New 3D Cloud Optical Scattering Simulations to Inform Development of Future Satellite-Based Lightning Mapping Missions.

Recent decades have seen a marked increase in the lightning science community’s ability to observe lightning from space. In particular, the use of optical imagers to monitor global lightning activity has improved both in the capabilities of the sensors and, when considering all the sensors together, in coverage area. However, there is still much to learn about the effects the cloud medium has on light as it travels from the lightning source to the detector, and this is an area of much active research. That is, how does the cloud’s structure and composition affect scattering of light from lightning? And, ultimately, what is the relationship between light detected by space-based imagers and the lightning source?

In pursuit of understanding lightning observations made by space-based imagers and what can be inferred about the cloud based on those observations, we investigate how changes in the cloud and lightning properties affect the light that escapes the top of the cloud. We use the Collaborative Model for Multiscale Atmospheric Simulation (COMMAS) to simulate an isolated thunderstorm, at high resolution (250-m horizontal grid spacing) based on an environmental sounding from a summer in Florida during the Convection and Precipitation/Electrification Experiment (CaPE). Lightning flashes are parameterized with a stochastic breakdown model to simulate realistic-looking, three-dimensional (3D) branched lightning channel geometry. The simulated storm was relatively small, producing lightning flashes for 24 minutes at a fairly low flash rate. To model the resultant optical emissions from the simulated lightning channels, we use the 3D cloud microphysics to constrain a Monte Carlo-based model of photon scattering within the cloud. The magnitude and location of light exiting the cloud is examined on a flash-by-flash basis to explore the sensitivity to the scattering approach as well as variations in flash and cloud properties (e.g., energy dissipated, propagation direction and length, polarity, ice water content, optical depth). We do these scattering simulations at wavelengths of 777.4-nm, which is the obligatory channel used by satellite-based detectors that map lightning, and 337-nm, which is a relatively new channel for satellite-based lightning mapping and also being considered for use in future missions to extract new information about the cloud properties through which the light propagates.