Quoted from: https://www3.epa.gov/ttn/scram/7thconf/iscprime/useguide.pdf 

The Electric Power Research Institute (EPRI) sponsored a study to develop and evaluate new, improved plume rise and building downwash algorithms suitable for integration into regulatory air quality models. The Plume Rise Model Enhancements (PRIME) model was designed to incorporate the two fundamental features associated with building downwash: enhanced plume dispersion coefficients due to the turbulent wake, and reduced plume rise caused by a combination of the descending streamlines in the lee of the building and the increased entrainment in the wake. The PRIME algorithms have been integrated into the ISCST model, and can be installed in other analytical, Gaussian-based models. A full description of PRIME, including the key model equations, is provided by Schulman et al. (1998).

Wind-tunnel and field studies have made it clear that incorporating estimates of wind speed, streamline deflection, and turbulence intensities in the wake, as well as the location of the source, are crucial to improving modeling simulations of the influence of buildings on groundlevel concentrations. This is the central approach used in PRIME; to explicitly treat the trajectory of the plume near the building, and to use the position of the plume relative to the building to calculate interactions with the building wake. PRIME calculates fields of turbulence intensity, wind speed, and the slopes of the mean streamlines as a function of the projected building dimensions. These fields gradually decay to ambient values downwind of the building. Coupled with a numerical plume rise model and these local values, PRIME determines the change in plume centerline location with downwind distance and the rate of plume dispersion. Plume rise incorporates the descent of the air containing the plume material, and rise of the plume relative to the streamlines due to buoyancy or momentum effects.

PRIME addresses the entire structure of the wake, from the cavity immediately downwind of the building, to the far wake (see Figure 1). The building cavity can be defined as the region bounded above by the separation streamline originating at the upwind edge of the roof, and bounded downwind of the building by the reattachment streamline. The cavity is bounded laterally by the streamlines emanating from the corners of the building. Depending on the building geometry, there can be a separate roof-top and downwind cavity, or a single recirculation cavity. The cavity downwind of the building is often called the near-wake. The wake beyond the reattachment streamline is called the far wake. The entire wake envelope bounds the building recirculating cavities and the far wake.