Observations of High Ground Flash Densities of Positive Lightning in Summertime Thunderstorms

Abstract

Observations of summertime thunderstorms indicate that positive polarity cloud-to-ground lightning activity can occur with rates as high as 67 flashes in 5 min and spatial densities up to 0.60 flashes per square kilometer per hour. All ground flashes in a storm may be positive for substantial periods. Using data from a nationwide network of magnetic direction finders, 24 storms with high ground flash densities of positive lightning were found on 11 days in June and July 1989 in the Great Plains of the United States. The periods of high-density positive lightning persisted an average of 4 h, longer than the lifetime of a typical single thunderstorm cell. In most cases, they occurred at or near the beginning of the storms' cloud-to-ground lightning activity. Supporting data suggest that the production of high rate and high percentage of positive ground flashes may be associated with exceptionally tall storms that exhibit a stage of early, rapid increase in radar echo-top height and product large hail.