HYPACT
Hybrid Particle And
Concentration Transport Model

HYPACT, developed by the the personnel at Mission Research Corporation and current ATMET personnel, represents the next generation of dispersion modeling systems. It combines the best features of grid-based Eulerian dispersion methodologies with Lagrangian particle dispersion modeling.

The HYPACT Lagrangian dispersion scheme is very flexible. Species can include gases, and a spectrum of aerosol sizes. The 2-D or 3-D wind and turbulence fields are provided by MRC/*ASTER's RAMS (Regional Atmospheric Modeling System) for forecast applications, or an observational network for diagnostic applications. A Lagrangian model is ideal for regimes in which the assumptions underlying Gaussian plume-based models are violated, such as highly sheared flows, recirculating coastal and mountain/valley wind systems, urban heat islands, plume fumigation and bifurcation.

Although the RAMS code can directly compute the dispersion of any number of "tracers" in an Eulerian framework, HYPACT has certain advantages because it combines in one code the best features of both the Lagrangian and Eulerian dispersion estimating methodologies. The advantage is greatest near a source region for tracers when the source is small and unresolvable on the Eulerian grid. A comparable Eulerian treatment would necessarily represent the source by a volume no smaller than one grid cell, and would immediately begin diffusing the tracer in adjacent cells. A Lagrangian approach, on the other hand, is fully capable of representing a source of any size, and of maintaining a concentrated, narrow plume downwind of the source until atmospheric dispersion dictates that it should broaden. In contrast, at large distances from the source, where the tracer plume is typically broad and well mixed, representation of the plume by Lagrangian particles can become inefficient due to the large number of particles required to achieve a smooth characterization of the plume. The hybrid Lagrangian and Eulerian approach used in HYPACT represents a tracer by Lagrangian particles near the source, but converts particles to Eulerian concentrations where appropriate at large distances downwind.

Sources in HYPACT can be single or multiple, instantaneous (explosive), continuous, or time varying for any of the specified species. Source geometry can include point, line area and volume sources of various orientations. The model domain can extend from an area as small as an industrial plant site to hundreds of kilometers (up to the size of the RAMS domain). The number of particles released is limited only by available memory and can exceed hundreds of thousands. Sources, species and emission scenarios can either be managed through the namelist, or through database files.

HYPACT is a modular FORTRAN 90 compliant code with new features being regularly added. Currently planned future modules include dry deposition, rain out, re-evaporation, plume rise, evaporative sources and chemical transformations. Further planned "house keeping" enhancements include the removal of inactive particles and memory reallocations, gridded source input (and background concentrations), various file input/output formats and history restarts. Other enhancements may include code parrallelization, an execution control GUI, and enhancement of visualization tools (REVU and RINGI). Collaborative development on any of these enhancements is welcome.