Precipitation Increases in the Low Hills of Southern Illinois: Part 1. Climatic and Network Studies

Abstract

Long-term precipitation records indicated that, on the average, 15% more warm season precipitation falls on the forested western Shawnee Hills of southern Illinois than falls on the rural farm flatlands at 120 m lower elevations both north and south of the hills. This precipitation difference with relatively little elevation change offered an interesting opportunity to study the effect of orographic and land-use differences upon convective precipitation. Initially, two methods differing in scale and time were used to delineate the bill anomaly and to investigate its causes. Extensive climatic studies of all available precipitation data revealed that the effect of the hills was most pronounced during the warm season when showers and thunderstorms are the major source of precipitation. A subsequent 5-yr study involving a dense recording raingage and wind recording network showed that the hill-related increases apparently came through enhancement of heavy showers, particularly those associated with squall-line and cold-frontal conditions. This led to an intensive field study in July 1970 described in the companion paper (Part 2).