Characterizing three types of negative narrow bipolar events in thunderstorms

Abstract

Data from fast antennas (FAs) with bandwidth of 16 Hz–2.5 MHz and VHF power sensors (Log-RF) with bandwidth of 186–192 MHz are used to examine negative narrow bipolar events, or NNBEs. The main focus is on low-altitude (<8.0 km) NNBEs that initiate negative cloud-to-ground (-CG) flashes; very few low-altitude NNBEs have been studied previously. For comparison, 24 high-altitude (>8.0 km) NNBEs are also examined. The low-altitude NNBEs are found to have two types called NNBE(L) and NNBE(H). NNBE(L)s have a bipolar FA waveform typical of NBEs while NNBE(H)s have a unipolar FA waveform. It is hypothesized that NNBE(H)s may be weak versions of NNBE(L)s in which the second, overshoot part of the bipolar waveform is too weak to detect amid the FA sensor noise. Together the 33 NNBE(L)s and NNBE(H)s occurred at an average altitude of 6.2 km (range 4.6–7.8 km), had average range-normalized (to 100 km) amplitude of 0.4 V/m (range 0.06–1.5 V/m), and had average VHF power of 130 W (range 1–1300 W). These low-altitude NNBE properties are substantially smaller and weaker than the same properties of the high-altitude NNBEs and of positive NBEs that initiate intracloud (IC) flashes; these analyses indicate that -CG flashes are easier to initiate than IC flashes. Visual inspection of the FA and Log-RF data of 868 -CG flashes showed that only 33 flashes (4%) were preceded by either an NNBE(L) or NNBE(H), so 96% of the -CG flashes investigated probably did not begin with an NNBE.