Extreme Hydrometeorological Events and the Urban Environment: Dissecting the 7 July 2004 Thunderstorm over the Baltimore, MD Metropolitan Region.

Observational analyses and mesoscale modeling studies using the Weather Research and Forecasting (WRF) model are used to dissect the mechanisms associated with record lightning, rainfall and flooding over the Baltimore metropolitan region on 7 July 2004. Storm evolution on 7 July 2004 exhibited characteristic features for warm season thunderstorms producing flash flooding over the Baltimore - Washington DC Metropolitan region. The storm system was initiated along the Blue Ridge, with model simulations suggesting that convergence- induced spin-up of a mesolow was responsible for initial thunderstorm development. Observations and model analyses show that thermal effects associated with Chesapeake Bay had a pronounced impact on storm evolution and rainfall distribution. Analyses of radar reflectivity and lightning observations suggest that the urban environment played a significant role in storm evolution and heavy rainfall distribution. Model analyses show that urban canopy effects from both the Washington DC urban region and the Baltimore urban region play an important role in determining the storm environment associated with heavy rainfall. Urban Heat Island effects did not play a significant role in the storm evolution. Observations from a vertically-pointing Lidar and disdrometer observations of raindrop distributions suggest that hygroscopic growth of aerosols may have played an important role in stimulating efficient precipitation growth mechanisms and production of extreme rainfall rates.