A First Look at Cloud Inhomogeneity and Its Effect on Lightning Optical Emission

Optical emission observed from lightning is the result of in-cloud scattering, lightning channel geometry, and optical energy of the source discharge. To better understand how scattering affects measured optical emission, we developed a Monte Carlo model using an inhomogeneous scattering profile from a simulated storm using two-moment bulk microphysics in the Weather Research and Forecasting model, including the concentration and size distribution of cloud droplets and ice particles. Within the Monte Carlo model lightning discharges are simulated as point and line sources. These idealized simulations indicate that for a lightning discharge at 7km altitude approximately 22% and 13% of light, emitted near and away (respectively) from a reflectivity core will reach cloud top in a thunderstorm. The vertical gradient of the microphysical environment impacts light reaching cloud top regardless of overall particle concentration. Increasing cloud ice modifies the gradient of the microphysical profile and modulates the profile gradient effect. Plain Language Summary Measurements of light emitted by lightning are impacted by the cloud, the shape of the lightning channel, and the brightness of the lightning event. In this study we work to understand the impact each of these has on the fraction of the light that reaches cloud top. Understanding the fraction of the light reaching cloud top in a thunderstorm and where it occurs can help us understand how satellite lightning imagers are performing and how light propagates in a cloud. We develop a model using randomization of the scatterers and a cloud adapted from a simulated thunderstorm. The model includes a range of water droplets, ice particles, and their aggregates that change with height and location in the storm. More simulated light is detected at cloud top when lightning is near the center of the thunderstorm and approximately one third less light reaches cloud top when the lightning is located away from the main part of the thunderstorm. The difference between the locations is due to how the scattering length increases or decreases with height, regardless of the overall concentration of all scatterers, including ice, rain, cloud droplets, graupel, hail, or snow.