The properties of optical lightning flashes and the clouds they illuminate

Abstract

Optical lightning sensors like the Optical Transient Detector and Lightning Imaging Sensor (LIS) measure total lightning across large swaths of the globe with high detection efficiency. With two upcoming missions that employ these sensors—LIS on the International Space Station and the Geostationary Lightning Mapper on the GOES‐R satellite—there has been increased interest in what these measurements can reveal about lightning and thunderstorms in addition to total flash activity. Optical lightning imagers are capable of observing the characteristics of individual flashes that include their sizes, durations, and radiative energies. However, it is important to exercise caution when interpreting trends in optical flash measurements because they can be affected by the scene. This study uses coincident measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite to examine the properties of LIS flashes and the surrounding cloud regions they illuminate. These combined measurements are used to assess to what extent optical flash characteristics can be used to make inferences about flash structure and energetics. Clouds illuminated by lightning over land and ocean regions that are otherwise similar based on TRMM measurements are identified. Even when LIS flashes occur in similar clouds and background radiances, oceanic flashes are still shown to be larger, brighter, longer lasting, more prone to horizontal propagation, and to contain more groups than their land‐based counterparts. This suggests that the optical trends noted in literature are not entirely the result of radiative transfer effects but rather stem from physical differences in the flashes.

Mapping the Lateral Development of Lightning Flashes From Orbit

“…Because the footprint area and characteristic footprint diameter metrics describe the size of the cloud illuminated by the flash, they depend on the structure and energetics of the discharge as well as the geometry and scattering properties of the surrounding cloud medium. This sensitivity to the cloud scene made the flash footprint ideal for constructing LIS illuminated cloud features in Peterson, Deierling, et al () to examine connections between flash energetics and morphology and the microphysics of the cloud medium. For example, that study showed oceanic flashes to be more energetic than land‐based flashes even when they occur at the same time of day and in similar types of clouds.…”

“…We define these features as LIS “series,” and they are meant to describe distinct optical pulses that are separated by “dark” periods with no groups detected. These features result from physical lightning processes such as propagation (Peterson, Deierling, et al, ) or continuing current (Bitzer, ). A series feature is defined as any collection of groups within the same flash separated in time by no more than 1 LIS frame.…”

“…The flash skeleton, by contrast, tracks lateral motions between groups regardless of radiance. The size of the skeleton can be described in terms of the maximum separation of groups, as in Peterson, Deierling, et al (), the total length of all skeleton branches (total propagation distance) or the area of the convex hull (CHULL) around the skeleton (white line in Figure ). Flash footprint and skeleton areas are used to calculate characteristic length scales reported in kilometers.…”

“…The primary motivations for examining the frame‐by‐frame evolution of individual flashes have been to validate independent lightning measurements (Rudlosky et al, ) and to identify important components of the flash such as the return stroke (Koshak, ) or continuing currents (Bitzer, ). Changes in the extent, position, and radiance of the optical signals are indicative of such physical lightning processes, but these signals are modified by scattering in the cloud medium (Peterson, Deierling, et al, ; Thomson & Krider, ). Thomas et al () found an excellent spatiotemporal agreement between LIS observations and Oklahoma LMA sources for flashes that extended into the upper part of a storm.…”

Characteristics of the initial stage and return stroke currents of rocket‐triggered lightning flashes in southern China

“…The transitional zones or stratiform regions of the thunderstorms typically featured weak convection and low‐frequency natural lightning activity. Several recent studies have reported that thunderstorm characterized by weak updrafts or the special regions of thunderstorm far away from the updraft might feature larger size of or stronger lightning discharges, such as ocean thunderstorms [ Peterson et al ., ], morning thunderstorms [ Chronis et al ., ; Zheng et al ., ], stratiform areas of thunderstorms [ Wang et al ., ], and forward anvils of supercell thunderstorms [ Bruning and MacGorman , ; Zheng and MacGroman , ], as there might be relatively large charge regions. Therefore, the stronger discharge of the TLFs in southern China might be associated with the large‐scale thunderstorm systems in Guangdong and the special triggering location in the transitional zones or stratiform regions of these thunderstorms.…”