Modeling the Transmission of Optical Lightning Signals Through Complex 3‐D Cloud Scenes

Abstract

Space-based lightning imagers have shown that complex cloud scenes consisting of multiple tall convective features, anvil clouds, and warm boundary cloud layers are illuminated by lightning in many different ways depending on where the lightning occurs and how energetic it is. Modifications to the optical lightning signals from radiative transfer in the cloud medium can lead to reductions in detection efficiency and location accuracy for these instruments and can also cause some of the optical signals that are detected to have unexpected spatial energy distributions. In this study, we perform Monte Carlo radiative transfer simulations of optical lightning emissions in clouds with complex 3-D geometries to shed some light on the origins of certain irregular radiance patterns that have been recorded from orbit. We show that diffuse reflections off nearby cloud faces can explain lightning signals in nonelectrified clouds, tall clouds can result in poor optical transmission and suppressed radiances that could lead to missed events and that particularly favorable viewing conditions can cause otherwise normal lightning to produce a superbolt that is orders of magnitude brighter than the same flash seen from a different angle. Plain Language Summary Lightning is detected from space using instruments that report rapid changes in cloud brightness from lightning illumination. However, this light can be modified by scattering and absorption in the cloud. Scattering off water drops causes portions of the signal to be diluted in space and delayed in time. What starts off as an impulsive point light source in the cloud may illuminate a region of the cloud-top that is 100 km across with a waveform that persists over significant fraction of a millisecond. Interactions between the optical lightning emissions and the cloud scene are particularly complex when the surrounding clouds do not take on a simple geometric shape. Clouds observed in nature often contain multiple vertical layers including warm boundary clouds and overhanging anvils. Understanding some of the more irregular spatial energy distributions recorded by space-based lightning sensors requires accounting for these complex geometries. In this study, we develop 3-D cloud models that approximate cloud structures found in nature and perform Monte Carlo radiative transfer simulations of how they are illuminated by lightning. In doing so, we confirm the suspected origins of irregular cloud illumination, such as reflections off of nearby cloud faces or particularly favorable viewing conditions allowing normal lightning to appear highly energetic.

Blue Flashes as Counterparts to Narrow Bipolar Events: The Optical Signal of Shallow In‐Cloud Discharges

“…10.1029/2021JD035013 6 of 13 (Peterson, 2020;Platt, 1997) also lead to absorption times significantly longer than the duration of our events. Hence here we assume τ A ≫ τ D .…”

“…The cloud tops are dominated by ice particles, which absorb radiation at 337 nm several orders of magnitude less efficiently than water (Warren & Brandt, 2008). Besides, the available estimates of the extinction coefficient (Peterson, 2020; Platt, 1997) also lead to absorption times significantly longer than the duration of our events. Hence here we assume τ A ≫ τ D .…”

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

“…The trends in Figure 8 might be explained by three simultaneous factors: (a) small dim groups originating in the lower charge layer being attenuated to the point where GLM does not easily resolve them, (b) the additional optical depth available for scattering broadening the optical signals from the lower charge layer -leading to larger groups (as we saw in Figure 7f of Peterson, 2020a), and (c) large, energetic groups arising from lightning at the edge of the thunderstorm where the optical signals can transmit through/ reflect off of thinner cloud layers to reach the satellite.…”

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

“…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 (Koshak et al., 1994; Light, Suszcynky, & Jacobson, 2001; M. Peterson, 2020; Thomson & Krider, 1982).…”