Positive Ground Flashes Produced by Low-Precipitation Thunderstorms in Oklahoma on 26 April 1984

Abstract

A group of thunderstorms developed in western Oklahoma during the afternoon of 26 April 1984. Two of these storms initially exhibited characteristics of low-precipitation (LP) thunderstorms. Lightning ground flashes produced by these storms were mostly positive. These storms split, with one right-moving component evolving into a tornadic supercell. Ground flashes produced by the supercell, however, were predominantly negative. The highest rate of positive ground flashes (1.5 min?1) occurred during LP storm splitting and merging, when about 84% of ground flashes were positive. The maximum total ground-strike rate was 3.4 min?1and occurred during the tornadic supercell phase and when all but one of 136 ground flashes were negative. Analysis of lightning ground-strike and radar reflectivity data reveals a concentration of positive ground flashes in areas of maximum reflectivity within the LP storms; furthermore, the concentration of positive flashes appeared during storm split. After storm splitting and merging, the number of positive ground flashes in all cells decreased. Recent studies suggested a relationship between high values of wind-shear magnitude within the cloud-bearing layer and the production of positive ground flashes. Analyses of soundings in the environments of the LP and supercell thunderstorms on this day show that the magnitude of the vector-averaged shear vector within the cloud-bearing layer was 4.8 ? 10?3 and 3.8 ? 10?3 s?1 respectively. Both are above the previously published thresholds hypothesized for positive flash production. Thus, our analysis suggests that strong shear may be a necessary, but not sufficient, condition for the production of positive ground flashes. Contrary to earlier reports in the literature, our data indicate the height of the ?10°C isotherm is not a key parameter for positive flash production in warm-season convection. Finally, a link between positive ground flashes and hail is again suggested.