The Urban-related Nocturnal Rainfall Anomaly at St. Louis

Abstract

Studies during the Metropolitan Meteorological Experiment (METROMEX) sought to define influences of St. Louis on the summer atmosphere that led to alterations in rainfall. These studies defined how city influences caused an afternoon maximum of rainfall cast of the city. Rain data indicated a second rain maximum northeast of the city during the 2000?2400 CDT period. Study of this nocturnal maximum revealed a 58% localized rain increase, relative to the mean rainfall in the 5200 km2 network. The anomaly was present in all summers from 1971?1975. The northeast rain maximum is preceded by a local increase beginning 2 h earlier and 30 km west over the urban-industrial area. Most northeast anomaly-related storms were found to move either from the southwest (from over the urban area) or from the west-northwest (from a major industrial area), and to produce heavy rainfall rates; 19 storms moved from St. Louis between 2100?2400 and these produced 69% of the rainfall in the maximum rainfall area. The afternoon and nocturnal maximums both occurred when the entire area was receiving relatively heavy rainfall indicating that urban influences are most effective during relatively heavy rainfall conditions. All of the nocturnal anomaly rainfall occurred with well-organized convective systems. The individual convective raincells which led to heavy rainfall in the anomaly typically began over the urban industrial area and ended in the anomaly area. The raincell areas, volumes, and intensifies were much greater than rural raincells. Collectively, the results strongly suggest that the nocturnal anomaly is a result of urban influence that affect a few of the heavier rain events.