What is the Role of Aerosol Particle Size Distributions on Precipitation in Coastal Urban Environments?

The research presented here is directed towards determining the effect of aerosol particle size distributions (PSD) on precipitation for the coastal urban environment of New York City (NYC). It is known that aerosols are intrinsically necessary for rainfall formation while urban environments also influence precipitation via convection enhancement; however, the partial contributions of each on urban precipitation are not yet known. This research aims to solve this challenge with the higher goal of improving our ability to forecast precipitation on complex urban environments. An explicit microphysics model coupled to the Regional Atmospheric Modeling System (RAMS) was used to represent several summer precipitation scenarios for NYC. In-situ aerosol PSD data from NASA's AERONET sunphotometer network were processed and ingested directly into RAMS to represent Cloud Condensation Nuclei (CCN) and initiation of microphysics for the selected events. The city was represented by high resolution land data acquired from the United States Geological Survey (USGS) updated in 2006. Anthropogenic heat from buildings and traffic were included in the urban parameterization. An ensemble of six numerical simulations for NYC and northern New Jersey were configured and run. The first two runs were month long runs for July 2007, the first without PSD updates, and the second with PSD updates. The third and fourth runs mirrored the first two simulations for a “No-City” case. Two more runs addressed one day localized precipitation events under City and No-City conditions respectively. Model results suggest that precipitation rates, rainfall amounts, and spatial precipitation patterns are very sensitive to in-situ PSD, while urban environments enhance convection leading to further activation of CCN. Model validation results indicate that forecasting of precipitation in the complex urban environment of NYC could be improved significantly by data assimilation of in-situ PSD.