How Hard Can It Rain? (Invited Presentation)

The largest measured rainfall rate at 1-minute time scale for the US of 1875 mm/h occurred in Unionville, MD on 4 July 1956. The 19 July 1942 Smethport, PA storm produced a measured 10-minute rainfall rate of 1000 mm/h. The D’Hannis, Texas storm of 31 May 1935 produced the world record rainfall rate of 203 mm/h over a time interval of 2.75 hours. These and other observed storms provide evidence on how hard it can rain and how it rains hard. The observational record shows that thunderstorms are the principal agents of these extreme rainfall rates and the most intense forms of convection, supercell thunderstorms, are agents of at least some. We will formulate and examine the hypothesis that supercell thunderstorms produce the largest short-term rainfall rates in the US. We will examine the physical processes that control the maximum instantaneous point rainfall rates from supercell thunderstorms and the spatial and temporal distribution of extreme rainfall from supercell thunderstorms through analyses of polarimetric radar fields and through numerical experiments based on the Weather Research and Forecasting model. We will use these analyses to formulate a conceptual model of extreme rainfall rates from supercell thunderstorms. Analyses of the 31 May 2013 and 6 May 2015 tornadic supercell storms in Oklahoma will be used to illustrate the structure and evolution of extreme rainfall rates and the physical processes that control how hard it can rain.

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