The Illumination of Thunderclouds by Lightning: Part 3: Retrieving
Optical Source Altitude
Abstract
Optical space-based lightning sensors such as the Geostationary
Lightning Mapper (GLM) detect and geolocate lightning by recording rapid
changes in cloud-top illumination. While lightning locations can be
determined to within a pixel on the GLM imaging array, these instruments
are not individually able to natively report lightning altitude. It has
previously been shown that thunderclouds are illuminated differently
based on the altitude of the optical source. In this study, we examine
how altitude information can be extracted from the spatial distributions
of GLM energy recorded from each optical pulse. We match GLM “groups”
with LMA source data that accurately report the 3-D positions of
coincident Radio-Frequency (RF) emitters. We then use machine learning
methods to predict the mean LMA source altitudes matched to GLM groups
using metrics from the optical data that describe the amplitude,
breadth, and texture of the group spatial energy distribution. The
resulting model can predict the LMA mean source altitude from GLM group
data with a median absolute error of < 1.5 km, which is
sufficient to determine the location of the charge layer where the
optical energy originated. This model is able to capture changes to the
source altitude distribution following convective invigoration or
maturation, and the GLM predictions can reveal the vertical structure of
individual flashes - enabling 3-D flash geolocation with GLM for the
first time. Additional work is required to account for differences in
thunderstorm charge / precipitation structures and viewing angle across
the GLM Field of View.