Modeling the Transmission of Optical Lightning Signals Through Complex 3‐D Cloud Scenes. Issue 23 (27th November 2020)

• Record Type: Journal Article
• Authors: Peterson, Michael
• Is Part Of: Journal of geophysical research. Volume 125:Issue 23(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-27
• Subjects: lightning -- radiative transfer -- Monte Carlo modeling -- thunderstorms -- optical detection -- superbolts; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2020JD033231
• Languages: English
• ISSNs: 2169-897X

Abstract

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. Key Points: Lightning measurements from space have revealed complex interactions between optical emissions and nearby clouds Monte Carlo radiative transfer simulations are conducted to examine how complex cloud scenes are illuminated by lightning Modeling results support the suggested origins of irregular spatial radiance patterns and unobscured lightning producing superbolts