The 29 June 2000 Supercell Observed during STEPS. Part II: Lightning and Charge Structure

Abstract

This second part of a two-part study examines the lightning and charge structure evolution of the 29 June 2000 tornadic supercell observed during the Severe Thunderstorm Electrification and Precipitation Study (STEPS). Data from the National Lightning Detection Network and the New Mexico Tech Lightning Mapping Array (LMA) are used to quantify the total and cloud-to-ground (CG) flash rates. Additionally, the LMA data are used to infer gross charge structure and to determine the origin locations and charge regions involved in the CG flashes. The total flash rate reached nearly 300 min?1 and was well correlated with radar-inferred updraft and graupel echo volumes. Intracloud flashes accounted for 95%?100% of the total lightning activity during any given minute. Nearly 90% of the CG flashes delivered a positive charge to ground (+CGs). The charge structure during the first 20 min of this storm consisted of a midlevel negative charge overlying lower positive charge with no evidence of an upper positive charge. The charge structure in the later (severe) phase was more complex but maintained what could be roughly described as an inverted tripole, dominated by a deep midlevel (5?9 km MSL) region of positive charge. The storm produced only two CG flashes (both positive) in the first 2 h of lightning activity, both of which occurred during a brief surge in updraft and hail production. Frequent +CG flashes began nearly coincident with dramatic increases in storm updraft, hail production, total flash rate, and the formation of an F1 tornado. The +CG flashes tended to cluster in or just downwind of the heaviest precipitation, which usually contained hail. The +CG flashes all originated between 5 and 9 km MSL, centered at 6.8 km (?10°C), and tapped LMA-inferred positive charge both in the precipitation core and (more often) in weaker reflectivity extending downwind. All but one of the ?CG flashes originated from >9 km MSL and tended to strike near the precipitation core.