Observed Lightning Morphology Relative to Modeled Space Charge and Electric Field Distributions in a Tornadic Storm

Abstract

This study uses a kinematic numerical cloud model that includes electrification and lightning discharge processes to investigate hypotheses concerning intracloud lightning flash rates in the Binger, Oklahoma, tornadic storm of 22 May 1981. MacGorman et al. have observed that intracloud (IC) flash rates in this storm's mesocyclone region peak when overall storm intensity is greatest and cloud-to-ground flash rates are low. They hypothesize that precipitation interactions involved in reflectivity growth at the 7?9-km level of the updraft are involved in precipitation charging and electrification. They also hypothesize that the intense convection in the mesocyclone region elevates the lower negative charge of the storm closer to upper positive charge, thereby enhancing IC flash rates. These hypotheses are tested by examining the charge and electric field distributions and charging rates produced by the kinematic model for the Binger storm. The model produces maximum electric field and net space charge magnitudes of around 200 kV m?1 and 1 nC m?3 in runs where the threshold for activating simulated lightning discharges was set at 200 kV m?1. The noninductive mechanism, driven by charge separation during rebounding collisions between ice particles and riming graupel, generally dominates the inductive mechanism in the model. Computed precipitation charging rates of up to ?5 ? 10?11 C m?3 s?1 are partially compensated by cloud particle charging from discharges in middle levels of the updraft. Simulated discharges add positive charge to cloud particles in the main negative precipitation charge region and negative charge to cloud particles in the upper positive precipitation charge region. The principal effect of lightning in the model is not to neutralize the charge on individual particles, but to partially mask the net charge carried by precipitation. The simulated discharges occur at a rate of 12 min?1, comparable to the peak observed IC flash rate of 13 min?1 within 10 km of the mesocyclone. The model results also suggest that lightning, combined with subsequent particle motions, creates new regions of charge comparable to those created by particle collisions.