Modeling the role of aerosols on warm-season rainfall in a coastal urban environment

Evidence of warm-season rainfall increases over and downwind of major cities such as Atlanta, Phoenix, Mexico City, St. Louis, Houston, and Chicago has been presented in several recent studies. This precipitation has been predominately attributed to the induced updraft of warm air masses associated with the urban heat island. However, aerosols are abundant in urban environments and it has been hypothesized that they play a role in the water balance of humid regions. High concentrations of cloud condensation nuclei (CCN) may reduce precipitation in humid urban environments. However, it is also noted that precipitation may also be increased due to the presence of large hygroscopic aerosols which are also known as giant cloud condensation nuclei (GCCN). The present research is directed towards improving our understanding of the role of aerosols in cloud processes in complex coastal urban environments through ground observations obtained from AERONET, and weather station data for the New York City/New Jersey (NY-NJ) region, in addition to analysis of synoptic and localized precipitation patterns using the urbanized mesoscale model known as the Regional Atmospheric Model System (RAMS) with a modified explicit microphysics model. The RAMS cloud microphysics module was modified to allow for activation of CCN and GCCN and for specifying initial conditions for the concentration of CCN and GCNN, which may include 3D fields. For this study, particle size distributions of aerosols from AERONET observations are ingested into the new RAMS microphysics code for the NY-NJ regions, and an ensemble of numerical runs was organized to discern between urban induced convective precipitation, aerosol-induced suppression of precipitation, or both, for several events in summer 2007. This approach is allowing discerning between localized precipitation attributed solely to the presence of atmospheric aerosols, and precipitation prompted by the presence of convection in the atmosphere for this coastal urban environment for warm seasons. Conclusions are drawn for other coastal urban regions and conditions.