A Hypothesis for Urban Rainfall Anomalies

Abstract

METROMEX was the first major field program aimed at studying the reality and causes of urban rainfall anomalies suggested in several climatological studies. The results from the 1971?74 METROMEX data portray statistically significant increases in summer rainfall, heavy (>2.5 cm) rainstorms, thunderstorms and hail in and just east (downstorm) of St. Louis. Examination of the rainfall yield of individual showers (cells), the spatial distribution of echo (rain) developments, and areal distribution of afternoon rain clearly point to the urban-industrial complex as the site for the favored initiation of the rain process under certain conditions. The greater frequency of rain initiations over the urban and industrial areas appear to be tied to three urban-related factors including thermodynamic effects leading to more clouds and greater incloud instability, mechanical and thermodynamic effects that produce confluence zones where clouds initiate, and enhancement of the coalescence process due to giant nuclei. Case studies reveal that once additional cells are produced, nature, coupled with the increased likelihood for merger with more storms per unit area, takes over and produces heavier rainfalls. Hence, the city is a focal point for both rain initiation and rain enhancement under conditions when rain is likely.