Urban aerosol effects on convective storm characteristics

Urban regions are sources of cloud condensation nuclei (CCN) and giant CCN (GCCN). Such regions may therefore have impacts on the microphysical, dynamical and precipitation characteristics of convective storms developing over and downwind of urban regions. Simulations of convective storm development over St. Louis, Missouri, have been performed used the Regional Atmospheric Modeling System (RAMS), in order to investigate the influence of urban aerosol on convective storm characteristics. CCN and GCCN concentrations upwind and downwind of St. Louis were estimated using data obtained during the METROMEX experiment. A suite of sensitivity tests have been conducted in which the CCN and GCCN concentrations over the city were varied from clean rural values to polluted urban values. Simulations were also run in which the urban region was removed, while the aerosol concentrations associated with the city were maintained. This allowed for an estimation of the relative importance of the urban aerosol effects to that of the urban heat island (UHI).

The results of these sensitivity tests indicate that the UHI has a greater influence on whether convective storms develop over and downwind of the city, than do variations in urban aerosol concentrations. However, given the presence of the urban region, variations in aerosol concentrations influence numerous convective storm characteristics. Enhanced urban aerosol concentrations result in a greater number of stronger convective updrafts. A similar trend is observed for the downdrafts associated with these storms. The storms tend to form slightly earlier in the presence of enhanced aerosol concentrations. The location of convective core development is also influenced by the presence of urban aerosol. Enhanced CCN concentrations tend to delay the onset of precipitation downwind of the city, while greater GCCN concentrations tend to enhance precipitation development. However, the accumulated volumetric precipitation at the end of the simulations is greater in all the cases in which the urban aerosol concentrations are enhanced.