The U.S. Great Plains are characterized by convective storms frequently having high percentages of positive cloud-to-ground lightning (+CG), often with high peak currents and accompanied by large hail (>2-4 cm). The literature contains many case studies of large hail associated with predominately +CG supercells, often exhibiting extremely high IC-to-CG ratios as well. However, significant hail is not always associated with elevated +CG percentages and high CG flash rates of either polarity. Recent studies of hail-producing storms in France noted a number of storms with low overall CG rates, small peak currents and low stroke multiplicities.

This paper examines the lightning characteristics of the intense supercells which formed on the evening of 22-23 June 2003 in south-central Nebraska during the BAMEX program. One cell (“the Aurora cell”) produced the largest recorded hail stone in U.S history. It fell at about 0005 UTC near Aurora, in Hamilton County, and measured 17.8 cm diameter and 47.6 cm circumference. During the first hour of this supercell’s life cycle, the percent positive reached nearly 50%, and then fell rapidly, where it remained. In the 5 minutes preceding the giant hailstone’s impact, only 5 of the 71 reported strokes were positive. Also, during this period, peak currents were unusually low for both polarities (12 kA for +CGs, and 9.1 kA for the –CGs). Another supercell ~100 km to the south (“the Superior cell”), contained the most intense mesocyclone ever recorded (Wakimoto, personal communication). This storm exhibited similar CG lightning characteristics, i.e., an initial burst of high +CGs percentages followed by strongly negative dominated CGs with both polarities showing very low mean peak currents.

Using new ELF transient analysis techniques developed during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) at Duke University (Cummer and Lyons 2004), we analyzed the impulsive charge moment changes of CG strokes occurring between 2355 and 0005 UTC for both the Aurora and Superior supercells. Out of 688 strokes, only 77 had sufficiently powerful signals to allow computation of impulse charge moment changes above the threshold of ~5 C km. There were only 14 strokes with impulse charge moment changes >50 C km. The largest was only 185 C km. This stands in contrast to previous investigations of hail producing supercells during STEPS in which not only many more +CGs with large peak currents were present, but impulse charge moment values >100-300 C km were not uncommon. While supercells rarely produce impulse charge moment values large enough to trigger sprites (>300-600 C km), these intense BAMEX supercells were characterized by (1) relatively low total flash rates, (2) low percentages of +CGs, (3) very small peak currents for strokes of both polarities, and (4) extremely low impulse charge moment changes, the majority being <5 C km. This suggests charge lowered to ground by individual strokes was typically on the order of only 1 C (Note: Continuing current contributions are not retrieved by this technique). The lightning metrics for these cells are now being investigated with greater temporal and spatial resolution. Unfortunately, no information is currently available on IC/CG ratios. The extremely high surface dewpoint and large CAPE values for these storm environments support recent contentions by E. Williams (personal communication) that positive CGs in convective cores are more likely to be found in high cloud base storms. This does not explain, however, the burst of +CGs early in the storms’ growth period. Many aspects of these storms’ electrical behavior merit considerable further attention.