Climatological Assessment of Urban Effects on Precipitation at St. Louis

Abstract

An investigation was made of urban effects on precipitation in and downwind of St. Louis through use of long-term climatic records within a radius of 50–75 mi of the city. Based upon radar climatological studies, two potential urban effect areas and two no-effect (control) areas were designated. Analyses were then performed to assess the urban effect on 1) total monthly and seasonal precipitation; 2) the frequency and intensity of daily precipitation; 3) the frequency of thunderstorms, hailstorms, and severe rainstorms; 4) wet-dry period rainfall; and 5) weekday-weekend precipitation (indicator of industrial-induced effect). Evidence of urban effects was found in all seasons, but appear to he strongest in late spring and summer. Results indicated an urban-induced increase in average summer rainfall ranging from 6 to 15% for distances up to 25 mi downwind of the city. The urban-increase mechanism apparently is most active on days of moderate to heavy intensifies in the natural rainfall. The urban effect was most pronounced in relatively wet summers, and indications of a suppression effect were found in dry summers. Evidence was found of an urban-induced increase in thunderstorm days within the city and of hail-day maxima downwind of the city. Overall, results appear to support thermal effects as the most important causative factor for urban modification of the natural precipitation distribution.