To accurately simulate the transport of a tracer released into a metropolitan area requires sufficiently high model resolution (1-10 meter grid cells) to resolve buildings and urban street "canyons". Within our group a modeling effort has been underway to develop a model---termed HIGRAD---capable of simulating flow at the high spatial resolution required within the urban environment. HIGRAD uses state-of-the-art numerical techniques to accurately simulate the regions of strong shear found near edges of buildings. HIGRAD also employs a newly developed radiation package which in addition to standard shortwave and longwave heating/cooling effects can account for the shadowing effects of building complexes on the urban flow field. Idealized simulations have been conducted which clearly illustrate the role radiation plays in transport and dispersion of tracers in an urban setting. For instance, idealized two-dimensional simulations reveal fewer boundary-layer eddies within cooler shaded regions and rather persistent eddies on the tops of buildings.
In addition to our idealized simulations, we have modeled a realistic urban environment. In particular, we have simulated the flow of a tracer past a group of buildings designed to mimic the Washington Mall area. Complex flow/building interactions were produced during the simulation and these interactions had a significant impact on the transport of the tracer. These simulations suggest that a tracer released in a street canyon which runs parallel to the wind can be quickly transported away from the source by the strong winds found in the urban canyon. In constrast, if a tracer is released upwind of a building, then its movement is slowed almost to the point of stagnation.