Spatial and Temporal Patterns of Thunderstorm Events that Produce Cloud-to-Ground Lightning in the Interior Southeastern United States

Abstract

Cloud-to-ground (CG) lightning data are used in this study to trace the daily patterns of thunderstorms in time and space across the topographically diverse southeastern United States. Four reoccurring patterns of thunderstorms (i.e., local, multilocal, regional, and widespread) are identified on the basis of the size of the region of CG lightning as well as the spatial pattern of the flashes within this region. To identify these patterns, hourly maps of CG flashes are produced over five summer seasons (June?August) and used to identify thunderstorm events on all days in which at least one CG lightning is observed. Thunderstorm events are defined by a temporally and spatially clustered hourly pattern of lightning flashes. The spatial pattern of lightning associated with each event is examined during the hour in which the flash density is the highest and is used to classify the event. The geographical and temporal patterns of each thunderstorm type are described. Also, flash densities are calculated at spatial scales ranging from 1- to 100-km radial distance. Over half of the identified thunderstorm events in the study were confined to the local scale and contained relatively few flashes. They were most common early in the morning and in the mountainous portions of the study area. Widespread events, on the other hand, showed a dense coverage of flashes within a given hour over a majority of the area. Although they occurred much less frequently (i.e., once every 8 days across most locales), they were responsible for the highest number of CG lightning flashes in the study region; furthermore, they produced the highest flash densities, both at the local and regional scale. A radar echo classification revealed that these events were not tied to mesoscale convective systems, but rather to the early afternoon development of numerous convective cell clusters and lines across the study area.