Though the horizontal grid resolution of operational mesoscale numerical models has significantly increased in recent years, to values on the order of kilometers, the resolution is still not sufficient to explicitly account for the effects of the urban environment. Thus, these mesoscale models often employ an area-averaged parameterization to account for sub-grid scale building effects. One such model, the Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS) developed by the Naval Research Laboratory (NRL), is used to simulate the urban environment of the Washington, DC metropolitan area.

COAMPS is a complete three-dimensional mesoscale prediction system that has been used for operational mesoscale forecasting since 1996. It consists of atmospheric and ocean data assimilation with data quality control, analysis, initialization, and a nonhydrostatic atmospheric forecast model with an imbedded aerosol/passive tracer model. Because the urban infrastructure can have a tremendous impact on surface and atmospheric dynamic and thermodynamic structure, an urban canopy parameterization (UCP) has been implemented recently into COAMPS. This parameterization accounts for effects due to building drag, turbulent production, radiation balance, and anthropogenic and rooftop heating, and makes use of an additional equation for the rooftop surface energy balance.

Simulations with COAMPS at 1-km horizontal resolution are compared to observations from the National Oceanic and Atmospheric Administration (NOAA) DCNet. DCNet is a new local weather network consisting of eight towers with mean and turbulence sensors mounted mainly on buildings throughout the Washington, DC metropolitan area. Simulations with variations in the model specification of land-use characterization and urban morphology are also conducted to assess the sensitivity of model simulations to changes in the lower boundary condition.

The impact of the urban canopy layer on mesoscale plume transport, as well as its development and evolution, will be shown using COAMPS simulations with the imbedded passive tracer model. Model tracer releases occurring both within and above the urban canopy will be compared to assess the impact of the urban canopy on transport of the plume.