Comparing MM5 radiative fluxes with observations gathered during the 1995 and 1999 Nashville southern oxidants studies

Abstract

The impact of radiative transfer processes on MM5 air‐quality forecasts during the Nashville Southern Oxidants Experiments is addressed. We find that an incorrect specification of aerosol scattering, aerosol absorption, and ozone absorption in the model can lead to positive errors in the instantaneous total solar irradiance at the surface of 100 W m−2 using the Dudhia solar parameterization. This increased solar irradiance upsets the surface energy balance in the model causing errors in the surface heat fluxes and mixing depth estimation. We also show that a systematic 80 W m−2 increase in the forecast downwelling long‐wave radiation can inhibit the formation of ground‐based nocturnal inversions in the model when the Dudhia long‐wave radiation routine is used. Thus we find that accurate specification of radiative processes, including some driven by local pollution, can be important for mesoscale meteorological forecasting. A method for including ozone absorption in MM5 is introduced, and optimum choices for running the model in aerosol rich geographic regions are discussed.