The dispersion of pollutants in an urban area are a complex phenomenon. Many scales of motion are responsible for the transport and diffusion of pollutants. During the US Department of Energy VTMX/URBAN2000 project, basin- and urban- scale flows and dispersion of passive tracers were measured simultaneously, in an attempt to quantify the vertical mixing that occurred during night-time, stable conditions.

We will present results from a two model simulations of the conditions during IOP-10, October 25-26, 2000, of the VTMX field program. First, a mesoscale model simulation of the Salt Lake City basin, using the Regional Atmospheric Modeling System (RAMS), will be discussed. The results from this model will be used as boundary conditions for a high-resolution large-eddy simulation (LES) of the downtown area using the HIGRAD LES code, being developed at Los Alamos National Laboratory. In the HIGRAD LES, the buildings and topography will be explicitly resolved. It is anticipated that incorporating the mesoscale information in the LES, including down-slope drainage flows, will have a large impact on the downtown dispersion. We will compare the results incorporating mesoscale information in the LES to the VTMX observations and previous LES's of the downtown area, using a horizontally homogeneous sounding for initialization and boundary conditions.

In addition to comparing with observations of the VTMX field program and model simulations, we will present preliminary results of using the LES to compute an equivalent drag coefficient for the urban area and compare it to the parameterizations used in the mesoscale model. Ultimately these simulations should prove useful in developing urban canopy parameterizations for use in mesoscale models.