URBAN AND LAKE EFFECTS ON SUMMER RAINFALL IN THE CHICAGO AREA

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Abstract
Rainfall data from three summers were studied to discern possible urban and lake influences on precipitation processes and quantity over the Chicago region and southern Lake Michigan. The stabilizing influence of Lake Michigan on convective rainfall is considerable, with fewer raincells and lower rain rates than over adjacent rural areas. The lake effect is realized largely in less well-organized and weaker convective conditions, whereas squall lines do not exhibit any great diminishment in rainfall production over the lake. Average over-lake rainfall more than 40 km east of the western shore was reduced, estimated as being 36% less than the shoreline average. The urban influences on rain are most pronounced in well-organized convective systems, leading to localized increases in raincell initiations, mergers of raincells, and larger more intense raincells than occur over nearby rural areas. The net result is 15% more rain, on the average, over Chicago, and 10 to 20% more rain extending 40 km east over southern Lake Michigan. Results agree with those obtained from St. Louis studies.