Numerical Simulations of the 29 June 2000 STEPS Supercell : Microphysics , Electrification , and Lightning

A three-dimensional dynamic cloud model incorporating air flow dynamics, microphysics, and thunderstorm electrification mechanisms was used to sim ulate the first three hours of the 29 June 2000 supercell from the Severe Thunderstorm Electri fica ion and Precipitation Study (STEPS). The 29 June storm contained high flash rates and prod uced predominately positive cloud-to-ground lightning, large hail, and an F1 tornado. F our different simulations of the storm were made, each one containing a different noninducti ve (NI) charging parameterization. The charge structure and thus lightning polarity of the simu lated storm was sensitive to the treatment of cloud water dependence in the different NI char ging schemes. The results from the simulations are compared with observations from STEPS, including balloon-borne electric field meter soundings and flash locations from the Lightning M apping Array. The observed “inverted” tripolar charge structure, which features a mai n positive charge region with lower and upper negative charge regions, was well approximated by the model. The polarity of the ground flashes was opposite that of the lowest charge region o f the inverted tripole in both the observed storm and the simulations. The convective intensi ty of the storm was indicated by the total flash rate in both the observations and simulations . Rather high correlations existed between the detrended time series for graupel volume and tot al flash rate. Updraft volume and updraft mass flux also were well correlated with total flas h rate, though there was little correlation between total flash rate and maximum updraft spe ed. Based on these correlations, it is likely that the best electrical representation of a sto rm that is reliable for storm intensity is the total flash rate.