How Hard Can It Rain? Cloudbursts of the Mid-Atlantic
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Dr. James Smith
William and Edna Macaleer Professor of Engineering
Monday December 21, 2020, 2 PM
The world record 1-minute rainfall accumulation of 31 mm occurred in Unionville, Maryland on July 4, 1956. Unionville is 30 km northwest of the setting for “cloudbursts” on July 30-31, 2016 and May 27, 2018 which produced catastrophic flooding in Ellicott City, Maryland. We use polarimetric radar analyses to examine how it rains hard at short time intervals (5 -180 minutes) and small spatial scales (1 – 100 km2). Analyses center on 10 storms in the Mid-Atlantic region which – like the Ellicott City storms – produced rainfall at 5 – 180 minute time scale approaching or exceeding 1000 year return interval accumulations. Analyses focusing on exceptional cloudbursts in the Mid-Atlantic are grounded in climatological assessments of extreme rainfall based on a 21-year, high-resolution (1 km, 15 minutes) radar rainfall record and cloud-to-ground lightning observations over the same time period. We examine the “environment” of cloudburst rainfall in terms of physical processes that control the structure, motion and evolution of convective elements that produce extreme rainfall over short durations and small areas. We also examine the role of “complex terrain” – land water boundaries, urban land cover and mountainous terrain – in determining the pronounced spatial heterogeneities of rainfall extremes in the Mid-Atlantic. Recent cloudbursts in the Mid-Atlantic have sharpened interest in assessing the potential for increasing frequency of cloudburst rainfall in a warming climate; we will point to key questions needed to assess changing extremes of short-duration rainfall.
Jim Smith is the William and Edna Macaleer Professor of Engineering at Princeton University. He received a BS in Mathematics from the University of Georgia and PhD in Environmental Engineering from The Johns Hopkins University. Smith’s research interests concern the hydrology and hydrometeorology of extreme floods. He is a Fellow of the American Geophysical Union (AGU) and American Meteorological Society (AMS) and received the Hydrologic Sciences Medal of AMS.
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