Merging of Convective Clouds: Cloud Initiation, Bridging, and Subsequent Growth

Abstract

This study examines the growth of radar echoes from the time of their initiation to several minutes after they have merged to ascertain what factors are important in determining the frequency of merging events, the manner in which echo cores join together, and the, effect of merging on subsequent echo core growth. Three-dimensional radar reflectivity data were examined for two convective periods, 25 July 1986 and 26 August 1986. It was found that echoes that merged were initially taller and slightly larger than those that dissipated without merging, suggesting they were more vigorous and thus more likely to grow and join with another echo. While meteorological conditions on the two days were quite different, 25% of the mergers occurred between young echoes, 65% occurred between a young echo and a parent storm, and 10% occurred between echo cores from two different systems. Previous case studies have indicated that merging is accomplished through differential storm motion or through the growth of a new echo core between adjacent cores. Echo cores in this study appeared to merge primarily through horizontal expansion. Two mechanisms were proposed to account for this expansion: moisture-laden oufflow air may have resulted in the presence of precipitation-sized drops at low altitudes in areas bridging the cores, and an intercell flow mechanism may have been at work at middle and upper levels. About 45%-70% of the cores were growing just after merger. Generally, the cores that grew were young, were growing prior to or at the time of merger, and thus were likely to continue growing.