Influences of the Local Environment on Supercell Cloud-to-Ground Lightning, Radar Characteristics, and Severe Weather on 2 June 1995

Abstract

Radar, cloud-to-ground (CG) lightning characteristics, and storm reports were documented for 20 long-lived supercell thunderstorms that occurred during a 6-h period in the west Texas Panhandle on 2?3 June 1995. These thunderstorms occurred in proximity to a preexisting mesoscale outflow boundary. Storms that remained on the warm side of the mesoscale outflow boundary and storms that formed directly on the boundary tended to produce weaker low-level rotation, lower maximum heights for the 40-dBZ echo top, and had the largest negative CG flash rates. The largest negative flash rate was produced as each storm was gradually weakening. In contrast, out of 11 boundary-crossing storms, several important radar-based measurands increased unambiguously after storms crossed the boundary: 40-dBZ echo-top height in 5 cases, radar reflectivity above the environmental freezing level in 6 cases, and low-level mesocyclone strength in 9 cases. Trends of the first two measurands were ambiguous for 4 of 11 cases affected by a ±15 min estimated boundary-position uncertainty. Five out of 11 storms dramatically increased their positive flash rate within 60 min after crossing the outflow boundary. These large positive flash rates were associated with descending reflectivity cores that were larger in magnitude and areal extent compared to other storms in this study. The local mesoscale environment and its horizontal variations of 0?3-km vertical wind profile, CAPE below the in-cloud freezing level, and boundary layer mixing ratio appeared to greatly influence storm structure and evolution. The observed environmental variations are hypothesized to support changes in charge structure that might lead to the observed changes in flash rate and polarity.