Holes in Optical Lightning Flashes: Identifying Poorly-Transmissive
Clouds in Lightning Imager Data
Abstract
Space-based optical lightning sensors including the Lightning Imaging
Sensor (LIS) and Geostationary Lightning Mapper (GLM) are pixelated
imagers that detect lightning as transient increases in cloud-top
illumination. Detection requires optical lightning emissions to escape
the cloud-top to space with sufficient energy to trigger a pixel on the
imaging array. Through scattering and absorption, certain clouds are
able to block most light from reaching the instrument, causing a
reduction in Detection Efficiency (DE). We use cases of radiant
lightning emissions that illuminate large cloud-top areas to examine
scenarios where clouds block light in only certain pixels on the imaging
array. In some cases, these anomalies in the spatial radiance
distribution from the lightning pulse leads to “holes” in the optical
lightning flash where certain pixels fail to trigger, entirely. Such
holes are identified algorithmically in the Tropical Rainfall Measuring
Mission (TRMM) satellite LIS record over the southern Continental United
States, and the microphysical properties of the coincident storm region
are queried. We find that holes primarily occur in tall (IR Tb
< 235 K) convection (87%) and overhanging anvil clouds
(10%). The remaining 3% of holes occur in moderate-to-weak convection
or in clear air breaks between stormclouds. We further demonstrate how
an algorithm that assesses the spatial radiance patterns from energetic
lightning pulses might be used to construct an optical transmission
gridded stoplight product for GLM that could help operators identify
clouds with a potentially-reduced DE.