Numerical simulations of the 29 June STEPS supercell

A three-dimensional dynamic thunderstorm electrification model was used to simulate the first three hours of the 29 June 2000 supercell from the Severe Thunderstorm Electrification and Precipitation Study (STEPS). The 29 June storm contained high lightning flash rates and produced predominately positive cloud-to-ground lightning, large hail, and a F1 tornado. The charge structure and thus lightning polarity of the simulated storm were sensitive to the treatment of cloud water dependence in the various noninductive charging schemes tested. The results from the simulations are compared with observations from STEPS including radar, balloon-borne electric field meter soundings and the Lightning Mapping Array. The "inverted'' tripolar charge structure featuring a main positive charge region with lower and upper negative charge regions, noted in observations, was well approximated by the model. Positive ground flashes occurred when the lowest charge region of the inverted tripole was negative in both the observed storm and the simulations. The convective intensity of the storm was mirrored by the total flash rate in both the observations and simulations. Correlations between detrended time series were best for graupel volume and total flash rate, updraft volume and updraft mass flux. All showed reasonable correlations with the total flash rate, though there was very little correlation with maximum updraft speed. Based on these correlations, it is likely that the best electrical indicator of storm intensity is the total flash rate.