Urban, Topographic and Diurnal Effects on Rainfall in the St. Louis Region

F. A. Huff Illinois State Water Survey, Urbana 61801

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J. L. Vogel Illinois State Water Survey, Urbana 61801

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Abstract

Analyses were made of the summer rainfall distribution (June-August) in 17 areas within the METROMEX network in St. Louis region. These were selected to represent areas subject to various types and degrees of inadvertent weather modification, plus a no-effect (control) area. The purpose was to determine the magnitude and relative intensity of urban and topographic effects on rain enhancement. Results indicated the urban enhancement was greater than that associated with hills and river bluffs in the experimental region. The urban enhancement maximized northeast of the St. Louis urban-industrial complex where it was computed to be approximately 30–35%. The bluffs effect was calculated to be approximately 14%, and the hill effect was 9% for all rainstorms combined during the five summers. The urban and topographic enhancements were most pronounced in heavy storms, defined as those producing gage amounts of 25 mm or more. The enhancement maximized in June in the region of maximum urban effect, but occurred in July or August in other network areas subjected to urban or topographic influences. The urban effect was found to be associated with stimulation of rain output in ongoing storms, as opposed to an increase in frequency of storm events. Analyses of diurnal distributions showed that the major high in the Edwardsville area was produced by a double maximum of nearly equivalent magnitude in late afternoon and late evening. Further evidence was found that the urban effect is substantially greater than the topographic effect. During the diurnal peak rainfall periods, both the rainfall frequency and rainfall amounts were greater in the urban-effect areas, but the enhancement was produced largely by greater rain amounts.

Abstract

Analyses were made of the summer rainfall distribution (June-August) in 17 areas within the METROMEX network in St. Louis region. These were selected to represent areas subject to various types and degrees of inadvertent weather modification, plus a no-effect (control) area. The purpose was to determine the magnitude and relative intensity of urban and topographic effects on rain enhancement. Results indicated the urban enhancement was greater than that associated with hills and river bluffs in the experimental region. The urban enhancement maximized northeast of the St. Louis urban-industrial complex where it was computed to be approximately 30–35%. The bluffs effect was calculated to be approximately 14%, and the hill effect was 9% for all rainstorms combined during the five summers. The urban and topographic enhancements were most pronounced in heavy storms, defined as those producing gage amounts of 25 mm or more. The enhancement maximized in June in the region of maximum urban effect, but occurred in July or August in other network areas subjected to urban or topographic influences. The urban effect was found to be associated with stimulation of rain output in ongoing storms, as opposed to an increase in frequency of storm events. Analyses of diurnal distributions showed that the major high in the Edwardsville area was produced by a double maximum of nearly equivalent magnitude in late afternoon and late evening. Further evidence was found that the urban effect is substantially greater than the topographic effect. During the diurnal peak rainfall periods, both the rainfall frequency and rainfall amounts were greater in the urban-effect areas, but the enhancement was produced largely by greater rain amounts.

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