HYPOTHESIS FOR URBAN RAINFALL ANOMALIES.

Abstract

The results from the 1971 - 74 METROMEX data portray statistically significant increases in summer rainfall, heavy ( greater than 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 in-cloud instability, mechanical and thermodynamic effects that produce confluence zones where clouds initiate, and enhancement of the coalescence process due to giant nuclei. The city is a focal point for both rain initiation and rain enhancement under conditions when rain is likely.