Lightning‐channel morphology revealed by return‐stroke radiation field waveforms

Abstract

Simultaneous video and wideband electric field recordings of 32 cloud‐to‐ground lightning flashes in Florida were analyzed to show that the formation of new channels to ground can be detected by examination of the return‐stroke radiation fields alone. The return‐stroke E and dE/dt waveforms were subjectively classified according to their fine structure. Then the video images were examined field by field to identify each waveform with a visible channel to ground. Fifty‐five correlated waveforms and channel images were obtained. Of these, all 34 first‐stroke waveforms (multiple jagged E peaks, noisy dE/dt), 8 of which were not radiated by the chronologically first stroke in the flash, came from new channels to ground (not previously seen on video). All 18 subsequent‐stroke waveforms (smoothly rounded E and quiet dE/dt after the initial peak) were radiated by old channels (illuminated by a previous stroke). Two double‐ground waveforms (two distinct first‐return‐stroke pulses separated by tens of microseconds or less) coincided with video fields showing two new channels. One “anomalous‐stroke” waveform (beginning like a first stroke and ending like a subsequent) was produced by a new channel segment to ground branching off an old channel. This waveform classification depends on the presence or absence of high‐frequency fine structure. Fourier analysis shows that first‐stroke waveforms contain about 18 dB more spectral power in the frequency interval from 500 kHz to at least 7 MHz than subsequent‐stroke waveforms for at least 13 μs after the main peak.

Wavelet analysis of lightning return stroke

“…With the same technique and in the 0.2-20 MHz band, Willet et al (1990) observed no significant differences between the first and subsequent stroke spectra, stepped and dart-stepped leaders spectra. Willet et al (1995) through the WWD technique found that first-stroke waveforms contain about 18 db more spectral power than subsequent-stroke waveforms, in the interval from 500 kHz to at least 7 MHz, for at least 13 ms after the main peak.…”

Multiple pulses in dE/dt and the fine-structure of E during the onset of first return strokes in cloud-to-ocean lightning

“…Submicrosecond measurements of the electric field, E, and dE/dt waveforms that are radiated during the onset of first return strokes in cloud-to-ocean lightning have been reported previously by Weidman and Krider (1978;1984), Krider et al (1996), Willett et al (1995;, and Willett and Krider (2000). Weidman and Krider (1978; noted that the shape of the initial E field, when recorded under conditions where there is minimal distortion due to the effects of ground-wave propagation, typically begins with a slow, concave front that lasts for several microseconds.…”

“…We have examined the dE/dt waveforms produced by the subsequent return strokes in our dataset, i.e., the strokes that come after the first in a flash and that do not appear to initiate new attachments to ground (e.g., Willett et al,1995), to see if there were multiple pulses in dE/dt during the onset of those strokes. The results are summarized in Table 2 together with our results for the Type A and Type B first strokes.…”

Characterization of Positive Cloud to Ground Flashes Observed in Indonesia

“…The main leader branch usually induces an upward leader from the ground and eventually produces a return stroke to ground termination. For negative CG flashes, several researchers have reported that the downward leaders with many branches are able to produce different ground terminations, which are separated by several tens to hundreds of meters [6][7][8][9][10][11][12]. Two ground terminations of a stroke occurring within several microseconds with a separation distance of hundreds of meters are difficult to identify by a lightning location system [13].…”