The Impacts of the Urban Environment on Extreme Rainfall from Warm Season Thunderstorm Systems

The spatial variability of extreme rainfall over urban environments and its change in time is examined from both a climatological and a case-study perspective. The areas of focus are the Washington D.C-Baltimore and New York City Metropolitan areas. Climatological analyses utilize Cloud-to-Ground lightning data from the National Lightning Detection Network (NLDN) and radar-rainfall products from the HydroNEXRAD project. The regional distribution of heavy convection, time-trends, initiation locations and extreme thunderstorm lifecycle characteristics are presented for both areas. Case studies include a collection of extreme events over the two Metropolitan areas that combine detailed observations and mesoscale modeling using the Weather Research and Forecasting (WRF) model with chemistry and cloud-aerosol interaction (WRF-Chem) capabilities. The impacts of the urban environment on extreme thunderstorm evolution, in terms of the Urban Heat Island (UHI), the Urban Canopy Layer (UCL), and urban aerosols are examined for this collection of events. High-resolution urban data is incorporated in the modeling scheme with the use of the Urban Canopy Model capability of WRF. Key results of the study include large differences in rainfall accumulation when aerosols and chemistry are included in the model that hint to the role of hygroscopic aerosols on extreme warm season thunderstorm evolution. The urban environment itself impacts the rainfall distribution and movement of extreme thunderstorms, in the urban vicinity, mainly through the impacts of the urban canopy layer and aerosols.