Modeling the Influences of Local Urban Aerosols on a Derecho in Birmingham, AL

This study uses the Regional Atmospheric Modeling System (RAMS) to simulate a derecho that crossed the Birmingham, AL metro area on June 14, 2009. The observed derecho modified significantly as it passed over the urban center: 1) the core of storm split around the city and 2) the entire mesoscale system turned southward (to the right) after passing over the urban area. The RAMS bin-emulating microphysical parameterization, with two urban aerosol modes, is coupled with an air quality model to produce local urban sources of cloud-nucleating aerosols near and downwind of Birmingham. The local sources produce a realistic, heterogeneous aerosol distribution compared to observations made by satellite. A sensitivity study is performed, varying the strength and composition of the urban sources, to evaluate the aerosol effects on the microphysics and mesoscale dynamics of the storm. While urban land-surface effects are more significant to the derecho modification, the aerosols are important to understanding localized effects. As the derecho encounters the edge of the urban aerosol field, higher aerosol concentrations enhance convective rain, accelerate cold-pool formation, and split the storm around the center of Birmingham. The trailing precipitation, primarily warm-rain, is suppressed by higher aerosol concentrations, decreasing the overall precipitation total in the urban area. Downwind of the urban center, higher aerosol concentrations on the north (left) side of the derecho suppress precipitation in that sector. Thus the storm redevelops more completely on the south (right) side and appears to turn southward. The most realistic model representations of the event were those with the local sources, and the detailed local urban-aerosol thunderstorm interaction was only possible with the heterogeneous aerosol distribution.