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Publication Abstract

Studies of Selected Precipitation Cases from METROMEX Editors: Changnon, Stanley A., Jr., and Richard G. Semonin, 1975  Illinois State Water Survey, Champaign, IL,  ISWS RI-81    Full Text Available

In the analysis of METROMEX field data, nine rain periods (case days) in 1972 and 1973 were chosen for in-depth investigations aimed to better define the urban-related mechanisms that cause the summer rainfall in and east of St. Louis and Wood River to be greater than that in surrounding areas. The nine cases were selected to represent a variety of summer precipitation conditions in the St. Louis area. These investigations identified several urban-related conditions that contributed to the observed precipitation alterations. Principal findings were that the urban areas: 1) initiated and thus increased the number of raincells, 2) intensified rain and severe weather from the urban-initiated cells by their subsequent merger with each other or a later system, 3) intensified rain and severe weather from existing cells passing over the urban areas by the effect of their heat, moisture, and/or aerosol plumes, 4) produced these effects only when rain-conducive synoptic conditions existed, and 5) operated with a 'domino' effect in which urban cells beget more urban cells on the same day and tend to reinforce developments in a multi-day sequence in the same locale. The case studies also revealed missing or inadequate measurements critical for METROMEX or other field projects and provided guidance for direction of the research.

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