Simulated three-dimensional branched lightning in a numerical thunderstorm model

[1] Lightning discharges are simulated by using a stochastic dielectric breakdown model within a numerical thunderstorm model with extensive parameterizations of electrification mechanisms. The lightning model simulates the macroscopic bidirectional extension of discharges as a step-by-step stochastic process. Discharge channels are propagated on a uniform grid, and the direction of propagation (including diagonals) for a particular step is chosen randomly, with the probability for choosing a particular direction depending on the net electric field. After each propagation step the electric fields are recomputed via Poisson's equation to account for the effect of the conducting channel. The lightning parameterization produces realistic looking, three-dimensional, branched lightning discharges. A variety of lightning types have been produced, including intracloud discharges, negative cloud-to-ground (CG) lightning, and positive CG lightning. The model simulations support the hypothesis that negative CG flashes occur only when a region of positive charge exists below the main negative charge region. Similarly, simulated positive CG flashes were found to occur only in regions of storms where the two significant charge layers closest to ground had roughly a "normal dipole'' structure (i.e., positive charge above negative).