Electrification and lightning in an idealized boundary-crossing supercell simulation of 2 June 1995

A non-hydrostatic cloud model was utilized to investigate how simulated supercell thunderstorms respond to changes in local environmental conditions. Of interest is how the kinematical and microphysical structure change and, in turn, how this influences storm charge separation, charge structure and cloud-to-ground (CG) lightning flash rate and dominant polarity. The model represents an idealized horizontally-varying environment initialized with soundings based upon observations from the 2 June 1995, VORTEX case.

The initial right-moving boundary-crossing supercell storm rapidly intensifies in updraft strength, updraft volume, and low-level rotation with related increases in radar reflectivity and 40 dBZ echo top height as it crosses the boundary. This is consistent with many of the 2 June 1995 observed storms. Graupel and hail increase dramatically as the riming accretion rate (RAR) is enhanced on the boundary’s cool side. The change is causal since the same storm in a no-boundary simulation fails to intensify. Later, when this storm precipitation region merges with that of another storm, larger relative increases in these kinematical and microphysical variables were found.

For a single kinematical and microphysical evolution, four non-inductive (NI) charging parameterizations were tested. Inductive charging was turned on. In all four cases, there was a general tendency for a strengthening right-moving storm which lofted its middle and upper charge regions and increased the local intra-cloud (IC) lightning flash rate 40 min after crossing. Coincident with this was a gradual deepening and strengthening the lower charge regions, which in turn increased the total CG (sum of +CG and –CG) flash rate by allowing leader propagation into the lower portions of the cloud. These lowest charge regions are produced primarily via inductive charging.

Although the detailed CG behavior varied between cases, all four NI schemes, when used with inductive charging, showed the same tendency for increased amounts of positive leaders coming closer to ground with time after crossing the boundary thereby increasing the probability of +CG. This is consistent with +CG increases on the cool side of the boundary in the observed storms. However, only two of four NI charging schemes produced greater amounts of +CG within 60 min after crossing the boundary. Eventually, after the reflectivity core merger, all four NI charging schemes produced increases in +CG flash rate, IC flash rate, charge density, and charge volume.