Accurate numerical prediction of airflow and tracer dispersion in urban areas depends, to a great extent, on the use of appropriate stability conditions. Due to the lack of relevant field measurements or sufficiently sophisticated turbulence models, modelers often assume that nearly neutral conditions are appropriate to use for the entire urban area being simulated. The main argument for such an assumption is that atmospheric stability (as defined by the Richardson number) is determined by both mechanical stresses and buoyant forcing but, for a typical urban setting with a given thermal stability or sensible heat flux, building-induced mechanical stresses can become so dominant to drive the resulting stability toward nearly neutral conditions. Results from our recent simulations of two Joint URBAN 2003 releases, using a computational fluid dynamics (CFD) model - FEM3MP, appear to support partially the assumption that urban areas tend toward neutral stability. More specifically, based on a model-data comparison for winds and concentration in the near field and velocity and turbulence profiles in the urban wake region, Chan and Lundquist (2005) and Lundquist and Chan (2005) observed that neutral stability assumption appears to be valid for intensive operation period (IOP) 9 (a nighttime release with moderate winds) and also appears to be valid for IOP 3 (a daytime release with strong buoyant forcing) in the urban core area but is less valid in the urban wake region. Our model, developed under the sponsorship of the U.S. Department of Energy (DOE) and Department of Homeland Security (DHS), is based on solving the three-dimensional, time-dependent, incompressible Navier-Stokes equations on massively parallel computer platforms. The numerical algorithm is based on finite-element discretization for effective treatment of complex building geometries and variable terrain, together with a semi-implicit projection scheme and modern iterative solvers developed by Gresho and Chan (1998) for efficient time integration. Physical processes treated in our code include turbulence modeling via Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) approaches described in Chan and Stevens (2000), atmospheric stability, aerosols, UV radiation decay, surface energy budgets, and vegetative canopies, etc. Predictions from our model are continuously being verified against measured data from wind tunnel and field experiments. Examples of such studies are discussed in Chan et al. (2001, 2004), Chan and Leach (2004), Calhoun et al. (2004, 2005), and Humphreys et al. (2004). In this study, the stability conditions associated with two more of the Joint URBAN 2003 releases are investigated. Through a model-data comparison of the wind and concentration fields, observed buoyancy production in the urban wake region, together with predicted values of turbulence kinetic energy (TKE) in various regions of the computational domain, a more definitive characterization of stability conditions associated with the simulated releases is presented. In the following, we first discuss briefly the field experiments being simulated, then present sample results from a model-data comparison for both the wind and concentration fields, examine the predicted TKE field and the observed buoyant forcing relative to the total TKE in the urban wake, and finally offer a few concluding remarks including the resulting stability conditions of the simulated releases
