The relationships that account for the observed anomalies in precipitation and severe weather over and downwind of urban regions are still not well understood, in spite of good field studies and data analyses. Air pollutants emanating from urban areas tend to be rich in cloud condensation nuclei (CCN). It is well known that enhanced CCN concentrations can result in narrower droplet spectra and thereby suppress warm rain processes. On the other hand, some urban areas can also be sources of giant CCN (GCCN) or ultra-giant particles, which can enhance warm rain processes. The relative contributions of these competing processes influences the amount of liquid water available for vertical transport and subsequent freezing in upper portions of the cloud. This in turns has an impact on the amount of latent heat released and the subsequent updraft development, which feeds back on the microphysical properties of convective storms.

In order to further investigate the impacts of CCN and GCCN on the microphysical and dynamical characteristics of thunderstorms developing downwind of urban regions, a number of numerical model simulations have been conducted using the Regional Atmospheric Modeling System (RAMS). The sophisticated "Town Energy Budget"(TEB) model of the urban land surface, and the two-moment bulk microphysics, which allows for the prognosis of CCN and GCCN concentrations, were both utilized. The results of these simulations indicate that urban-forced convergence downwind of the city, rather than the presence of greater aerosol concentrations, determines whether or not thunderstorms actually develop in the downwind region. However, once convection is initiated, the urban-enhanced aerosols can have a significant effect on the dynamics, microphysics and precipitation produced by these storms. The effects of these aerosols influence the rate and amount of liquid water and ice produced within these storms, the accumulated surface precipitation, the strength and timing of both updrafts and downdrafts, and the strength and influence of the associated cold pools. These modeling studies also demonstrate that the impacts of urban aerosol on downwind thunderstorms tend to decrease with increasing background aerosol concentrations. Results from these simulations and the implications of these results for expanding urban regions will be presented.