The Evolution and Structure of Extreme Optical Lightning Flashes

Abstract

This study documents the composition, morphology, and motion of extreme optical lightning flashes observed by the Lightning Imaging Sensor (LIS). The furthest separation of LIS events (groups) in any flash is 135 km (89 km), the flash with the largest footprint had an illuminated area of 10,604 km2, and the most dendritic flash has 234 visible branches. The longest‐duration convective LIS flash lasted 28 s and is overgrouped and not physical. The longest‐duration convective‐to‐stratiform propagating flash lasted 7.4 s, while the longest‐duration entirely stratiform flash lasted 4.3 s. The longest series of nearly consecutive groups in time lasted 242 ms. The most radiant recorded LIS group (i.e., “superbolt”) is 735 times more radiant than the average group. Factors that impact these optical measures of flash morphology and evolution are discussed. While it is apparent that LIS can record the horizontal development of the lightning channel in some cases, radiative transfer within the cloud limits the flash extent and level of detail measured from orbit. These analyses nonetheless suggest that lightning imagers such as LIS and Geostationary Lightning Mapper can complement ground‐based lightning locating systems for studying physical lightning phenomena across large geospatial domains.

Mapping the Lateral Development of Lightning Flashes From Orbit

“…As a result, then the flash footprint will be significantly wider (tangentially) than it is long (radially). The top LIS flash in terms of event separation from Figure 4 in Peterson, Rudlosky, et al () shows this exact case where the flash footprint even curves around the edge of the convective feature.…”

“…This structure describes 30% of all flashes. All of the complex flashes with extreme branch counts from Peterson, Rudlosky, et al () were propagating flashes. Of these, only 4.5% contain a single branch (Figure d) while 4 in 10 contain 20 or more branches.…”

“…Branch counts for LIS flashes are shown in Figures c and d. Though Peterson, Rudlosky, et al () found extreme cases with multiple hundreds of branches, these flashes are rare as 90% of flashes contain less than 20 branches. The most frequent flash skeleton type is a linear feature that consists of a single branch.…”

“…Not only will weak optical signals at cloud top that fall below the instrument background threshold be excluded but also particularly radiant pulses can illuminate a large area of cloud far beyond the extent of the flash before or after that point. The largest of these occur near the edges of convective cloud features where the optical energy escaping the side of the cloud can illuminate the top of a lower cloud deck (Peterson, Rudlosky, et al, ). When mapping optical flashes with optical sensors, it is necessary to distinguish between flash developments that result from physical lightning processes—that is, strokes, K‐changes, and leader development—and those that occur due to scattering across the cloud scene.…”

Meteorological Imagery for the Geostationary Lightning Mapper

“…The qualitative meteorological interpretation suggested that lightning in the trailing, more stratified clouds to the rear of the convective line was larger. Utilizing LIS data, Peterson et al () and Peterson et al () introduced the idea of propagating flashes and suggested that they were a separate class of discharge where group centroids moved in a preferential direction away from the first group so as to elongate the flash footprint. They also diagnosed another class of very bright flash with large illuminated area, so‐called “superbolts,” in which the group centroids did not propagate.…”