Lightning relative to other tornadic storm parameters on 3 May 1999

Besides cloud-to-ground lightning data from the National Lightning Detection Network (NLDN), limited cloud flash data are available for the 3 May 1999 tornado outbreak. The cloud flash data are from a new long-range subnetwork that processes cloud flash signals in the same radio frequency band used by the rest of the NLDN. Because most cloud flashes radiate weakly in this band and few of the new stations had been installed by May 3, cloud flash detection efficiency was low. Probably no more than 2% of the cloud flashes that occurred (those producing the strongest signals) were detected.

Flash rates for both ground flashes and these strong cloud flashes have been compared with the time of tornadoes and with radar-derived storm parameters, including Vertically Integrated Liquid (VIL), the maximum height of 30-dBZ echoes, and maximum estimated hail size. Ground flashes that lower the usual negative charge to ground were fairly infrequent throughout the period we examined, with flash rates being only occasionally as large as 1 per minute. Ground flashes that lowered positive charge to ground (the normally anomalous polarity) were more frequent throughout almost the entire period. Beginning approximately when the tornado that became F5 touched down, positive ground flash rates increased rapidly to almost 5 per minute (attained at roughly the time that the tornado first became F5), decreased almost as rapidly over a 30-minute period, and then increased to a peak of 2.4 per minute during the next 15 minutes, before decreasing again. Flash rates of strong cloud flashes were particularly interesting, because they increased to their largest value (2 per minute) during the F3 and F2 tornadoes immediately preceding the tornado that became F5. Cloud flash rates then decreased over the 15-minute period immediately preceding touchdown of the F5 tornado, increased again to approximately 2 per minute during the first 5 minutes of the tornado, and then decreased to a fairly steady rate of roughly 0.5 per minute for the rest of the period in which the storm was tornadic.

The top of 30-dBZ reflectivity echoes was highest (approximately 14 km MSL) during the period in which cloud flash rates were also largest and then decreased steadily from shortly after the F5 tornado began until shortly after the tornado dissipated. VIL also was largest (63 kg ^m-2) during the period shortly before the F5 tornado. However, VIL again attained the same value near the time of the large peak in +CG flash rates, roughly coincident with the tornado first becoming F5, and then declined fairly steadily during the rest of the tornado's life.