Urban centers are concentrated areas with large populations and are the most likely areas for either accidental or intentional release of toxic agents. With this in mind, the Department of Energy, through the Chemical and Biological National Security Program (CBNP) has sponsored the development of a suite of models that include urban effects on the thermodynamics, wind fields and turbulence generation. Furthermore, the CBNP sponsored a major field campaign, URBAN 2000 (Allwine et al. 2001), that was conducted in Salt Lake City in October 2000.

URBAN 2000 was intended to provide data to evaluate the CBNP models. The experiment was designed to provide data across a range of length scales, including the flow fields around single buildings, in a downtown urban area, and across the entire urban basin that includes Salt Lake City. An inert tracer, SF6 was released in a downtown area, with sampling arcs of bulk samplers at several distances downwind. Instrumentation included a variety of meteorological sensors to characterize the circulation in and around the urban area.

An urban parameterization (Chin et al., 2000) was added to a mesoscale meteorological forecast model to account for the drag effects and thermodynamic effects of the city infrastructure. Modifications were made to the mean momentum and turbulence kinetic energy equations, to the radiative transfer in the model and to the surface energy budget. The forecast wind field and atmospheric stability are used to drive a Lagrangian particle model. Simulations with and without the urban parameterization in the mesoscale model are compared to the observed SF6 concentrations in the sampling arcs to illustrate the urban effects on the wind field and thus the transport of a released agent in the urban area.