Cell Development and Merger in an Illinois Thunderstorm Observed by Doppler Radar

Abstract

A reflectivity and triple-Doppler radar study of the development of several cells and their successive union within a nonsevere thunderstorm is presented. Two characteristic separations were found between the newly formed cells and the parent thunderstorm, with the closer cells forming in response to the collapse of an active cell and the more distant cells forming in a previously existing storm-modified area characterized by mesoscale convergence and rain cooled air. The manner in which these cells evolved appeared to be partially related to differences in the environment in which they formed. As suggested by Peterson, the cells that formed closer to the main storm resembled the ?weakly evolving? cells of Foote and Frank. The updraft of the ?weakly evolving? cell analyzed here merged with the updraft in a cell in the main storm as one cell was decreasing in intensity and the other was increasing. Later in the life cycle of the storm, two cells which initially formed further away from the main storm appeared more like classical ?strongly evolving? cells. While the vertical air velocity analyses of these cells were incomplete, a trend towards the maintenance of a discrete cell updraft was noted. The ways in which the reflectivity cores of these two cells became merged with the main storm differed. In one case the development of a new cell between two existing cells produced the merger, in the second case differential cell motion played an important role. Additionally, periods of significant intercell flow at 4 km coincided with the times when the midlevel reflectivity bond linking the cell cores showed a rapid intensification. It is proposed that the intercell flow is a result of radial outflow observed at heights above the maximum updraft level in the actively growing echoes. The strengthening of the reflectivity bridge may have been the result of both particle transfer and environmental modification brought about by this radial outflow.