Numerically simulated lightning production in severe storms

Thunderstorm simulations were performed for a strong, airmass storm and three types of supercell storms, and several different electrification parameterizations were used for each type of storm. The main focus of our analysis was to examine characteristics of the resulting charge distributions and lightning. Of particular interest are the several supercell storm simulations that produced predominantly positive cloud-to-ground flashes.

The numerical cloud model used for the simulations is three dimensional and includes detailed bulk microphysics, with separate categories for cloud water, rain, cloud ice (columns, plates, and rimed), snow aggregates, frozen drops, three graupel densities, and two size ranges for hail. Lightning discharges were produced by a stochastic dielectric breakdown model that extends flashes bidirectionally in a step-by-step manner and creates realistic, fractal- like branch structure. The simulations produced a wide variety of lightning types, including horizontally extensive bilevel intracloud flashes, positive and negative cloud-to-ground flashes, and intracloud discharges involving charge layers at the cloud boundary. Simulated flashes sometimes reversed the net charge density locally, and this added complexity to the charge structures of the storms.

The highest density of lightning activity always occurred in the convective regions of the simulated storms, consistent with observations. However, in many cases, large anvils contained enough charge for initiation to occur several tens of kilometers from deep convection. Though lightning has been observed in these regions of anvils, initiation of flashes there has not yet been documented in published observations. Furthermore, suitable observations have not yet been obtained to document the occurrence of lightning in multiple layers of charge throughout the vertical depth of the anvil, as seen in our simulations.

All lightning originated between regions of opposite charge, as has been inferred previously for cloud flashes and negative cloud-to- ground flashes. However, positive cloud-to-ground flashes also were initiated between opposite charges in our simulations, a relationship not suggested previously. Positive cloud-to-ground lightning occurred only when the lowest significant charge region near the initiation point was negative (i.e. roughly a positive dipole structure about the initiation point).