Aerosol and Urban Influences on Rainfall and Reflectivity Coverage During a Warm-Season Convective Event in Coastal Louisiana

Aerosol influences on local thunderstorms are complex and often contradictory. Within aerosol-laden city environments, these interactions are further complicated by additional thermodynamic and kinematic effects related to the urban landscape. Observational studies are limited in their ability to distinguish the different influences of urban land cover and aerosols on thunderstorms, but numerical modeling can alleviate the problem by systematically accounting for each variable separately. This study used the NASA Unified-Weather Research and Forecasting (NU-WRF) model to simulate frontal, scattered thunderstorms during a summer day in coastal Louisiana. The study period was simulated by iteratively including and excluding atmospheric chemistry and the physical attributes of the urban land cover, producing a total of four simulations. Three sub-zones, distinguished by their thunderstorm activity, level of urbanization, and aerosol concentrations, were selected for more detailed analysis to examine the sensitivity of precipitation and thunderstorm coverage to the experimental conditions (i.e., atmospheric chemistry and urban landscape). The aerosol and urban landscape simulations demonstrated a minimal impact on storm initiation timing, suggesting that these processes were secondary to the synoptic atmospheric conditions driving the initial development of convection. However, the quantity of precipitation and reflectivity coverage was more sensitive to the experimental modeling scenarios, with the individual simulations (i.e., aerosol or urban effects only) frequently overproducing precipitation compared to observations. Whether the aerosol or urban effects were the more influential factor was regionally dependent and varied between sub-zones. When both aerosol and urban effects were activated, the precipitation and reflectivity coverage aligned more closely with NASA’s Integrated Multi-SatellitE Retrievals for GPM(IMERG) estimated rainfall across much of the study area. Over the heavily urbanizedcoastal region, the combined model run with both aerosol and urban effects overproduced the rainfall and reflectivity coverage, whereas on a regional scale it modulated rainfall quantity, creating a non-linear precipitation reduction in most regions. These results suggest that urban and aerosol properties weakly influence the meso-beta meteorological pattern but can drive meso-gamma-scale changes that accumulate over larger areas.