The validity of omitting stability considerations when simulating transport and dispersion in the urban environment is explored using observations from the Joint URBAN 2003 field experiment and computational fluid dynamics simulations of that experiment. Four releases of sulfur hexafluoride, during two daytime and two nighttime intensive observing periods, are
simulated using the building resolving computational fluid dynamics model, FEM3MP to solve
the Reynolds Averaged Navier-Stokes equations with two options of turbulence parameterizations. One option omits stability effects but has a superior turbulence
parameterization using a non-linear eddy viscosity (NEV) approach, while the other considers buoyancy effects with a simple linear eddy viscosity (LEV) approach for turbulence
parameterization. Model performance metrics are calculated by comparison with observed winds and tracer data in the downtown area, and with observed winds and turbulence kinetic energy (TKE) profiles at a location immediately downwind of the central business district (CBD) in the
area we label as the urban shadow. Model predictions of winds, concentrations, profiles of wind
speed, wind direction, and friction velocity are generally consistent with and compare reasonably
well with the field observations. Simulations using the NEV turbulence parameterization generally exhibit better agreement with observations.