PRECIPITATION AND COLD AIR PRODUCTION IN MESOSCALE THUNDERSTORM SYSTEMS

Abstract

This paper presents a proposed mechanism of cold air production associated with precipitation. A dome of cold air is produced by the evaporation of raindrops falling beneath the cloud base. A quantitative relationship between the evaporated rain and the produced excess mass of cold air was obtained, which showed that the mass is directly proportional to the evaporation. The coefficient of proportionality is a dimensionless number which varies between 0 and 1 depending on the temperature lapse rate originally existing beneath the cloud base. Results of mesoanalyses of squall lines and thunderstorms were used to estimate the actual amount of evaporation. The mass ratio of evaporated rain to the surface rain was found to increase with the height of the cloud base, reaching 1.0 at a cloud base of 9000 ft. In-cloud evaporation obtained by Braham (1952) showed a very good agreement with the values obtained in this study.