Holes in Optical Lightning Flashes: Identifying Poorly-Transmissive Clouds in Lightning Imager Data

Abstract

Recent analyses of geostationary lightning mapper (GLM: Goodman et al., 2013;Rudlosky et al,. 2019) observations from NOAA's Geostationary Operational Environmental Satellites (GOES) have revealed that while GLM meets its required specifications for detection over 24 h (Bateman & Mach, 2020), there are drastic reductions in DE in certain storms compared to ground-based radio-frequency lightning locating systems (i.e., Bitzer, 2019;Rutledge et al., 2019;Said & Murphey, 2019;Thomas, 2019). Differences in instrument performance between GLM and the ground networks has been attributed to detection physics. Radio-Frequency (RF) emissions from lightning escape the cloud unimpeded, while the optical emissions that GLM measures interact with the cloud medium through scattering and absorption. Computational models have shown how optical emissions are diluted in space and delayed in time as the result of scattering in various cloud geometries (

The Illumination of Thunderclouds by Lightning: 3. Retrieving Optical Source Altitude

“…Our discussion of “optical repeater” flashes in Peterson et al. (2021a) and previous analyses of groups with complex spatial radiance distributions (Peterson, 2020b) further showed that radiance patterns were consistent between subsequent illuminations of the same cloud layer. However, these pictures of cloud illumination would change if the flash moved into a different layer, for example, during cases in Peterson et al.…”

“…At the same time, radiative transfer effects can also cause groups to underestimate the scale of the lightning source if the cloud is able to block radiant energy from reaching orbit. In extreme cases, particularly opaque clouds generate “holes” in the group footprint where the cloud regions surrounding the poorly transmissive cloud are illuminated while its center remains dark and free of events (Peterson, 2020b).…”

Combined Optical and Radio‐Frequency Perspectives on a Hybrid Cloud‐To‐Ground Lightning Flash Observed by the FORTE Satellite

“…We have noted this behavior before with LIS flashes (Figure 1 in Peterson & Liu, 2013 is one example) where coincident radar data show that such optical holes correspond to dense convective cells embedded in the flash footprint that seem to block radiance from reaching the satellite. Dark pixels causing “holes” in otherwise contiguous flash footprints during intense low‐altitude processes provides further evidence that clouds modify the optical signals recorded from orbit – even to the point of preventing detection (Peterson, 2021). The pixels corresponding to the hole were illuminated during earlier periods of the flash dominated by high‐altitude IC pulses (including in Figure 7c), and thus the hole is not observed in Figure 4c.…”

The Illumination of Thunderclouds by Lightning: 1. The Extent and Altitude of Optical Lightning Sources

“…The horizontal extent of the illuminated lightning channels and their vertical altitudes are not consistent between flashes – or even at different times within the same flash. Spatial variations in cloud composition cause the optical emissions to preferentially transmit through certain cloud regions compared to others. An extreme case of this is when “holes” occur in LIS or GLM groups where the clouds surrounding a particularly opaque region are simultaneously illuminated while the central region remains dark (Peterson, 2020b). This occurs in both dense convective clouds and with overhanging anvil clouds that are illuminated from below. If the optical emissions encounter a cloud boundary, they can access neighboring clouds and take a “shortcut” path to the satellite compared to transmitting through the full optical depth of cloud above the source (Peterson, 2020a).…”