Urban effects on the distribution of heavy convective rainfall

Abstract

Data from a network of 225 recording rain gages in 5500 km2, operated during the summers of 1971–1973 in the St. Louis area, were used to investigate potential urban effects upon the distribution of heavy rainfalls. The primary purpose was to determine quantitatively the effect of inadvertent weather modification upon the frequency and magnitude of short‐duration, intense rain rates that are pertinent to urban and suburban sewer design and of concern in applications of planned weather modification to alleviate agricultural and municipal water supply shortages. Emphasis was placed upon analysis of (1) rain cells, the basic convective unit responsible for the production of heavy short‐duration rates in thunderstorm‐dominated climates, such as the Midwest, and (2) the distribution of heavy rainstorms producing 25 mm or more of rainfall within the network. Comparison of urban‐affected and rural (nonaffected) rain cells was the primary evaluation tool in evaluating the urban effect. Results showed a substantially greater water yield from the urban‐affected cells compared with that from the rural (control) cells. The excess was greatest in moderate to heavy storms and apparently resulted most often from enhancement of the natural atmospheric rainfall processes in organized weather systems. The 25‐mm rainstorms showed a pronounced maximum of occurrence in a region that frequently lies downwind of two urban‐industrial areas, St. Louis and Alton‐Wood River. It was concluded that the frequency distribution of heavy rainfalls may vary significantly between urban, suburban, and rural areas in large urban‐industrial regions and that this may necessitate reevaluation of sewer design storm parameters in current use.