Explicitly Simulated Electrification and Lightning within a Tropical Cyclone Based on the Environment of Hurricane Isaac (2012)

This work analyzes a high-resolution 350-m simulation of the electrification processes within a hurricane in conjunction with available total lightning observations to augment the general understanding of some of the key cloud-scale electrification processes within these systems. The general environment and trends of Hurricane Isaac (2012), whose lightning activity was observed by the Earth Networks Total Lightning Network, were utilized to produce a reasonable tropical cyclone simulation. The numerical model in this work employs explicit electrification and lightning parameterizations within the Weather Research and Forecasting Model. Overall, simulated storm-total flash origin density rates remain comparable to the observations. Because simulated reflectivities were larger and echo tops were higher in the eyewall than observed, the model consistently overestimated lightning rates there. The gross vertical charge structure in the eyewall resembled a normal tripole or a positive dipole, depending on the location. The negative charge at middle levels and positive at upper levels arose primarily from noninductive charging between graupel and ice crystals/snow. As some graupel melted into rain, the main midlevel negative charge region extended down to the surface in some places. The large volume of positively charged snow aloft caused a radially extensive negative screening layer to form on the lighter ice crystals above it. Akin to continental storms and tropical convection, lightning activity in the eyewall was well correlated with the ice water path (r > 0.7) followed by the graupel + hail path (r ≈ 0.7) and composite reflectivity at temperatures less than −10°C and the snow + ice path (r ≈ 0.5). Relative maxima in updraft volume, graupel volume, and total lightning rates in the eyewall all were coincident with the end of an intensification phase.