Quoted from: http://www.atmet.com/html/docs/hypact/1.2.0/hypact.htm 

The HYPACT (HYbrid Particle And Concentration Transport) model code has been developed to simulate the motion of atmospheric tracers under the influence of atmospheric flow, including turbulence.  In its current form, it is set up to be driven by meteorological output from the RAMS (Regional Atmospheric Modeling System), but the basic design allows the flexibility, with minor code changes, to provide a variety of other inputs as well, including observational data.

Although RAMS can itself perform a similar function by simulating the motions of any number of Eulerian tracer fields, HYPACT has certain advantages because of the ability to run the dispersion as a post-processing step and because of its Lagrangian formulation.  The advantage is greatest near a source region for the tracer when the source is of small scale and unresolved on the Eulerian grid.  RAMS would necessarily represent the source by a volume no smaller than one grid cell, and would immediately begin diffusing the tracer to the adjacent cells.  A Lagrangian model, on the other hand, is capable of representing a source of any size, and of maintaining a concentrated, narrow tracer plume downwind of the source until atmospheric dispersion dictates that the plume should broaden.  A unique feature of HYPACT is that once a plume of Lagrangian tracer particles becomes sufficiently broad downstream from a source region, it can be converted to a well-resolved concentration field and advected using an Eulerian formulation.  This hybrid approach allows high particle emission rates to be specified for a source to achieve good plume resolution, without retaining all particles so far into the future that excessively large numbers of them accumulate.  Tracer sources that are well resolved on a grid may alternatively be specified as gridded sources in HYPACT and transported entirely by a Eulerian formulation.  HYPACT carries out Eulerian tracer prediction much faster than RAMS because it does not predict its own wind, pressure, temperature, moisture, or turbulence fields.

In order to run HYPACT from RAMS, the latter must be run first to generate a series of output "analysis" files.  These contain wind, potential temperature, and turbulent kinetic energy fields at the output times of the atmospheric simulation.  HYPACT reads these data from the analysis files and interpolates them in time between file times (for each HYPACT timestep) for determining transport and diffusion.  For Eulerian concentration prediction, HYPACT assigns the time-interpolated data to a series of nested grids identical with those of the RAMS simulation that generated the analysis files.  Concentrations are predicted on these grids given user-specified initial values and source characteristics.  For Lagrangian particle prediction, the time-interpolated wind and turbulence data are also interpolated in space to the location of each Lagrangian tracer particle.  If nested grids are used in the RAMS simulation, the finest grid data defined at the location of each particle is used for the interpolation.  The particles are moved through space and time based on the interpolated wind velocity plus a random motion scaled to the local turbulent intensity.  A gravitational settling velocity may also be superimposed on the particle motion.

HYPACT sources, species and emission can either be defined in the HYPACT namelists, or in database files.  This is designed to allow both quick and dirty runs, and a range of sophisticated scenarios from common data.

In addition to Eulerian concentration field outputs on the RAMS grids, concentration can be periodically diagnosed from the particle distribution on the RAMS grids (assuming a specified mass represented by each particle), and the combined Eulerian and Lagrangian concentration fields computed and output.  The concentration is useful for contouring plume morphologies, determining instantaneous or time-integrated dosages at specified locations, computing chemical reaction rates between different tracer fields, etc.  The raw particle files can also be output, allowing the user to diagnose the particle concentrations on a finer concentration grid.  Concentration grid diagnosis and NCAR Graphics plotting features were included in the 0.2 beta version of HYPACT, however, there are now included in REVU (RAMS/HYPACT Evaluation and Visualization Utilities).  For details on the use of this tool, please refer to the REVU User's Guide.

Similar to RAMS, HYPACT is in a state of continual evolution.  This document is the fourth edition of the HYPACT User’s Guide, describing the model as of August 2001.  HYPACT version 1.2.0 is FORTRAN 90 compliant and includes several RAMS modules and the RAMS and Utilities libraries.  UNIX/Linux make commands and a system of make files and makefile includes are used for compiling the code.  The make files are detailed in the section describing how to compile the HYPACT model.

Execution of HYPACT is controlled by a set of namelist variables usually contained in a file named HYPACT_IN.  In order to operate HYPACT, the user should be acquainted with the two primary means of setting parameters that control its functions.  These are:

1.       The various configuration parameters contained in hcommons.h, which define several array dimensions controlling the capacity of the model.

2.       The variables in the HYPACT_IN namelist file.

The following sections of this document describes the function and use of each of the parameters and namelist variables, and how to set appropriate values for them.