Electric field change and VHF waveforms of Positive Narrow Bipolar Events in Mississippi thunderstorms

Abstract

Positive Narrow Bipolar Events (+NBEs) in Mississippi thunderstorms were studied using fast antennas (FA, bandwidth 16 Hz - 2.6 MHz) and VHF antennas (Log-RF, bandwidth 186–192 MHz). The waveform characteristics of 201 positive NBEs were determined using both sensors. The +NBEs were classified in two ways: by FA waveform into Types A-D and by +NBE occurrence relative to other lightning events into three groups called Isolated, Not-Isolated, and INBE. (An INBE initiates an intracloud flash.) The FA waveform properties of 188 positive NBEs were mainly in reasonable agreement with previous studies. The VHF waveform properties of +NBEs have not been studied previously. The VHF powers of 201 positive NBE ranged from 0.1–88.4 kW with an average value of 7.8 kW. Types C and D positive NBEs tended to be more energetic (average VHF powers of 9.2 and 13.2 kW) than Types A and B (average powers of 1.9 and 4.0 kW). The INBE group of +NBEs had a larger range of VHF powers (0.2–88.4 kW) than the combined power range of the Not-Isolated and Isolated groups (0.1–26.7 kW). The INBE group also had a larger average peak power (9.9 kW) than the Not-Isolated and Isolated Groups (4.0 and 8.7 kW, respectively). However, 53% of INBEs had peak VHF power < 5 kW, so an INBE does not require a large power to initiate an intracloud flash. For 90% of the 201 positive NBEs the magnitude of the time difference between the FA peak amplitude and the Log-RF peak power was ≤2 μs, but the FA peak amplitude showed almost no correlation with the Log-RF peak power.