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Effects of St. Louis on Convective Cloud Heights

Roscoe R. Braham Jr.Cloud Physics Laboratory, The University of Chicago, Chicago, Ill. 60637

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Daniel WilsonCloud Physics Laboratory, The University of Chicago, Chicago, Ill. 60637

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Abstract

Heights and locations of the tallest 3 cm radar echoes at each half-hour through the convective parts of 140 summer days were measured during Project METROMEX. Comparison of the area-weighted frequencies of echoes occurring over and downwind of St. Louis, Mo., with those over a large nearby rural area, shows a substantial enhancement in the frequency of tall echoes over the city and near-down-wind areas. This enhancement comes mainly during the late morning and early afternoon and possibly again during the evening hours. The data also show a substantially different echo height distribution over urban and rural areas. Whereas the rural height distribution is distinctly bimodal, the urban height distribution shows no such bimodal character. This means that urban clouds are frequently able to penetrate mid-level arresting levels which limit the growth of rural clouds. These observations suggest an important role for urban-enhanced cloud dynamics in causing the St. Louis rainfall anomaly.

Abstract

Heights and locations of the tallest 3 cm radar echoes at each half-hour through the convective parts of 140 summer days were measured during Project METROMEX. Comparison of the area-weighted frequencies of echoes occurring over and downwind of St. Louis, Mo., with those over a large nearby rural area, shows a substantial enhancement in the frequency of tall echoes over the city and near-down-wind areas. This enhancement comes mainly during the late morning and early afternoon and possibly again during the evening hours. The data also show a substantially different echo height distribution over urban and rural areas. Whereas the rural height distribution is distinctly bimodal, the urban height distribution shows no such bimodal character. This means that urban clouds are frequently able to penetrate mid-level arresting levels which limit the growth of rural clouds. These observations suggest an important role for urban-enhanced cloud dynamics in causing the St. Louis rainfall anomaly.

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