Lightning characteristics relative to mesoscale convective system decay in the Central and Southeast United States

Abstract

The launch of the next generation of geostationary satellites with the Geostationary Lightning Mapper makes it possible to analyze lightning flash characteristics over the life cycle of large-scale convection. The relationship between lightning activity trends and storm strength provides insights into thunderstorm growth and decay using lightning data as a diagnostic and predictive tool. Infrequent lightning occurrence in stratiform regions of mesoscale convective systems (MCSs) is an under realized hazard to people, aviation, and defense.

This study selected twenty-four MCS cases from the warm seasons of 2020 through 2022 for locations east of the Rocky Mountains in the contiguous United States (CONUS) to examine the partitioning of lightning within the convective, stratiform, and “hybrid” convection to stratiform mixed regions to diagnose the tendencies of lightning propagation between the convective and stratiform regions within MCSs. 94.3% of all flashes in the 24 MCS case database originated from a location with reflectivity values greater than 30 dBZ and 70.1% of the largest 5% of flashes are originating in regions of 40 dBZ reflectivity or higher.

This study shows that the flash frequency and flash size relationship, similar to other studies on more isolated convective, extends to the largest size flashes in MCSs. During peak flash frequency, the largest flash sizes occur in the stratiform regions of MCS systems because of active charging from the convective and stratiform regions. As the convective portions of the system decrease, the result is fewer flashes propagating from the convection to the stratiform region which allows larger electric field build up within the stratiform area. This weakening leads to a reduction in charge advected rearward from the convective line into the stratiform, leaving only the in-situ charge separation mechanism within the stratiform region.

Radar-derived microphysical characteristics were compared with large lightning flashes during the mature to weakening stages of the 5 May 2021 MCS in the southeastern United States to understand the microphysical underpinnings for large, infrequent, stratiform MCS flashes that pose a large safety threat. The results presented in this study demonstrate that graupel does not need to be present at the location of a lightning flash initiation with a well-established electric field in MCSs. Six of the largest 5% of flashes initiate in stratiform regions with little to no graupel mass. These flashes initiate in locations along the leading edge of the stratiform, close to convective regions where charged hydrometeors are advected rearward from the forward propagating convective line. Two examples were presented to demonstrate how the loss of depolarization artifacts in ZDR signaled the last lightning flash in specific portions of the MCS. The inclusion of depolarization streaks in ZDR capture areas with strong electric fields where infrequent lightning may initiate and propagate.

Algorithms that seek to automatically identify lightning occurrence for lightning safety should consider the inclusion of include depolarization streaks for infrequent flashes as well as the traditionally included metrics for graupel. These inclusions will create more robust algorithms to support better lightning predictions for the benefit of safety.