Simulating High Lightning Flash Rates at Upper Levels in Strong and Severe Thunderstorms

Abstract:

Lightning mapping systems have detected high lightning flash rates at the top of convective cores above the environmental -30°C level, particularly in severe storms. Previous 3-D model simulations have reproduced similarly high flash rates using bulk (one or two moment) microphysics, but the details of the charge separation were not closely examined. Recent work has raised questions as to whether the charging processes have been adequately simulated, both in terms of grid resolution and microphysical processes.

New simulations of a severe supercell storm with Takahashi spectral bin microphysics and 4x better grid resolution (250m horizontal and 125 vertical spacing) are better able to capture the steep vertical temperature gradients (on the order of 8°C per km) and the process of freezing of larger liquid particles. The results suggest that high charge separation rates can occur as larger drops freeze homogeneously while smaller droplets remain liquid. This process far outstrips other small ice particle production processes like vapor nucleation or ice multiplication. This intense and vertically narrow charging zone then leads to sufficiently fast electric field growth that favors smaller flashes. These results form the hypothesis that high rates of spatially small lightning flashes result from a high density charging current in a relatively small volume, such that initiation electric field thresholds are met quickly before larger net charge regions have time to form.

Results will also be presented for bulk microphysics with an updated droplet freezing scheme that emulates the freezing of droplets based on diameter and ambient temperature. An important consideration in both schemes is how to handle newly-freezing droplets in the calculation of riming rate (or cloud water content) in the charge separation rate equations.