Ultra-high speed video observations of intracloud lightning flash initiation

Abstract

This study describes results from video observations of five intracloud flashes located ≤ 20 km from the camera and recorded with 6.1 µs exposure time and 6.66 µs frame intervals. Video data are supported with electric field change (E-change) and VHF measurements, with emphasis on the flash initiating event (IE) and initial breakdown (IB) stage. In four of the five flashes, the IE is accompanied by weak luminosity, ≤ 5% above background, lasting for 300–500 µs. Two of these four IEs were positive Narrow Bipolar Events (NBEs) with VHF powers of 43 and 990 W; these are the first (known) data showing visible light detected with a positive NBE. Two other IEs with weak luminosity had powers of 0.5 and 1 W, and the IE with no detected luminosity had a VHF power of 3 W. A typical IB cluster consists of several narrow pulses and one classic pulse in E-change data (along with many VHF pulses), and each example flash has 2–10 IB clusters in the first 5–50 ms. The luminosity of IB clusters was substantially greater than IE luminosity, ranging from 10 to 40% above background in four examples, while for one flash with 10 IB clusters, the luminosity range was 35–360% above background (average 190%). Luminosity durations of IB clusters were 520–1750 µs with average 1210 µs. For both IEs and IB clusters, increases in the detected luminosity were closely timed with substantial VHF emissions and decreased when VHF emissions weakened.

Transition in Optical and Radio Features During the Early Development of Negative Intracloud Leader

“…In addition, the initial event of this flash appears at around 156.8 ms corresponding to only a subtle burst of the 337 nm radiance in Figure 2c with no distinguishable signal occurs in the 777.4 nm band, consistent with observations from the high‐speed videos that only a little or no visible luminosity coincident with the initial events (Stolzenburg et al., 2021). Here, we further suggest the weak luminosity of this initial event could be created by streamers due to the absence of 777.4 nm radiance (Soler et al., 2020).…”

“…Rare optical observations of cloud‐to‐air flashes (Edens et al., 2014; Krehbiel et al., 2008; Stolzenburg et al., 2021) have discussed the highly intermittent features, the light‐emitting patterns, and the escaping behaviors of cloud‐to‐air flashes. For IC flashes, luminosity measurements from ground‐based photometric instruments have been presented (Stolzenburg et al., 2016, 2022; Wilkes et al., 2016).…”

New insights into the lightning discharge processes

“…This was confirmed by groundlevel observations of Jacobson et al (2013), (table 5), who reported that out of 193 lower-level CIDs (see figure 8(a)) only 2 (1%) were accompanied by optical (350-1100 nm) signals and none out of 24 upper-level CIDs did so. More recently, Stolzenburg et al (2021), (figures 5 and 11), from ultra-high speed (6.1 μs exposure time) video observations, reported on faint luminosity enhancements in the 350-1060 nm (with peak at 670 nm) range associated with two lower-level CIDs.…”

Secondary Fast Breakdown in Narrow Bipolar Events

“…Stolzenburg et al. ( 2021 ) analyzed five IC flashes and in four of them reported faint light emissions (⩽5% above background cumulative intensity) at the time of the IEs. Recently López et al.…”

“…However, the physical mechanism of fast breakdowns of the initial type NBEs (Rison et al, 2016;Wu et al, 2014) remains poorly understood. Stolzenburg et al (2021) analyzed five IC flashes and in four of them reported faint light emissions (⩽5% above background cumulative intensity) at the time of the IEs. Recently López et al (2022) investigated the initiation of four lightning flashes detected from LMA and simultaneous optical satellite observations and found that, in three of the flashes, the detection of blue emissions without any red luminosity supports the pure streamer nature of fast breakdown processes at the flash initiation.…”