Introduction

Transport and dispersion models are major tools to help an emergency manager determine where an accidental release of a toxic gas into the atmosphere is moving and how fast the concentration is decreasing. These models help the manager determine who is at risk so that appropriate evacuations can be instituted, if necessary. The models also help the manager determine from which direction emergency personnel should approach to avoid exposure themselves. ARLFRD uses two transport and dispersion models for emergency planning and preparedness, known as MDIFF and HYSPLIT.

MDIFF

MDIFF is a transport and dispersion model built to handle accidental gaseous releases at DOE's Idaho National Laboratory. MDIFF is incorporated into the INLViz display system that is used for emergency response at the INL.

MDIFF is based on the well known MESODIF puff dispersion model, the grandfather of many transport and dispersion models in use around the world today. MESODIF was one of the first puff diffusion models ever developed for use on modern computers and was developed right here at FRD.

The puff transport algorithm is conceptually very simple: 1) every release of material is represented as a series of puffs, 2) each puff is allowed to move and grow independently, and 3) concentrations are calculated as the sum of the concentrations due to all of the puffs.

MDIFF is used to model short-term episodes. It operates using five-minute data from the INL Mesonet. The model does have some limitations and is showing its age. It lacks certain features such as deposition, radiological dose computations, and the ability to account for vertical wind shear. These features could have been added to MDIFF, but ARLFRD decided that it made more sense to transition to the more widely used NOAA HYSPLIT model instead. This would leverage ARLFRD's limited staff resources by interfacing with a much larger HYSPLIT community. MDIFF will continue to be available during the transition to HYSPLIT.

More information about MDIFF is contained in a NOAA Tech. Memo available here.

HYSPLIT Decision Support Tool

ARLFRD also has at its disposal the the NOAA HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) model. HYSPLIT is a complete system for computing air parcel trajectories for complex dispersion and deposition simulations. The dispersion of a pollutant is calculated by assuming either puff or particle dispersion or a combination of the two. Full puff mode is similar to MDIFF. In full particle mode, a cloud of individual particles is released at the source. Each particle is advected by both the mean wind field and a randomized turbulent component. Particles at different altitudes can move in different directions, so the model more accurately accounts for the effects of vertical wind shear. HYSPLIT can also be run in a hybrid mode in which the horizontal dispersion uses a puff algorithm whereas the vertical dispersion uses the particle approach.

More about the HYSPLIT model is available here.