Non-reactive Lagrangian T&D (transport and diffusion) models involve simple linear or first-order chemistry, handle multiple sources by superposition, and have a detailed treatment of horizontal plume transport and diffusion. These models attempt to recreate in detail the mesoscale meteorological variations that dominate plume dispersion on appropriate transport scales. Most Lagrangian plume models operate under the assumption that long-range transport and diffusion is dominated by plume meander due to mesoscale variations in the wind field. That is, the models assume that the characteristic scale of flow controlling the plumes transport and diffusion are greater than the plume dimension rather than smaller scale turbulent eddies. Thus, Lagrangian models implicitly require high-resolution wind data in order to successfully simulate transport and diffusion processes. The use of Eulerian (grid-based) mesoscale models to produce the required meteorological data is becoming a popular practice.
TASC has developed an interface designed to couple the PSU/NCAR MM5 (Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model Version 5) model to a Lagrangian T&D model. Graphical User Interfaces (GUIs) are used to quickly and efficiently setup the MM5 model grid to operate over a selected region of interest. MM5 is then run to re-create (or predict) high-resolution wind fields given more sparce observational initial data. The model uses a global 30 second terrain database to resolve the critical effects of terrain on airflow. The Four-Dimensional Data Assimilation (FDDA) technique can be used to incorporate weather observations so that the model forecast adheres to pre-specified analysis fields during the model run. Finally, a Lagrangian trajectory plume model is used to represent one or more continuous plumes as a succession of discrete plume elements emitted by user-specified point sources. These plume elements are independently advected and diffused by the spatially and temporally varying wind field. Each element carries with it an independent time history including such characteristics as plume chemistry, dry deposition and scavenging.
The time-averaged ground level impact of the plume at a given point is calculated by combining the contributions from all elements that independently traverse that point during the specified averaging time. The approach therefore allows for determination of local-scale impacts of individual point sources at finer resolution than most grid models (including MM5) can achieve economically. In this paper, we will apply the coupled approach to the transport and diffusion of passive chemical and biological agents.