Evolution of Cloud-to-Ground Lightning Characteristics and Storm Structure in the Spearman, Texas, Tornadic Supercells of 31 May 1990

Abstract

On 31 May 1990, four tornadic supercell storms formed sequentially near the intersection of a dryline and an outflow boundary in the northern Texas panhandle. ?Staccato? lightning flashes, which have been hypothesized to be positive ground flashes, were observed beneath the anvil of one storm during the most violent tornado that the storm produced. Evidence was found from a lightning mapping system that at least some of the staccato flashes were negative ground flashes. Although the four supercell storms on this day formed in approximately the same area, traveled over roughly the same region, and produced tornadoes and large hail, the relationship between the genesis and evolution of tornadoes and the polarity and flash rates of ground flashes varied widely, as in previous studies. The second of the supercell storms had low-precipitation supercell characteristics; the third and fourth did not. In previously studied storms, ground flash activity in low-precipitation supercell storms has always been dominated by positive ground flashes. However, all ground flashes detected in the second, low-precipitation storm were negative ground flashes. Positive ground flashes dominated ground flash activity in the third and fourth supercell storms for roughly their first hour, after which the dominant polarity switched to negative. In the third storm, the maximum positive ground flash rate before this polarity reversal was 1 min?1 and the most intense tornado produced by the storm occurred before the maximum positive ground flash rate. In the fourth storm, positive ground flash rates increased to 7.4 min?1 over a period of 30 min early in the storm, followed by a rapid decrease to 0 min?1 over the next 10 min; the most intense tornado produced by the fourth storm occurred during the lull in ground flash rates following the large maximum. These observations are consistent with a previously reported tendency for a storm dominated by positive ground flashes to produce its most violent tornado after it attains its maximum positive ground flash rate, whenever the rate is in excess of 1.5 min?1.