Effects of City Size on Thunderstorm Evolution Revealed Through a Multi-Radar Climatology of the Central United States

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Five years of 0.01° latitude x 0.01° longitude multi-radar multi-sensor grids of composite reflectivity and vertically-integrated signals from maximum expected size of hail (MESH) and vertically integrated liquid (VIL) were created to examine the role of city size on thunderstorm occurrence and strength around four cities: Dallas/Ft. Worth, TX; Minneapolis/St. Paul, MN; Oklahoma City, OK; and Omaha, NE. A storm-tracking algorithm identified thunderstorm areas every minute and connected them together to form tracks. These tracks defined the upwind and downwind regions around each city on a storm-by-storm basis and were analyzed in two ways: (1) by sampling the maximum value every 10 min. and (2) accumulating the spatial footprint over its lifetime. Beyond examining all events, a subset of events corresponding to favorable conditions for urban modification were explored. This urban favorable (UF) subset consisted of non-supercells occurring in the late afternoon/evening in the meteorological summer on weak synoptically forced days. When examining all thunderstorm events, regions at variable ranges upwind of all four cities generally had higher areal mean values of reflectivity, MESH, and VIL compared to downwind areas. In the UF subset, the larger cities (Dallas/Ft. Worth and Minneapolis/St. Paul) had a 24-50% increase in the number of downwind thunderstorms, resulting in a higher areal mean reflectivity, MESH, and VIL in this region. The smaller cities (Oklahoma City and Omaha) did not show such a downwind enhancement in thunderstorm occurrence and strength for the radar variables examined. This pattern suggests that larger cities could increase thunderstorm occurrence and intensity downwind of the prevailing flow under unique environmental conditions.

Publication Title

Journal of Applied Meteorology and Climatology

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American Meteorlogical Society