WARM SEASON THUNDERSTORMS AND THE FLOOD CLIMATOLOGY OF URBAN ENVIRONMENTS

The climatology of warm season thunderstorms is closely linked to the climatology of flash floods in urban regions of the eastern US. An important consequence of urbanization is to increase the sensitivity of urban watersheds to the distribution of short-duration (1 - 60 minutes) rainfall rates. The distribution of extreme short-duration rainfall rates, which are dominated by warm season thunderstorm systems in the eastern US, is poorly characterized in urban environments. Climatological analyses of thunderstorms and flash floods in the Baltimore metropolitan region are based on volume scan reflectivity observations from the Sterling, Virginia WSR-88D radar, cloud-to-ground (CG) lightning observations from the National Lightning Detection Network (NLDN) and discharge observations from a dense network of stream gaging stations. Analyses address the alteration of thunderstorm rainfall associated with the effects of urbanization (including frictional effects associated with urban canopy, urban heat island effects and the effects of urban aerosols on storm microphysics and dynamics). It is shown that there are pronounced spatial heterogeneities in thunderstorm rainfall over the Baltimore-Washington region. It is also shown that warm season thunderstorm systems that produce flash floods in Baltimore, exhibit characteristic life cycles that include storm initiation over the Blue Ridge, propagation and upscale growth over the Piedmont region, and interaction with the urban environment and sea breeze circulation systems in the Baltimore metropolitan region. Observations of lightning and convective rainfall also show pronounced year-to-year variation, with the period from 2003-2006 characterized by an unusually high frequency of thunderstorms and flash flooding. Implications for urban flood response and the evolution of urban drainage networks are examined.