distribution within the scatterometer cell. These results show that, especially at low and moderate wind speed, rain data such as the Special Sensor Microwave/Imager (SSM/I) rain fields are too coarse for correction of Normalized Radar Cross Section (NRCS) and that high-resolution rain data (such as the Tropical Rainfall Mapping Mission (TRMM) ones) are necessary. They also show that a good rain flagging is still an important issue for the operational use of Ku-band scatterometer data. A succeeding paper will present an example of application of the model for the correction of QuikScat data using TRMM rain data within a tropical cyclone.

Senan2003

Intraseasonal ''monsoon jets'' in the equatorial Indian Ocean
R. Senan and D. Sengupta and B. N. Goswami
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] The zonal wind in the equatorial Indian Ocean (EqIO) is westerly almost throughout the year. It has a strong semiannual cycle and drives the spring and fall Wyrtki jets. In addition, high resolution daily satellite winds show ``westerly wind bursts'' lasting 10-40 days, associated with atmospheric convection in the eastern EqIO. These bursts have the potential to produce intraseasonal eastward equatorial jets in the ocean. Using an ocean model driven by QuikSCAT scatterometer winds, we show that strong westerly bursts associated with summer monsoon intraseasonal oscillations can drive ''monsoon jets'' in the eastern EqIO, which have been observed recently. Although there are distinct equatorial wind bursts associated with Madden-Julian oscillations in January March, they do not produce equatorial jets in the ocean. The role of ocean dynamics in producing the selective response of the ocean is discussed.

Lin2003

First evidence for the detection of natural surface films by the QuikSCAT scatterometer
I. I. Lin and W. Alpers and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] For the first time it is demonstrated that with the QuikSCAT scatterometer it is possible to detect natural surface films resulting from enhanced biological activity in the ocean. It is shown for two regions in the Norwegian and Baltic Sea that areas of strongly reduced Normalized Radar Cross Section (NRCS) are associated with areas of enhanced chlorophyll-a concentration as evidenced by quasi-simultaneously acquired SeaWiFS data. This result has two implications. Firstly, it opens up the possibility to map globally natural surface film coverage using QuikSCAT data. Secondly, it demonstrates that in ocean areas with high biological activity the presence of natural surface films can give rise to significant errors in wind vector retrieval when using the current QuikSCAT wind retrieval algorithm.

ONeill2003

Observations of SST-induced perturbations of the wind stress field over the Southern Ocean on seasonal timescales
L. W. O'Neill and D. B. Chelton and S. K. Esbensen
JOURNAL OF CLIMATE  16  2340--2354  (2003)

The surface wind stress response to sea surface temperature (SST) over the latitude range 30degrees-60degreesS in the Southern Ocean is described from the National Aeronautics and Space Administration's QuikSCAT scatterometer observations of wind stress and Reynolds analyses of SST during the 2-yr period August 1999 to July 2001. While ocean-atmosphere coupling at midlatitudes has previously been documented from several case studies, this is the first study to quantify this relation over the entire Southern Ocean. The spatial structures of the surface wind perturbations with wavelengths shorter than 10degrees latitude by 30degrees longitude are closely related to persistent spatial variations of the SST field on the same scales. The wind stress curl and divergence are shown to be linearly related, respectively, to the crosswind and downwind components of the SST gradient. The curl response has a magnitude only about half that of the divergence response. This observed coupling is consistent with the hypothesis that SST modification of marine atmospheric boundary layer (MABL) stability affects vertical turbulent mixing of momentum, inducing perturbations in the surface winds. The nonequivalence between the responses of the curl and divergence to the crosswind and downwind SST gradients suggests that secondary circulations in the MABL may also play an important role by producing significant perturbations in the surface wind field near SST fronts that are distinct from the vertical turbulent transfer of momentum. The importance of the wind stress curl in driving Ekman vertical velocity in the open ocean implies that the coupling between winds and SST may have important feedback effects on upper ocean processes near SST fronts.

Nakamura2003

Variation of underwater noise in the tomography experiments at the Central Pacific
T. Nakamura and H. Fujimori and I. Nakano
JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES \& REVIEW PAPERS  42  3194--3197  (2003)

Tomography experiments using seven 200 Hz transceivers were conducted from January to December 2000 around the date line in the Central Pacific Ocean. The surface buoy for, data communication of transceiver No. 6 was separated from the mooring system and began to drift on September 12, 2000. In this report, variation of underwater noise is investigated around that day. As a result, an increase of noise level more than 15 dB was found on September 11 in comparison to the stationary level. To investigate the reason for the extraordinary noise level, wind data by ``QuikSCAT'' of NASA were examined. Wind speed was approximately 10 m/s at the area and did not affect the noise level. As many line spectra were observed in the underwater noise and a steep slope was shown in the inclination data of the mooring on that day, separation of the buoy may have occurred by an artificial phenomenon.

Hu2003

Oceanic thermal and biological responses to Santa Ana winds
H. Hu and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] Ocean surface wind vectors with improved spatial resolutions were derived from the National Aeronautics and Space Administration's (NASA's) Quick Scatterometer (QuikSCAT) satellite. They allow us to examine the details and extent of oceanic influence of a Santa Ana event - a strong offshore and downslope wind in southern California that may spread wide fires, damage properties, and endanger aviation. The oceanic thermal and biological responses to the surface wind jets were observed with other spaceborne sensors. It is found that surface wind jets reduce sea surface temperatures and increase biological productivities. Spacebased measurements demonstrate the inadequacy of current operational numerical weather prediction (NWP) models to accurately and consistently predict the characteristics of Santa Ana winds over the coastal ocean.

Patoux2003

Global pressure fields from scatterometer winds
J. Patoux and R. C. Foster and R. A. Brown
JOURNAL OF APPLIED METEOROLOGY  42  813--826  (2003)

A method is presented for computing global surface pressure fields from satellite scatterometer winds. Pressure gradients are estimated using a two-layer similarity planetary boundary layer model in the midlatitudes and a mixed-layer model in the Tropics. A global pressure field is then fit to the pressure gradients by least squares optimization. A series of surface pressure fields calculated from SeaWinds-on-QuikSCAT (Quick Scatterometer) measurements are compared with numerical weather analyses and buoy measurements. Surface pressure observations in the tropical oceans are scarce and come largely from ships of opportunity. At present no buoy in the Atlantic Ocean and only 10 buoys in the Pacific Ocean have pressure sensors. The method presented here suggests that 0.5degrees-resolution maps of sea surface pressure can be readily retrieved from available satellite remote sensing data every 12 h in near-real time. It is shown that these fields are at least of comparable quality to the ECMWF analyses.

Goswami2003

A note on the deficiency of NCEP/NCAR reanalysis surface winds over the equatorial Indian Ocean
B. N. Goswami and D. Sengupta
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  108    (2003)

The seasonal cycle and intraseasonal variability of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP) reanalysis surface winds over the Indian Ocean (IO) are assessed by comparing them with in situ surface observations from two moored buoys and winds from the SeaWinds scatterometer on the QuikSCAT satellite. The buoys are located in the central Bay of Bengal and eastern Arabian Sea. Both QuikSCAT and NCEP wind products reproduce closely the seasonal cycle and intraseasonal variability (10-60 day) in the in situ observations. In the equatorial IO, however, the seasonal mean NCEP wind speeds can be 2-3 m s(-1) smaller and the amplitude of intraseasonal variability only about half that of QuikSCAT winds. The systematic errors of NCEP zonal winds are comparable to the annual mean or amplitude of the seasonal cycle in the equatorial IO. It is suggested that the systematic error of mean and intraseasonal variability of reanalysis winds is related to systematic error in the NCEP analysis of precipitation.

Torres2003

Spatial patterns of wind and sea surface temperature in the Galician upwelling region
R. Torres and E. D. Barton and P. Miller and E. Fanjul
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  108    (2003)

[1] Cape Finisterre is the most northwest point in the Galician region and separates the meridional west and zonal north coasts of Galicia. The wind field there has high spatial and temporal variability throughout the year. No clear seasonal signal is evident; upwelling and nonupwelling patterns alternate in all seasons. Two outstanding features of the Galician region are persistent upwelling near Cape Finisterre even when not present farther south and alternation of strong upwelling on north and west coasts. Up to now, explanations have relied upon particular dynamics of oceanic flow past Finisterre. We find that major features of upwelling around Finisterre are related to strong spatial structure in the wind field. Analysis of QuikScat wind data for July 1999 to May 2001 shows strong repeatable patterns in the synoptic wind field. These wind patterns emerge as the combination of the two dominant modes in a complex empirical orthogonal function (CEOF) analysis representing over 85\% of the variance. Summer wind patterns give rise to characteristic distributions of upwelling along the coast and favor development of filaments in particular locations. The wind measured at Finisterre itself, often used as a general indicator of upwelling conditions around the Galician coast, is not always representative of the overall wind field. The relevant wind fields allowed a qualitative explanation of temperature structure seen in sea surface temperature images, and of differences in both the upwelling and downwelling regimes between the two years.

Draper2003

An advanced ambiguity selection algorithm for SeaWinds
D. W. Draper and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  41  538--547  (2003)

SeaWinds on QuikSCAT, a spaceborne Ku-band scatterometer, estimates ocean winds via the relationship between the normalized radar backscatter and the vector wind. Scatterometer wind retrieval generates several possible wind vector solutions or ambiguities at each resolution cell, requiring a separate ambiguity selection step to give a unique solution. In processing SeaWinds on QuikSCAT data, the ambiguity selection is ``nudged'' or initialized using numerical weather prediction winds. We describe a sophisticated new ambiguity selection approach developed at Brigham Young University (BYU) that does not require nudging. The BYU method utilizes a low-order data-driven Karhunen-Loeve (KL) wind field model to promote self-consistency. Ambiguity selected winds from the BYU method and standard SeaWinds processing are compared over a set of 102 revs. A manual examination of the data suggests that the nonnudging BYU method selects a more self-consistent wind field in the absence of cyclonic storms. Over a set of cyclonic storm regions, BYU performs better in 9\% of the cases and worse in 20\% of the cases. Overall, the BYU algorithm selects 93\% of the same ambiguities as the standard dataset. This comparison helps validate both nonnudging and nudging techniques and indicates that SeaWinds ambiguity selection can be generally accomplished without nudging.

Spencer2003

High-resolution measurements with a spaceborne pencil-beam scatterometer using combined range/doppler discrimination techniques
M. W. Spencer and W. Y. Tsai and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  41  567--581  (2003)

Conically scanning pencil-beam scatterometer systems, such as the recently launched SeaWinds radar, constitute an important class of instruments for spaceborne climate observation. In addition to ocean winds, scatterometer data are being applied to a wide range of land and cryospheric applications. A key issue for future scatterometer missions is improved spatial resolution. Pencil-beam scatterometers to date have been real-aperture systems where only range discrimination is used, resulting in a relatively coarse resolution of approximately 25 km. In this paper, the addition of Doppler discrimination techniques is proposed to meet the need for higher resolution. Here, the unique issues associated with the simultaneous application of range and Doppler processing to a conically scanning radar are addressed, and expressions for the theoretical measurement performance of such a system are derived. Important differences with side-looking imaging radars, which also may employ Doppler techniques, are highlighted. Conceptual design examples based on scatterometer missions of current interest are provided to illustrate this new high-resolution scatterometer approach. It is shown that spatial resolution of pencil-beam scatterometer systems can be improved by an order of magnitude by utilizing combined range/Doppler discrimination techniques, while maintaining the wide-swath and constant incidence angle neaaaaeded for many geophysical measurements.

Bourassa2003

SeaWinds validation with research vessels
M. A. Bourassa and D. M. Legler and J. J. O'Brien and S. R. Smith
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  108    (2003)

[1] The accuracy of the SeaWinds scatterometer's vector winds is assessed through comparison with research vessel observations. Factors that contribute to uncertainty in scatterometer winds are isolated and examined as functions of wind speed. For SeaWinds on QuikSCAT, ambiguity selection is found to be near perfect for surface wind speed (w) > 8 m s(-1); however, ambiguity selection errors cause directional uncertainty to exceed 20 for w < ?5 m s(-1). These average uncertainties for wind speed and direction are found to be 0.45 m s(-1) and 5$^\circ$ for the QSCAT-1 model function and 0.3 m s(-1) and 3$^\circ$ for the Ku-2000 model function. The QuikSCAT winds are examined as vectors through two new approaches. The first is a method for determining vector correlations that considers uncertainty in the comparison data set. The second approach is a wind speed-dependent model for the uncertainty in the magnitude of vector errors. For the QSCAT-1 (Ku-2000) model function this approach shows ambiguity selection dominates uncertainty for 2.5 < w < 5.5 m s(-1) (0.6 < w < 5.5 m s(-1)), uncertainty in wind speed dominates for w < 2.5 m s(-1) and 5.5 < w < 7.5 m s(-1) (w < 0.6 m s(-1) and 5.5 < w < 18 m s(-1)), and uncertainty in wind direction (for correctly selected ambiguities) dominates for w > 7.5 m s(-1) (w > 18 m s(-1)). This approach also shows that spatial variability in the wind direction, related to inexact spatial co-location, is likely to dominate rms differences between scatterometer wind vectors and in situ comparison measurements for w > 4.5 m s(-1). The techniques used herein are applicable to any validation effort with uncertainty in the comparison data set or with inexact co-location.

Lin2003

Satellite observations of modulation of surface winds by typhoon-induced upper ocean cooling
I. I. Lin and W. T. Liu and C. C. Wu and J. C. H. Chiang and C. H. Sui
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] Two remote sensing data sets, the Tropical Rainfall Measurement Mission Sea Surface Temperature (SST) and the NASA QuikSCAT ocean surface wind vectors, are analysed to study ocean-atmosphere interactions in cold SST regions formed in the trail of two typhoon events. Anomalously cold SST patches up to 6degreesC below the surrounding warm tropical ocean SST are found along the trail of typhoon tracks as cold, deep waters are entrained up to the mixed layer due to typhoon forcing. In both typhoon events, significant and systematic weakening of surface wind speed is found over cold SST patches relative to surface wind speed in surrounding regions. The wind speed anomalies disappear as the patches recover to the level of the surrounding SST. The results are consistent with the mechanism proposed by Wallace et al. that surface winds are modulated by SST via stability. As wind within the well-mixed boundary layer moves over the cold patch, boundary layer stability increases, vertical mixing is suppressed, and the vertical wind shear increases; reduction in surface wind speed is caused. In particular, our result shows that this mechanism can act on relatively small spatial (approximate to 100 km) and short (approximate to 1 day) time scales.

Gille2003

Measuring the sea breeze from QuikSCAT scatterometry
S. T. Gille and S. G. L. Smith and S. M. Lee
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

[1] Differences between morning and evening winds from QuikSCAT scatterometer measurements are analyzed to diagnose the diurnal variability of the wind over the ocean. A statistically significant signal, associated with the sea breeze, is present along most of the world's coastlines. Significant diurnal variability is also present mid-ocean in the easterly trade wind belts.

Carr2003

CO2 exchange coefficients from remotely sensed wind speed measurements: SSM/I versus QuikSCAT in 2000 (vol 29, DOI no. 10.1029/2002GL015068, 2002)
M. E. Carr and W. Q. Tang and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

Wu2003

Design and calibration of the SeaWinds scatterometer
C. L. Wu and Y. Liu and K. H. Kellogg and K. S. Pak and R. L. Glenister
IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS  39  94--109  (2003)

The SeaWinds Scatterometer is a Ku-band Earth orbiting remote sensing radar. It has a! 1 m dish antenna shared by two beams with respective nadir look angles of 40 and 46 deg, scanning azimuthally to provide greater than 90\% daffy coverage of the Earth at an altitude of 800 km. The first sensor was launched in 1999 and produces sea surface wind field to 2 m/s accuracy at 25 km resolution. The design and calibration of the SeaWinds radar is described here.

Chao2003

A high-resolution surface vector wind product for coastal oceans: Blending satellite scatterometer measurements with regional mesoscale atmospheric model simulations
Y. Chao and Z. J. Li and J. C. Kindle and J. D. Paduan and F. P. Chavez
GEOPHYSICAL RESEARCH LETTERS  30    (2003)

A 2-dimensional variational method is used to blend the satellite scatterometer measured (QuikSCAT) and regional mesoscale atmospheric model simulated (COAMPS) surface vector winds for coastal central California. The approach is distinct from existing methods in that it considers errors from both measurements and models. When compared with independent in situ observations, the blended wind product shows consistently higher correlation and smaller RMS errors than QuikSCAT or COAMPS winds. The proposed algorithm can be implemented over any part of the world ocean. It should be a valuable tool for describing small-scale atmospheric processes in coastal zones and for forcing high-resolution coastal ocean models.

Goswami2003

Indian Ocean surface winds from NCMRWF analysis as compared to QuikSCAT and moored buoy winds
B. N. Goswami and E. N. Rajagopal
PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-EARTH AND PLANETARY SCIENCES  112  61--77  (2003)

The quality of the surface wind analysis at the National Centre for Medium Range Weather Forecasts (NCMRWF), New Delhi over the tropical Indian Ocean and its improvement, in 2001 are examined by comparing it with in situ buoy measurements and satellite derived surface winds from NASA QuikSCAT satellite (QSCT) during 1999, 2000 and 2001. The NCMRWF surface winds suffered from easterly bias of 1.0-1.5 ms(-1) in the equatorial Indian Ocean (IO) and northerly bias of 2.0-3.0 ms(-1) in the south equatorial IO during 1999 and 2000 compared to QSCT winds. The amplitude of daily variability was also underestimated compared to that in QSCT. In particular, the amplitude of daily variability of NCMRWF winds in the eastern equatorial IO was only about 60\% of that of QSCT during 1999 and 2000. The NCMRWF surface winds during 2001 have significantly improved with the bias of the mean analyzed winds considerably reduced everywhere bringing it to within 0.5 ms(-1) of QSCT winds in the equatorial IO. The amplitude and phase of daily and intraseasonal variability are very close to that in QSCT almost everywhere during 2001. It is shown that the weakness in the surface wind analysis during 1999 and 2000 and its improvement in 2001 are related to the weakness in simulation of precipitation by the forecast model in the equatorial IO and its improvement in 2001.

Chen2002

A dual-frequency approach for retrieving sea surface wind speed from TOPEX altimetry
G. Chen and B. Chapron and R. Ezraty and D. Vandemark
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  107    (2002)

More than a dozen of wind speed (U) algorithms have been proposed during the past 2 decades, as a result of a continuing effort to improve altimeter wind measurement. The progress in terms of accuracy, however, is seen to be rather slow. The reported root mean square (RMS) error of prevailing algorithms varies mostly between 1.6 and 2.0 m/s for the dominant wind regime. As far as the TOPEX altimeter is concerned, three measured quantities, namely, the radar cross sections from Ku and C band (sigma(Ku) and sigma(C)), as well as the significant wave height (H-s), have been used in previous algorithm developments, resulting in a variety of single-, dual-, and three-parameter model functions. On the basis of the finding of a banded dependency of the U-sigma(Ku) relationship on sigma(C) a new approach for retrieving altimeter wind speed, termed linear composite method (LCM), is proposed in this study. The LCM model function appears as a set of sigma(C)-dependent linear relations between U and sigma(Ku). A unique advantage of this approach is that it allows the algorithm to be tuned or expanded for a given range of wind speed without affecting the rest. Over 1.7 million coincident TOPEX/NASA scatterometer (NSCAT) and TOPEX/QuikSCAT data covering a period of 2.5 years are used to adjust the model. Validation against extensive buoy measurements indicates that the LCM algorithm is almost unbiased and has an overall RMS error of 1.56 m/s, which is 12\% lower compared to the algorithm in operational use [Witter and Chelton, 1991]. In addition, a small (2.5-6\%, depending on the reference data set) but significant improvement is found for the LCM when compared to the most recent dual- parameter algorithm [Gourrion et al., 2002].

Boukabara2002

Physically based modeling of QuikSCAT SeaWinds passive microwave measurements for rain detection
S. A. Boukabara and R. N. Hoffman and C. Grassotti and S. M. Leidner
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES  107    (2002)

[1] We present a method for detecting rain-contaminated wind vector cells in QuikSCAT SeaWinds scatterometer observations. This rain detection method uses passive measurements of microwave brightness temperature obtained as a signal processing by-product from the standard SeaWinds active scatterometer measurements. The rain flag is developed theoretically first by calibrating the SeaWinds brightness temperatures using Special Sensor Microwave Imager (SSM/I) observations and then by using physically based simulations including the effects of both rain and ice precipitation. Rain retrievals are validated by comparison to SSM/I-observed rain rates and to other independently produced SeaWinds rain flags and produce rain maps that agree well with the SSM/I estimates. The rain detection method may be used to complement existing rain flags in the current operational QuikSCAT data product. In addition, an atmospheric correction algorithm was developed to dynamically adjust the backscatter coefficient measurements for variations in water vapor and cloud liquid water; results are not significantly different from the climatological correction currently implemented.

Draper2002

An assessment of SeaWinds on QuikSCAT wind retrieval
D. W. Draper and D. G. Long
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  107    (2002)

[1] The scatterometer ocean wind retrieval process produces several possible solutions or ambiguities at each point, requiring a separate ambiguity selection step to infer a unique wind vector field. An ambiguity selection error occurs when the selected wind vector is not the closest ambiguity to the true wind. The current ambiguity selection routine for SeaWinds is ad hoc, but performs well under most circumstances. Factors such as instrument noise and rain can also cause the estimated wind flow to deviate from the true wind. A quality assurance (QA) analysis is performed to assess the ambiguity selection effectiveness and noise level of the retrieved wind using a low-order wind field model. The wind field model is data-driven and shown to be rather insensitive to the training data set. The QA analysis demonstrates that the SeaWinds ambiguity selection process is at least 95\% effective. Ambiguity selection errors are correlated with storms and rain corruption. A subjective analysis on a set of cyclonic storm passes confirms that the wind retrieval is somewhat less effective in storm regions.

Grodsky2003

The intertropical convergence zone in the South Atlantic and the equatorial cold tongue
S. A. Grodsky and J. A. Carton
JOURNAL OF CLIMATE  16  723--733  (2003)

Recent observations from the QuikSCAT and Tropical Rainfall Measuring Mission satellites, as well as a longer record of Special Sensor Microwave Imager winds are used to investigate the existence and dynamics of a Southern Hemisphere partner to the intertropical convergence zone in the tropical Atlantic Ocean. The southern intertropical convergence zone extends eastward from the coast of Brazil in the latitude band 10degrees-3degreesS and is associated with seasonal precipitation exceeding 6 cm month(-1) during peak months over a part of the ocean characterized by high surface salinity. It appears in austral winter when cool equatorial upwelling causes an anomalous northeastward pressure gradient to develop in the planetary boundary layer close to the equator. The result is a zonal band of surface wind convergence that exceeds 10(-6) s(-1), with rainfall stronger than 2 mm day(-1), and an associated decrease in ocean surface salinity of 0.2 parts per thousand.

Kelly2002

Scatterometer winds explain damped Rossby waves
K. A. Kelly and L. A. Thompson
GEOPHYSICAL RESEARCH LETTERS  29    (2002)

[1] Westward propagating waves in the North Pacific Ocean from 10-16degreesN are overwhelmed by a zonally coherent response at the annual period, as observed in sea surface height (SSH) anomalies from the TOPEX/ POSEIDON altimeter. SSH from a simple model of wind- forced Rossby waves and from seasonal heating are compared with observed SSH to understand the processes responsible for the observed signal. The seasonal heating cycle is out- of- phase and too weak to explain the SSH. The oceanic response to wind stress curl forcing more closely resembles the observations, but the response to NCEP Reanalysis winds does not show a strong annual cycle. Wind stress curl from the QuikSCAT/ SeaWinds scatterometer has a strong and zonally coherent annual cycle that produces a corresonding strong annual signal in SSH. The model forced by scatterometer winds demonstrates that the response to Ekman pumping is the source of the strong annual cycle in the SSH.

Sura2003

Stochastic analysis of Southern and Pacific Ocean sea surface winds
P. Sura
JOURNAL OF THE ATMOSPHERIC SCIENCES  60  654--666  (2003)

This paper shows that the synoptic variability of zonal and meridional midlatitude Pacific and Southern Ocean sea surface winds can be well described by a univariate stochastic dynamical system directly derived from data. The method used to analyze blended Quick Scatterometer (QuikSCAT)-NCEP winds is a general method to estimate drift and diffusion coefficients of a continuous stationary Markovian system. Almost trivially, the deterministic part consists of a simple, nearly linear damping term. More importantly, the stochastic part appears to be a state-dependent white noise term, that is, multiplicative noise. The need for a multiplicative noise term to describe the variability of midlatitude winds can be interpreted by the fact that the variability of midlatitude winds increases with increasing wind speed. The results indicate that a complete stochastic description of midlatitude winds requires a state-dependent white noise term, that is, multiplicative noise. A simple Ornstein-Uhlenbeck process is not sufficient to describe the wind data within a stochastic framework. The method used fails for tropical regions, suggesting that tropical variability might be non-Markovian.

Portabella2002

Characterization of residual information for SeaWinds quality control
M. Portabella and A. Stoffelen
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  2747--2759  (2002)

Recent work has shown the important properties of the wind inversion residual or maximum-likelihood estimator (MLE) for quality Control (QC) of QuikSCAT Hierarchichal Data Format (HDF) observations. Since March 2000, the QuikSCAT near-real-time (NRT) Binary Universal Format Representation (BUFR) product is available. As this product is used for numerical weather prediction (NWP) assimilation purposes, a QC procedure for the BUFR product is needed. We study the behavior of the MLE in order to determine whether the HDF QC procedure is appropriate for BUFR data. A comparison using real HDF and BUFR data reveals that the MLE distributions of HDF and BUFR differ and are actually poorly correlated. One important difference between BUFR and HDF is the amount of signal averaging prior to wind inversion. The averaging reduces the number of observations used in the wind retrieval for the BUFR product as compared to HDF. We show with a simple example that different MLE distributions are indeed expected due to this averaging. We also run a simulation in order to link theory and reality and better understand the behavior of the MLE. Despite the different MLE behavior in BUFR and HDF, the quality of the retrieved winds, as compared with the European Centre for Medium-Range Weather Forecasts winds, is very similar. We develop an MLE-based QC procedure for BUFR, similarly to the one in HDF, and we compare both. The skill of the QC in BUFR is again very similar to the one in HDF, showing that despite the different MLE behavior in both formats, the properties of the MLE as a QC indicator remain very similar.

Liu2002

Double intertropical convergence zones - a new look using scatterometer
W. T. Liu and X. S. Xie
GEOPHYSICAL RESEARCH LETTERS  29    (2002)

The high-resolution wind vectors observed by the space-based scatterometer QuikSCAT, from 1999 to 2002, show that the double intertropical convergence zones (ITCZ) exist in the Atlantic and the eastern Pacific oceans for most of the annual cycle, and are far more extensive than previously recognized. For most of the time, the southern ITCZ is weaker than the northern one. The stronger ITCZ occurs when the northerly trade winds meet the southerly trade winds over warm water, resulting in deep convection. The weaker ITCZ over cooler water is caused by the deceleration of the surface winds as they approach the cold upwelling water near the equator. Decreases in vertical mixing and increases in vertical wind shear in the atmospheric boundary layer are suggested to be the causes of the deceleration of the trade winds as they move from warmer to colder water.

Hu2002

QuikSCAT reveals the surface circulation of the Catalina Eddy
H. Hu and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  29    (2002)

[1] The Catalina Eddy, a small, recurring cyclonic vortex in the ocean off Los Angeles, is of keen interest to local weather forecasters because of the moderating oceanic effect it brings to the city. Its small size and shallow vertical extent have made it difficult to monitor and predict using conventional data. The microwave scatterometer on the QuikSCAT spacecraft has generated high-resolution surface wind vectors that provide the first visualization of the complete cyclonic flow of the eddy. Moreover, the superior performance of the QuikSCAT scatterometer demonstrates the relative inaccuracy and inconsistency of predictions of the eddy based upon numerical weather prediction models.

Pan2002

Observation of western boundary current atmospheric convergence zones using scatterometer winds
J. Y. Pan and X. H. Yan and Q. N. Zheng and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  29    (2002)

[1] A merged scatterometer wind data set from ERS-1/2, NSCAT, and QuikSCAT missions was used to observe the atmospheric convergence zones (ACZs) caused by the western boundary currents, the Gulf Stream and Kuroshio. The long-term means of the atmospheric convergence show the ACZs' spatial features, which are related to the precipitation patterns in these regions. Seasonal images of the ACZs were produced to show annual cycles of the ACZs, indicating that the intensities of the ACZs over these two regions strengthen in winter and weaken in summer. Furthermore, we calculated the total convergence over the Gulf Stream and Kuroshio ACZs, which is defined as the integral of the convergence over the ACZs. The interannual variability of the total convergence was extracted by using a multi-stage filter, revealing that in response to strong El Nino events in 1991-92 and 1997-98, the total convergence reached maxima.

Thompson2002

Orbit determination for the QuikSCAT spacecraft
B. F. Thompson and M. C. Meek and K. L. Gold and P. Axelrad and G. H. Born and D. G. Kubitschek
JOURNAL OF SPACECRAFT AND ROCKETS  39  852--858  (2002)

An operational orbit determination system for QuikSCAT has been developed to meet the requirement for 100-m (3sigma) positioning knowledge. This is nominally accomplished by processing global positioning system (GPS) position solutions in a dynamic filter. The operational orbit determination system produced 24-h overlapping arc position errors between 15 and 25 m (root-sum-square) and 3-h arc overlaps between 5 and 6 m (root-sum-square) for seven-day and one-day arcs, respectively. We also investigated the use of short segments of GPS pseudorange and carrier phase data and obtained results that differ by less than 10 m from the nominal orbit solutions. A third investigation considered the feasibility of a backup orbit determination system using antenna azimuth and elevation angles from three ground tracking stations. The methods and results of processing these three data types are presented.

Pan2003

Interpretation of scatterometer ocean surface wind vector EOFs over the Northwestern Pacific
J. Y. Pan and X. H. Yan and Q. N. Zheng and W. T. Liu and V. V. Klemas
REMOTE SENSING OF ENVIRONMENT  84  53--68  (2003)

Satellite scatterometer winds over the northwestern Pacific were analyzed with the vector empirical orthogonal function (VEOF) method. The Hilbert-Huang transform (HHT), a newly developed non-linear and non-stationary time series data processing method, was also employed in the analysis. A combination of European Remote Sensing Satellite (ERS) - 1/2 scatterometer, NASA Scatterometer (NSCAT) and NASA's Quick Scatterometer (QuikSCAT) winds covering the period from January 1992 to April 2000 and the area of 0-50degreesN, 100-148degreesE constitutes the baseline for this study. The results indicate that annual cycles dominate the two leading VEOF modes. The first VEOF shows the East Asian monsoon features and the second represents a spring-autumn oscillation. We removed the annual signal from the data set and calculated the interannual VEOFs. The first interannual VEOF represents the interannual variability existing in the spring-autumn oscillation. The temporal mode is correlated with the Southern Oscillation Index (Sol), but has a half-year lag with respect to the SOI. The spatial mode of the first interannual VEOF reflects the response of the tropical and extratropical winds to ENSO events. The second interannual VEOF is another ENSO related mode, and the temporal VEOF mode is correlated with the SOI with a correlation coefficient of 0.78, revealing the wind variability over mid-latitudes, which is associated with ENSO events. Further analysis indicated that the wind variability over the coast of East Asia represents anomalies of a Hadley cell. The quasi-biennial oscillation (QBO) was found in the temporal mode, indicating and verifying that the QBO in the wind fields is related to ENSO events. The third VEOF shows the interannaul variability in the winter-summer mode and displays the interannual variability of the East Asian monsoon. The three leading interannual VEOFs are statistically meaningful as confirmed by a significance test. (C) 2002 Elsevier Science Inc. All rights reserved.

Gourrion2002

A two-parameter wind speed algorithm for Ku-band altimeters
J. Gourrion and D. Vandemark and S. Bailey and B. Chapron and G. P. Gommenginger and P. G. Challenor and M. A. Srokosz
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  19  2030--2048  (2002)

Globally distributed crossovers of altimeter and scatterometer observations clearly demonstrate that ocean altimeter backscatter correlates with both the near-surface wind speed and the sea state. Satellite data from TOPEX/Poseidon and NSCAT are used to develop an empirical altimeter wind speed model that attenuates the sea-state signature and improves upon the present operational altimeter wind model. The inversion is defined using a multilayer perceptron neural network with altimeter-derived backscatter and significant wave height as inputs. Comparisons between this new model and past single input routines indicates that the rms wind error is reduced by 10\%-15\% in tandem with the lowering of wind error residuals dependent on the sea state. Both model intercomparison and validation of the new routine are detailed, including the use of large independent data compilations that include the SeaWinds and ERS scatterometers, ECMWF wind fields, and buoy measurements. The model provides consistent improvement against these varied sources with a wind-independent bias below 0.3 m s(-1). The continuous form of the defined function, along with the global data used in its derivation, suggest an algorithm suitable for operational application to Ku-band altimeters. Further model improvement through wave height inclusion is limited due to an inherent multivaluedness between any single realization of the altimeter measurement pair [sigma(o), H-s] and observed near-surface winds. This ambiguity indicates that H-s is a limited proxy for variable gravity wave properties that impact upon altimeter backscatter.

Ebuchi2002

Evaluation of wind vectors observed by QuikSCAT/SeaWinds using ocean buoy data
N. Ebuchi and H. C. Graber and M. J. Caruso
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  19  2049--2062  (2002)

Wind vectors observed by the QuikSCAT/SeaWinds satellite mission are validated by comparing with wind and wave data from ocean buoys. Effects of oceanographic and atmospheric environment on scatterometer measurements are also assessed using the buoy data. Three versions of QuikSCAT/SeaWinds wind data were collocated with buoy observations operated by the National Data Buoy Center (NDBC), Tropical Atmosphere Ocean (TAO), and Pilot Research Moored Array in the Tropical Atlantic (PIRATA) projects, and the Japan Meteorological Agency (JMA). Only buoys located offshore and in deep water were analyzed. The temporal and spatial differences between the QuikSCAT/SeaWinds and buoy observations were limited to less than 30 min and 25 km. The buoy wind speeds were converted to equivalent neutral winds at a height of 10 m above the sea surface. The comparisons show that the wind speeds and directions observed by QuikSCAT/SeaWinds agree well with the buoy data. The root-mean-squared differences of the wind speed and direction for the standard wind data products are 1.01 m s(-1) and 23degrees, respectively, while no significant dependencies on the wind speed or cross-track cell location are discernible. In addition, the dependencies of wind speed residuals on oceanographic and atmospheric parameters observed by buoys are examined using the collocated data. A weak positive correlation of the wind speed residuals with the significant wave height is found, while dependencies on the sea surface temperature or atmospheric stability are not physically significant.

Stiles2002

Impact of rain on spaceborne Ku-band wind scatterometer data
B. W. Stiles and S. H. Yueh
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  1973--1983  (2002)

The accuracy of Ku-band ocean wind scatterometers (i.e., NSCAT and SeaWinds) is impacted to varying degrees by rain. In order to determine how to best flag rain-contaminated wind vector cells and ultimately to calibrate out the effects of rain as much as possible, we must understand the impact of rain on the backscatter measurements that are used to retrieve wind vectors. This study uses collocated SSM/I rain rate measurements, NCEP wind fields, and SeaWinds on QuikSCAT backscatter measurements to empirically fit a simple theoretical model of the effect of rain on sigma(o), and to check the validity of that model. The chief findings of the study are 1) horizontal polarization measurements are more sensitive to rain than vertical polarization, 2) sensitivity to rain varies dramatically with wind speed, and 3) the additional backscatter due to rain overshadows the rain-related attenuation.

Carr2002

CO2 exchange coefficients from remotely sensed wind speed measurements: SSM/I versus QuikSCAT in 2000
M. E. Carr and W. Q. Tang and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  29    (2002)

We compare here the air-sea exchange coefficient for CO2 estimated with monthly mean wind speed measured by the Special Sensing Microwave Imager (SSM/I), K-S, and by the scatterometer QuikSCAT, K-Q, for the year 2000. K-S and K-Q present the same patterns, although are larger than in similar to65\% of the world ocean. Zonal mean K-S are consistently larger, except similar to50degreesS and north of 10degreesS in the Indian Ocean. Global oceanic uptake, F-Q, estimated using K-Q and climatological Deltap(CO2) ranges from 0.43 (July) to 2.6 Gt C y(-1) (December). The global sink estimated from SSM/I is similar to10\% larger than F-Q for most of the year. This comparison supports the use of SSM/I to quantify interannual variability of the global exchange coefficient of CO2.

Meissner2002

An updated analysis of the ocean surface wind direction signal in passive microwave brightness temperatures
T. Meissner and F. Wentz
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  1230--1240  (2002)

We analyze the wind direction signal for vertically (v) and horizontally (h) polarized microwave radiation at 37 GHz, 19 GHz, and 11 GHz and an Earth incidence angle of 53degrees. We use brightness temperatures from SSM/I and TMI and wind vectors from buoys and the QUIKSCAT scatterometer. The wind vectors are space and time collocated with the radiometer measurements. Water vapor, cloud water and sea surface temperature are obtained from independent measurements and are uncorrelated with the wind direction. We find a wind direction signal that is noticeably smaller at low and moderate wind speeds than a previous analysis had indicated. We attribute the discrepancy to errors in the atmospheric parameters that were present in data set of the earlier study. We show that the polarization combination 2v - h is almost insensitive to atmospheric changes and agrees with the earlier results. The strength of our new signals agrees well with recent JPL aircraft radiometer measurements. It is significantly smaller than the prediction of the two-scale sea surface emission model for low and intermediate wind speeds.

Zhao2002

Validation of sea ice motion from QuikSCAT with those from SSM/I and buoy
Y. H. Zhao and A. K. Liu and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  1241--1246  (2002)

Arctic sea ice motion for the period from October 1999 to March 2000 derived from QuikSCAT and special sensor microwave/imager (SSM/I) data using the wavelet analysis method agrees well with ocean buoy observations. Results from QuikSCAT and SSM/I are compatible when compared with buoy observations and complement each other. Sea ice drift merged from daily results from QuikSCAT, SSM/I, and buoy data gives more complete coverage of sea ice motion. Based on observations of six months of sea ice motion maps, the sea ice motion maps in the Arctic derived from QuikSCAT data appear to have smoother (less noisy) patterns than those from NSCAT, especially in boundary areas, possibly due to constant radar scanning incidence angle. For late summer, QuikSCAT data can provide good sea ice motion information in the Arctic as early as the beginning of September. For early summer, QuikSCAT can provide at least partial sea ice motion information until mid-June. In the Antarctic, a case study shows that sea ice motion derived from QuikSCAT data is consistent with pressure field contours.

Portabella2002

A comparison of KNMI quality control and JPL rain flag for SeaWinds
M. Portabella and A. Stoffelen
CANADIAN JOURNAL OF REMOTE SENSING  28  424--430  (2002)

In the past few years, scatterometer winds have been successfully assimilated in weather analysis. A good assessment of the information content of these winds is particularly important for such activities. Besides retrieval problems in cases of a confused sea state, a particularly acute problem of Ku-band scatterometry is the sensitivity to rain. Elimination of poor-quality data is therefore a prerequisite for the successful use of the new National Aeronautics and Space Administration (NASA) scatterometer, QuikSCAT. This issue has been the topic of recent work. On the one hand, the Royal Dutch Meteorological Institute (KNMI) has developed a quality-control (QC) procedure that detects and rejects the poor-quality QuikSCAT data (including rain contamination). On the other hand, the Jet Propulsion Laboratory (JPL) has developed a ``rain flag'' for QuikSCAT. In this paper, we test the KNMI QC against the JPL rain flag to improve QC for QuikSCAT. Collocations with the European Centre for Medium-range Weather Forecasts (ECMWF) winds and special sensor microwave imager (SSM/I) rain data are used for validation purposes. The results show that the KNMI QC is more efficient in rejecting poor-quality data than the JPL rain flag, whereas the latter is more efficient in rejecting rain-contaminated data than the former. The JPL rain flag, however, rejects too much of the consistent wind data in dynamically active areas. The KNMI QC is a good QC procedure in the parts of the swath where the wind retrieval ability of QuikSCAT is high. In the nadir region, however, the KNMI QC efficiency and the wind retrieval skill are relatively low. In the nadir region, the KNMI QC needs additional information from the JPL rain flag to reject rain-contaminated data.

Sharp2002

Early detection of tropical cyclones using seawinds-derived vorticity
R. J. Sharp and M. A. Bourassa and J. J. O'Brien
BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY  83  879--889  (2002)

Yueh2002

Polarimetric radar remote sensing of ocean surface wind
S. H. Yueh and W. J. Wilson and S. Dinardo
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  793--800  (2002)

Experimental data are presented to support the development of a new concept for ocean wind velocity measurement (speed and direction) with the polarimetric microwave radar technology. This new concept has strong potential for improving the wind direction accuracy and extending the useful swath width by up to 30\% for follow-on NASA spaceborne scatterometer mission to SeaWinds series. The key issue is whether there is a relationship between the polarization state of ocean backscatter and surface wind velocity at NASA scatterometer frequencies (13 GHz). An airborne Ku-band polarimetric scatterometer (POLSCAT) was developed for proof-of-concept measurements. A set of aircraft flights indicated repeatable wind direction signals in the POLSCAT observations of sea surfaces at 9-11 m/s wind speed. The correlation coefficients between co- and cross-polarized radar response of ocean surfaces have a peak-to-peak amplitude of about 0.4 and are shown to have an odd-symmetry with respect to the wind direction, unlike the normalized radar cross sections.

Katsaros2002

Microwave remote sensing of tropical cyclones from space
K. B. Katsaros and P. W. Vachon and W. T. Liu and P. G. Black
JOURNAL OF OCEANOGRAPHY  58  137--151  (2002)

This article reviews several microwave instruments employed in research and analysis of tropical cyclones (TCs), typhoons, and hurricanes. The instruments discussed include scatterometers, microwave radiometers, synthetic aperture radars (SARs), and rain radar from space. Examples of the particular contribution by one or more of these instruments in analysis of several storms illustrate the comprehensive new views provided by the SeaWinds scatterometers, the detailed high-resolution wind field provided by RADARSAT-1 SAR, particularly inside and in the vicinity of hurricane ``eyes,'' and the presence of secondary flows in the region between rainbands in TCs. The high spatial resolution of precipitation data from the Tropical Rainfall Measuring Mission's rain radar, combined with scatterometer or SAR data, give a significant improvement in the details that can be seen from space, at the surface, and in the precipitating areas of TCs. The microwave instruments provide a penetrating view below the upper level cirrus clouds.

Nghiem2001

Detection of snowmelt regions on the Greenland ice sheet using diurnal backscatter change
S. V. Nghiem and K. Steffen and R. Kwok and W. Y. Tsai
JOURNAL OF GLACIOLOGY  47  539--547  (2001)

Snowmelt regions on Greenland ice are mapped daily with the SeaWinds wideswath Ku-band (13.4 GHz) scatterometer on the QuikSCAT satellite. The approach exploits the high temporal resolution of SeaWinds/QuikSCAT data for the melt mapping using diurnal backscatter change independent of the absolute calibration. The results reveal several pronounced melting and refreezing events, and effects of topography are evident in the melt patterns. The spatial resolution is sufficient to identify melt features on the Sukkertoppen Iskappe west of the main ice sheet. An anomalous warming event, caused by downward mixing of warm air, is detected in late September 1999 over the west flank of the southern Greenland ice sheet. Time-series images of melt regions are presented over the period from summer to the fall freeze-up. The satellite observations are verified with in situ measurements from the Greenland Climate Network stations.

Weissman2002

Effects of rain rate and wind magnitude on SeaWinds scatterometer wind speed errors
D. E. Weissman and M. A. Bourassa and J. Tongue
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  19  738--746  (2002)

Rain within the footprint of the SeaWinds scatterometer on the QuikSCAT satellite causes more significant errors than existed with its predecessor, the NASA scatterometer (NSCAT) on Advanced Earth Observing Satellite-I (ADEOS-I). Empirical relations are developed that show how the rain-induced errors in the scatterometer wind magnitude depend on both the rain rate and on the wind magnitude. These relations are developed with collocated National Data Buoy Center (NDBC) buoy measurements (to provide accurate sea surface winds) and simultaneous Next Generation Weather Radar (NEXRAD) observations of rain reflectivity. An analysis, based on electromagnetic scattering theory, interprets the dependence of the scatterometer wind errors on volumetric rain rate over a range of wind and rain conditions. These results demonstrate that the satellite scatterometer responds to rain in a manner similar to that of meteorological radars, with a Z-R relationship. These observations and results indicate that the combined (wind and rain) normalized radar cross section will lead to erroneously large wind estimates when the rain-related radar cross section exceeds a particular level that depends on the rain rate and surface wind speed.

Tran2002

The dependence of Nadir ocean surface emissivity on wind vector as measured with microwave radiometer
N. Tran and D. Vandemark and C. S. Ruf and B. Chapron
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  515--523  (2002)

Global brightness temperature observations of TOPEX/Poseidon microwave radiometer (TMR) at 18, 21, and 37 GHz have been collocated with near-simultaneous SeaWinds wind vector data as well as with monthly sea surface temperature and salinity products. The combined data allow us to study the dependence of zenith-directed ocean surface emissivity, at each frequency, upon both wind speed and direction. Results show a clear two-branch wind speed dependence; weak and linear below 7 m . s(-1) with an increase in sensitivity above that point. The observed emissivity also depends on the angle between the wind direction and TMR's antenna polarization orientation, providing satellite confirmation of aircraft-derived results. There is little change in these wind vector dependencies with frequency.

Yeh2002

Kinematic characteristics of a mei-yu front detected by the QuikSCAT oceanic winds
H. C. Yeh and G. T. J. Chen and W. T. Liu
MONTHLY WEATHER REVIEW  130  700--711  (2002)

Based on conventional surface observations and NASA Quick Scatterometer (QuikSCAT) data, a heavy rainfall event that occurred in the Taiwan mei-yu season was chosen to further study the kinematic characteristics of the accompanying surface front. With the help of the QuikSCAT oceanic surface winds, it was found that the location and propagation of a mei-yu front over the ocean to the east of Taiwan during 10-12 June 2000 are better represented by the frontal wind shift line, which was located approximately on the leading edge of the baroclinic zone. The mesoscale system with cyclonic circulation embedded within the frontal zone was clearly shown in the wind field and kinematic parameters (horizontal divergence and vorticity) as well as satellite clouds and rainfall estimations. The conditional instability of the second kind (CISK) process was suggested to be responsible for the intensification of the mei-yu front and the frontal disturbance over the ocean. Under the influence of island topography over Taiwan and the mountains over southern China, the frontal wind shift line distorted and receded within the Taiwan Strait after the mei-yu front reached northern Taiwan. In the later period, the front moved southward faster to the area immediately east of Taiwan as compared to that over southwestern Taiwan and the Taiwan Strait due to earlier arrival of strong postfrontal northerly winds. The localized southwesterly jetlike feature was observed to the southeast of Taiwan and persisted until the arrival of the strong northerly winds. These localized phenomena were detected by the QuikSCAT oceanic winds.

Drinkwater2001

Greenland snow accumulation estimates from satellite radar scatterometer data
M. R. Drinkwater and D. G. Long and A. W. Bingham
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES  106  33935--33950  (2001)

Data collected by the C band ERS-2 wind scatterometer (EScat), the Ku band ADEOS-1 NASA scatterometer (NSCAT), and the Ku band SeaWinds on QuikScat (QSCAT) satellite instruments are used to illustrate spatiotemporal variability in snow accumulation on the Greenland ice sheet. Microwave radar backscatter images of Greenland are derived using the scatterometer image reconstruction (SIR) method at 3-day intervals over the periods 1991-1998 and 1996-1997 for EScat and NSCAT, respectively. The backscatter coefficient sigmadegrees normalized to 40degrees incidence, A, and gradient in backscatter, B, in the range 20degrees-60degrees are compared with historical snow accumulation data and recent measurements made in the Program for Arctic Regional Climate Assessment (PARCA) shallow snow pits. Empirical relationships derived from these comparisons reveal different exponential relationships between C and Ku band A values and dry snow zone mean annual accumulation, Q. Frequency difference images between overlapping scatterometer images suggest that C band data are more sensitive to snow layering and buried inhomogeneities, whereas Ku band data are more sensitive to volume scattering from recently accumulated snow. Direct comparisons between NSCAT B values and in situ Q measurements show a linear relationship between In (Q) and B, with a negative rank correlation of R = -0.8. The root-mean-square residual in fitting regression line equation In (Q) = 3.08 - 17.83B to the data is 0.05-m snow water equivalent. This simple Ku band empirical relationship is exploited to investigate decadal changes in dry snow zone accumulation between Seasat (1978) and NSCAT (1996). Additional comparisons between NSCAT and recent QSCAT (1999) data reveal significant upslope shifts in the dry snow line along the southwestern flank of the ice sheet. Recent acceleration in the increase in intensity of scattering is observed in the percolation zone, suggesting increased melting between 2000- and 3000-m elevation in the southern half of the ice sheet.

Stiles2002

Direction interval retrieval with thresholded nudging: A method for improving the accuracy of QuikSCAT winds
B. W. Stiles and B. D. Pollard and R. S. Dunbar
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  40  79--89  (2002)

The SeaWinds scatterometer was developed by NASA JPL, Pasadena, CA, to measure the speed and direction of ocean surface winds. It was then launched onboard the QuikSCAT spacecraft. The accuracy of the majority of the swath and the size of the swath are such that the SeaWinds on QuikSCAT Mission (QSCAT) meets its science requirements despite shortcomings at certain cross-track positions. Nonetheless, it is desirable to modify the baseline processing in order to improve the quality of the less accurate portions of the swath, in particular near the far swath and nadir. Two disparate problems have been identified for these regions. At far swath, ambiguity removal skill is degraded due to the absence of inner beam measurements, limited azimuth diversity and boundary effects. Near nadir, due to nonoptimal measurement geometry, (measurement azimuths approximately 180degrees apart) there is a marked decrease in directional accuracy even when ambiguity removal works correctly. Two algorithms have been developed: direction interval retrieval (DIR) to address the nadir performance issue and thresholded nudging (TN) to improve ambiguity removal at far swath. We illustrate the impact of the two techniques by exhibiting prelaunch simulation results and postlaunch statistical performance metrics with respect to ECMWF wind fields and buoy data.

Patoux2002

A gradient wind correction for surface pressure fields retrieved from scatterometer winds
J. Patoux and R. A. Brown
JOURNAL OF APPLIED METEOROLOGY  41  133--143  (2002)

Given a field of geostrophic winds and at least one pressure observation, a pressure field can be computed. If the winds are in reality gradient winds, then a correction must be applied to calculate the actual geostrophic winds. Here a method is proposed for including a gradient wind correction in the retrieval of geostrophic winds from Quick Scatterometer (QuikSCAT) surface measurements with a planetary boundary layer model. This correction translates into a better estimate of the corresponding surface pressure fields. The scheme is assessed by comparing these pressure fields to buoy measurements in the Gulf of Alaska and to radiosonde measurements in Hurricane Floyd. The gradient wind correction has a curvature component and a time-dependent component. Their relative magnitude is evaluated.

Yueh2001

QuikSCAT geophysical model function for tropical cyclones and application to hurricane floyd
S. H. Yueh and B. W. Stiles and W. Y. Tsai and H. Hu and W. T. Liu
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  39  2601--2612  (2001)

The QuikSCAT radar measurements of several tropical cyclones in 1999 have been studied to develop the geophysical model function (GMF) of Ku-band radar sigma(0)s for extreme high wind conditions. To account for the effects of precipitation, we analyze the co-located rain rates from the Special Sensor Microwave/Imager (SSM/I) and propose the rain rate as a parameter of the GMF The analysis indicates the deficiency of the NSCAT2 GMF developed for the NASA scatterometer, which overestimates the ocean sigma(0) for tropical cyclones and ignores the influence of rain. It is suggested that the QuikSCAT sigma(0) is sensitive to the wind speed of up to about 40-50 m.s(-1). We introduce modifications to the NSCAT2 GMF and apply the modified GMF to the QuikSCAT observations of Hurricane Floyd. The QuikSCAT wind estimates for Hurricane Floyd in 1999 was improved with the maximum wind speed reaching above 60 m/s. We perform an error analysis by comparing the QuikSCAT winds with the analyses fields from the National Oceanic and Atmospheric Administration (NOAA) Hurricane Research Division (HRD). The reasonable agreement between the improved QuikSCAT winds and the HRD analyses supports the applications of scatterometer wind retrievals for hurricanes.

Born2001

An analytical theory for orbit determination
G. H. Born and D. B. Goldstein and B. Thompson
JOURNAL OF THE ASTRONAUTICAL SCIENCES  49  345--361  (2001)

The QuikScat spacecraft was launched in June 1999 and is currently measuring vector winds over the global oceans with a spatial resolution of 25 km and a temporal resolution of one day. The University of Colorado is responsible for mission operations, including production of the ephemeris used to produce the science data records. The navigation solutions from the onboard Motorola Vice Roy receiver are smoothed in a post-processing mode to produce the operational ephemeris. The objective of this study is to develop a simple yet effective method of smoothing the navigation solutions from the onboard GPS receiver in a post-processing mode to provide a medium accuracy continuous ephemeris. The navigation solutions prior to Selective Availability being turned off in May, 2000 had noise at the 100 ra level. Currently the noise level is around 10 in. However, this noise is high frequency relative to the dominant orbit perturbations periods. Hence, orbit determination accuracy with SA off is not significantly different from that presented here. The approach used is to augment a J(2) analytical theory for near-circular orbits to include additional frequencies due primarily to tesseral harmonics. Accuracy of the theory was tested through fits to simulated and actual QuikScat GPS receiver data.

Patoux2001

A scheme for improving scatterometer surface wind fields
J. Patoux and R. A. Brown
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES  106  23985--23994  (2001)

A method is presented for improving QuikSCAT surface wind fields. The University of Washington Planetary Boundary Layer model is used to retrieve a surface pressure field from any swath of QuikSCAT surface wind vectors. An alternate set of surface wind vectors is computed from the newly calculated pressure field. The latter can be smoothed and the process can be iterated. New surface wind vectors can be calculated where ambiguity removal fails and where measurements are missing. The present methodology preserves boundary layer dynamics and is an improvement over a statistical filter.

Patoux2001

Spectral analysis of QuikSCAT surface winds and two-dimensional turbulence
J. Patoux and R. A. Brown
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES  106  23995--24005  (2001)

A spectral decomposition of QuikSCAT surface wind vectors reveals different levels of variance and different values of the spectral slope in various regions of the world ocean for the 12 months investigated. The traditional considerations on the factors affecting the shape of the spectra are reviewed and compared to the results. In particular, the influence of large-scale synoptic systems is shown by comparing the steeper and more energetic spectra of the midlatitudes to the shallower spectra of the tropics. Similarly, the signature of convection is investigated by comparing spectra in the tropical convectively active and dry zones of the Pacific Ocean. Spectra of vorticity and divergence are calculated, along with spectral vorticity-to-divergence ratios. Their spatial and temporal variations are discussed. It is hypothesized that when convection is enhanced in the tropics, the spectral analysis captures the mesoscale/synoptic structures in which convection is embedded and that the spectra exhibit some of the characteristics of their midlatitude counterparts (i.e., steeper and more energetic).

Pan2001

Vector empirical orthogonal function modes of the ocean surface wind variability derived from satellite scatterometer data
J. Y. Pan and X. H. Yan and Q. Zheng and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  28  3951--3954  (2001)

Ocean surface winds derived from NSCAT, QuikSCAT and ERS-1/2 scatterometer observations during a period from January 1992 to April 2000 were analyzed using the vector empirical orthogonal function (VEOF) method. With the boreal winter and summer oscillation, the first VEOF is dominated by the Indian and East Asian monsoons and also shows an annual cycle of the trade winds. The second VEOF represents the boreal autumn and spring oscillation, and reveals a transition state between winter and summer. The third VEOF indicates the wind variability associated with El Nino Southern Oscillation (ENSO) events, because the temporal mode has a high correlation coefficient of 0.8 with the Southern Oscillation Index (SOI). Further more, the third mode reveals the teleconnection of the Indian monsoon and wind variability over high latitude oceans', such as the Aleutian Low system, with ENSO events.

Chen2001

Applying satellite remote sensing to predicting 1999-2000 La Nina
D. Chen
REMOTE SENSING OF ENVIRONMENT  77  275--278  (2001)

The usability of altimeter sea level data (TOPEX/POSEIDON) and scatterometer wind data (QuikSCAT) in El Nino and the Southern Oscillation (ENSO) prediction is investigated with the latest version of the Lamont forecast model. The emphasis of this study is on the effectiveness of these data sets in initializing the model to forecast the 1999-2000 La Nina conditions. Both the altimeter and scatterometer observations helped to improve the model, with the former being more effective for this period. It is possible and extremely useful to apply these data to real-time ENSO forecasting. In principle, it is advisable to assimilate multiple data sets so that they can complement one another in providing the correct initial conditions for the model. (C) 2001 Elsevier Science Inc. All rights reserved.

Portabella2001

Rain detection and quality control of SeaWinds
M. Portabella and A. Stoffelen
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  18  1171--1183  (2001)

A good assessment of the information content of scatterometer winds is particularly important in order to assimilate them in weather analysis. Besides retrieval problems in cases of a confused sea state, a particularly acute problem of Ku-band scatterometry is the sensitivity to rain. Elimination of poor quality data is therefore a prerequisite for the successful use of the National Aeronautics and Space Administration (NASA) Scatterometer (NSCAT) or QuikSCAT winds. Following the quality control for the European Remote-Sensing Satellite and NSCAT scatterometers performed at the Royal Netherlands Meteorological Institute, the authors further develop this methodology for QuikSCAT and define a quality indicator called the normalized residual (Rn). In order to characterize and validate the normalized residual, the authors use collocated Special Sensor Microwave Imager rain and European Centre for Medium-Range Weather Forecasts wind data. The results show indeed correlation between Rn and data quality. A wind speed dependent Rn threshold is shown to be adequate in terms of rejecting poor quality data (particularly rain) and keeping fair quality data. This opens the way to a quantitative use of SeaWinds measurements in weather analysis.

Schlax2001

Sampling errors in wind fields constructed from single and tandem scatterometer datasets
M. G. Schlax and D. B. Chelton and M. H. Freilich
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY  18  1014--1036  (2001)

Sampling patterns and sampling errors from various scatterometer datasets are examined. Four single and two tandem scatterometer mission scenarios are considered. The single scatterometer missions are ERS (with a single, narrow swath), NSCAT and ASCAT (dual swaths), and QuikSCAT (a single, broad swath obtained from the SeaWinds instrument). The two tandem scenarios are combinations of the broad-swath SeaWinds scatterometer with ASCAT and QuikSCAT. The dense, nearly uniform distribution of measurements within swaths, combined with the relatively sparse, nonuniform placement of the swaths themselves create complicated space-time sampling patterns. The temporal sampling of all of the missions is characterized by bursts of closely spaced samples separated by longer gaps and is highly variable in both latitude and longitude. Sampling errors are quantified by the expected squared bias of particular linear estimates of component winds. Modifications to a previous method that allow more efficient expected squared bias calculations are presented and applied. Sampling errors depend strongly on both the details of the temporal sampling of each mission and the assumed temporal scales of variability in the wind field but are relatively insensitive to different spatial scales of variability. With the exception of ERS, all of the scatterometer scenarios can be used to make low-resolution (3 degrees and 12 days) wind component maps with errors at or below the 1 m s(-1) level. Only datasets from the broad-swath and tandem mission scenarios can be used for higher-resolution maps with similar levels of error, emphasizing the importance of the improved spatial and temporal coverage of those missions. A brief discussion of measurement errors concludes that sampling error is generally the dominant term in the overall error budget for maps constructed from scatterometer datasets.

Polito2001

Oceanic and atmospheric anomalies of tropical instability waves
P. S. Polito and J. P. Ryan and W. T. Liu and F. P. Chavez
GEOPHYSICAL RESEARCH LETTERS  28  2233--2236  (2001)

Tropical instability waves (TIWs) are detected in remotely-sensed sea surface height (SSH), temperature (SST), and wind records of the eastern equatorial Pacific. Analyses of TIW anomaly relationships reveal strong dynamical influence of TIWs within approximately 5 degrees of the equator. The first influence is advective heat flux. The primary forcing of TIW SST anomalies is advection of the meridional temperature gradient by TIW currents. The second influence is modification of the wind stress and Ekman pumping fields by TIW surface ocean currents. By affecting surface stress and hence roughness, TIW currents in this low-wind region introduce a significant bias in scatterometer vector wind measurement. This bias is evident in both NSCAT and QuikSCAT winds. The difference between wind measurements from TAO moorings and scatterometers is phase-locked with TIW SST oscillations. These results have important implications for scatterometry and for understanding tropical dynamics, thermodynamics and biogeochemistry.

Chelton2001

Observations of coupling between surface wind stress and sea surface temperature in the eastern tropical Pacific
D. B. Chelton and S. K. Esbensen and G. Schlax and N. Thum and M. H. Freilich and F. J. Wentz and C. L. Gentemann and M. J. McPhaden and P. S. Schopf
JOURNAL OF CLIMATE  14  1479--1498  (2001)

Satellite measurements of surface wind stress from the QuikSCAT scatterometer and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission Microwave Imager are analyzed for the three-month period 21 July-20 October 1999 to investigate ocean-atmosphere coupling in the eastern tropical Pacific. Oceanic tropical instability waves (TIWs) with periods of 20-40 days and wavelengths of 1000-2000 km perturb the SST fronts that bracket both sides of the equatorial cold tongue, which is centered near 1 degreesS to the east of 130 degreesW. These perturbations are characterized by cusp-shaped features that propagate systematically westward on both sides of the equator. The space-time structures of these SST perturbations are reproduced with remarkable detail in the surface wind stress field. The wind stress divergence is shown to be linearly related to the downwind component of the SST gradient with a response on the south side of the cold tongue that is about twice that on the north side. The wind stress curl is linearly related to the crosswind component of the SST gradient with a response that is approximately half that of the wind stress divergence response to the downwind SST gradient. The perturbed SST and wind stress fields propagate synchronously westward with the TIWs. This close coupling between SST and wind stress supports the Wallace et al. hypothesis that surface winds vary in response to SST modification of atmospheric boundary layer stability.

Katsaros2001

QuikSCAT's SeaWinds facilitates early identification of tropical depressions in 1999 hurricane season
K. B. Katsaros and E. B. Forde and P. Chang and W. T. Liu
GEOPHYSICAL RESEARCH LETTERS  28  1043--1046  (2001)

Far from land and surface ship observations, most tropical depressions are identified by examining images from geostationary satellites for the presence of rotation of the convective cloud masses. During the 1999 hurricane season, surface wind vectors obtained by the SeaWinds scatterometer on the QuikSCAT satellite for the tropical Atlantic and Caribbean Sea were examined to test the hypothesis that developing tropical depressions (TDs) could be observed with this satellite sensor, before identification by the traditional means. QuikSCAT was able to detect the presence of closed circulation in the surface winds before the systems were designated as depressions. The satellite's unprecedented large swath width of 1800 km allows twice a day observation of most of the tropical oceans. SeaWinds data can, therefore, provide valuable guidance that are an important addition to the tools available to the tropical cyclone forecasting community.

Early2001

Image reconstruction and enhanced resolution imaging from irregular samples
D. S. Early and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  39  291--302  (2001)

While high resolution, regularly gridded observations are generally preferred in remote sensing, actual observations are often not evenly sampled and have lower-than-desired resolution, Hence, there is an interest in resolution enhancement and image reconstruction. This paper discusses a general theory and techniques for image reconstruction and creating enhanced resolution images from irregularly sampled data. Using irregular sampling theory, we consider how the frequency content in aperture function-attenuated sidelobes can be recovered from oversampled data using reconstruction techniques, thus taking advantage of the high frequency content of measurements made with nonideal aperture filters, We show that with minor modification, the algebraic reconstruction technique (ART) is functionality equivalent to Grochenig's irregular sampling reconstruction algorithm. Using simple Monte Carlo simulations, we compare and contrast the performance of additive ART, multiplicative ART, and the scatterometer image reconstruction (SIR) (a derivative of multiplicative ART) algorithms with and without noise, The reconstruction theory and techniques have applications with a variety of sensors and can enable enhanced resolution image production from many nonimaging sensors. The technique is illustrated with ERS-2 and SeaWinds scatterometer data.

Lin2000

An analysis of a rotating, range-gated, fanbeam spaceborne scatterometer concept
C. C. Lin and B. Romen and J. J. W. Wilson and F. Impagnatiello and P. S. Park
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  38  2114--2121  (2000)

A new simple scatterometer concept combines the advantages of both the fixed, multiple beam, sidelooking radar such as AMI-Wind (ERS-1/2) and NSCAT (ADEOS), and the conically scanning pencil-beam radar such as SeaWinds, A wide, fanbeam antenna is rotated around a vertical axis with a slow rotation rate. For a satellite at an altitude of 725 km, the antenna footprint sweeps a circular donut of 1500 km diameter. Such a slow conical scan combined with the motion of the satellite at approximate to 7 km/s ground speed results in highly overlapping successive sweeps such that an image pixel is revisited up to 10 similar to 11 times during an overpass. The pixels in the radial direction are resolved by range-gating the radar echo. Depending on the across-track position of the imaged pixel, the measurement acquisitions during an overpass consist of a set of sigma degrees at different combinations of the azimuth and incident angles. A preliminary optimization of the system resulted in a C-band radar concept with a 15 km multiple-look spatial resolution and global coverage in two days. A sketch of the developed concept, preliminary system design, and predicted performance are described.

Kim2000

A physical-model-based, field-wise and self-contained algorithm for removing directional ambiguities of ocean surface winds retrieved from scatterometer measurements
Y. J. Kim
GEOPHYSICAL RESEARCH LETTERS  27  2665--2668  (2000)

An algorithm is introduced to remove the directional ambiguities in ocean surface winds measured by scatterometers, which requires scatterometer data only. It is based on two Versions of PBL (planetary boundary layer) models and a lowpass filter. A pressure field is first derived from the median-filtered scatterometer winds, is then noise-filtered, and is finally converted back to the winds, respectively, by an inverted PBL model, a smoothing algorithm, and a PBL model. The derived wind field is used to remove the directional ambiguities in the scatterometer data. This new algorithm is applied to Hurricane Eugene and produces results comparable to those from the current standard ambiguity removal algorithm for NASA/JPL SeaWinds project, which requires external numerical weather forecast/analyses data.

Liu2000

Atmospheric manifestation of tropical instability wave observed by QuikSCAT and tropical rain measuring mission
W. T. Liu and X. S. Xie and P. S. Polito and S. P. Xie and H. Hashizume
GEOPHYSICAL RESEARCH LETTERS  27  2545--2548  (2000)

Observations from two new spaceborne microwave instruments in 1999 clearly reveal the atmospheric manifestation of tropical instability waves north of the Pacific equatorial cold tongue. A unique zonal-temporal bandpass filter enables the isolation of the propagating signals and the determination of their phase differences. The phase differences between the propagation of wind and sea surface temperature (SST) signals observed from space and the vertical wind profiles measured from a research ship are consistent with the hypothesis that the coupling between wind and SST is caused by buoyancy instability and mixing, which reduces the wind shear in the atmospheric boundary layer. The coupling causes higher evaporative cooling over the warm phase and infers a negative thermal feedback.

Figa2000

On the assimilation of Ku-band scatterometer winds for weather analysis and forecasting
J. Figa and A. Stoffelen
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  38  1893--1902  (2000)

Following the successful assimilation of European remote sensing satellite (ERS) scatterometer winds for weather analysis and forecasting, we further develop this methodology for the assimilation of the NASA scatterometer (NSCAT) and QuikSCAT Ku-band scatterometer data. Besides retrieval problems in cases of a confused sea state, the quality control (QC) developed here identifies cases with rain on a mind vector cell (WVC) by WVC basis. The elimination of such geophysical conditions is a prerequisite to arrive at a successful assimilation of Ku band scatterometer data. Moreover, me propose to assimilate ambiguous winds rather than radar backscatter measurements, as is being done at most meteorological centers assimilating ERS scatterometer data. After our quality assessment, NSCAT winds still have more difficult ambiguity removal properties than ERS winds, A further testing of the data assimilation method proposed is being carried out at the European Center for Medium-range Weather Forecasts in NSCAT impact experiments. A normalized wind inversion residual is used for QC, In order to determine a threshold for the rejection of poor quality wind solutions, the inversion residual and the wind vector departure from the ECMWF model, are correlated. We end up rejecting around 7.4\% of wind vector solutions and 4.2\% of the NSCAT WVC's, In order to perform a qualitative assessment of this rejection, comparisons to collocated SSM/I rain and ECMWF minds are used. Confused sea state and presence of rain seem to be the most likely causes for the rejection of WVC's, As expected, the remaining number of ambiguities is larger than in the case of the ERS scatterometer but dependent on wind direction. The proposed cost function for the assimilation of NSCAT winds in weather analysis contains up to four ambiguous minds, the derived QC information, and the probability of each wind solution, We believe that the results of our study can be successfully extended for the interpretation and retrieval of good quality winds from QuikSCAT and their assimilation in weather analysis.

Tsai2000

Polarimetric scatterometry: A promising technique for improving ocean surface wind measurements from space
W. Y. Tsai and S. V. Nghiem and J. N. Huddleston and M. W. Spencer and B. W. Stiles and R. D. West
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  38  1903--1921  (2000)

Spaceborne wind scatterometers provide useful measurements of ocean surface winds and are important to climatological studies and operational weather forecasting. Past and currently planned scatterometers use measurements of the copolarized backscatter cross-section at different azimuth angles to infer ocean surface wind speed and direction. Although successful, current scatterometer designs have limitations such as degraded wind performance in the near-nadir and outer regions of the measurement swath and a reliance on external wind information for vector ambiguity, removal, Theoretical studies of scattering from the mind-induced ocean surface indicate that polarimetric measurements provide orthogonal and complementary directional information to aid the wind retrieval process. In this paper, potential benefits of making polarimetric backscatter measurements to improve wind retrieval performance are addressed, To investigate the performance of a polarimetric scatterometer, a modified version of the SeaWinds end-to-end simulator at the Jet Propulsion Laboratory (JPL), Pasadena, CA, is employed, To model the effect of realistic measurement errors, expressions for polarimetric measurement variance and bias are derived. It is shown that a polarimetric scatterometer can be realized with straightforward and inexpensive modifications to a current scanning pencil-beam scatterometer system such as SeaWinds, Simulation results show that such a system can improve wind performance in the nadir region and eliminate the reliance on external wind information. The mechanism by which the addition of polarimetric measurements improves wind vector retrieval is discussed in detail. Field experiments are suggested to better characterize the polarimetric scattering properties of the wind-modulated ocean surface for future applications to wind scatterometry.

Spencer2000

Improved resolution backscatter measurements with the SeaWinds pencil-beam scatterometer
M. W. Spencer and C. L. Wu and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  38  89--104  (2000)

The SeaWinds scatterometer was launched on the NASA QuikSCAT spacecraft in June 1999 and is planned for the Japanese ADEOS-II mission in 2000. In addition to generating a global Ku-band backscatter data set useful for a variety of climate studies, these flights will provide ocean-surface wind estimates for use in operational weather forecasting. SeaWinds employs a compact `` pencil-beam '' design rather than the ``fan-beam'' approach previously used with SASS on Seasat, NSCAT on ADEOS-I, and the AMI scatterometer on ERS-1, 2, As originally envisioned and reported, the resolution of the SeaWinds backscatter measurements were to be antenna-beamwidth limited. In order to satisfy an emerging demand for higher resolution backscatter data, however, the SeaWinds signal-processing design has been significantly modified. Here, the various options considered for improving the resolution of the SeaWinds measurements are discussed, and the selected:hardware modification (the addition of deramp processing for range discrimination) is described. The radar equation specific to a rotating pencil-beam scatterometer with digital range filtering is developed, and the new challenges associated with calibrating the resulting improved resolution measurements are discussed. A formulation for assessing the variance of the measurements due to fading and thermal noise is presented. Finally, the utility of improved resolution SeaWinds measurements for land and ice studies is demonstrated by simulated enhanced-resolution imaging of a synthetic Earth backscatter scene.

Oliphant1999

Accuracy of scatterometer-derived winds using the Cramer-Rao bound
T. E. Oliphant and D. G. Long
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  37  2642--2652  (1999)

A wind scatterometer makes measurements of the normalized radar-backscatter coefficient sigma degrees of the ocean surface. To retrieve the wind, a geophysical model function (GMF), which relates sigma degrees to the near-surface wind, is used. The wind vector can be estimated using maximum-likelihood techniques from several sigma degrees measurements made at different azimuth angles. The probability density of the measured sigma degrees is assumed to be Gaussian with a variance that depends on the true sigma degrees and therefore, depends on the wind through the GMF, With this model for wind estimation, the Cramer-Rao (C-R) bound is derived for wind estimation, and its implications for wind retrieval are discussed, As part of this discussion, the role of geophysical modeling error is considered and shown to play a significant role in the performance of near-surface wind estimates, The C-R bound is illustrated using parameters from the ERS AMI, NSCAT, and SeaWinds scatterometers.

Scharton1999

Combined loads, vibration, and modal testing on the QuikSCAT spacecraft
T. Scharton and M. Vujcich
JOURNAL OF THE IEST  42  36--42  (1999)

Combining the quasi-static loads, workmanship verification, and model validation tests of aerospace hardware into a single vibration test sequence can reduce schedule and cost considerably. The enabling factor in the implementation of the combined dynamic testing approach is the measurement of the dynamic forces exerted on the test item by the shaker. The dynamic testing of the QuikSCAT spacecraft is discussed as an example of a successful combined loads, workmanship, and model validation test program.

Hussein1999

Application of cylindrical near-field measurement technique to the calibration of spaceborne radar antennas: NASA scatterometer and SeaWinds
Z. A. Hussein and Y. Rahmat-Samii
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING  37  360--373  (1999)

Modern spaceborne radar scatterometers, such as the NASA scatterometer (NSCAT) and SeaWinds radar instruments, require precise determination of the normalized backscattered radar cross section within a few tenth of a decibel, This is needed to achieve the desired wind velocity and direction measurement accuracy of 2 mis and 20 degrees, respectively. This high level of precision demands a priori prelaunch accurate knowledge and determination of the radar antenna's absolute gain and relative radiation patterns characteristics over wide angular range. such characterizations may be performed on a far-field range, compact range, or in an indoor near-field measurement Facility, Among the unique advantage of the near-held measurement is that most of the information of the radar antenna radiation properties fan be obtained anywhere outside the near-field measurement surface. Two recently designed radar scatterometers are considered in this paper, NASA scatterometer (NSCAT) and SeaWinds, to demonstrate the utility of a newly completed cylindrical near-field measurement range. As an example of an advanced calibration methodology, the data based on a recently measured JPL/NASA scatterometer (NSCAT) radar antenna are used to experimentally demonstrate the role of the probe pattern compensation, probe multiple reflection effects, probe mispositioning effects, scan area truncation effects, etc, A measurement lest on a standard gain horn (SGH) has been performed to achieve and verify the absolute gain calibration accuracy. A comparison between direct far-field measured data and those obtained from cylindrical near-field measurements for the SeaWinds radar antenna was found in excellent agreement. Tt is demonstrated that the near-field measurement technique is a viable approach in accurately characterizing the performance of spaceborne radar antennas.