Initial electric field changes of lightning flashes in two thunderstorms

Abstract

The beginning of all 75 lightning flashes in two small thunderstorms was investigated using an array of electric field change (E‐change) meters and an array of VHF sensors with the goal of determining if an initial E‐change (IEC) preceded the initial breakdown (IB) pulses in each flash. IECs were found at the beginning of all 62 flashes in Storm 1 and all 13 flashes in Storm 2. Hence, it is concluded that an IEC is a fundamental part of most or all lightning initiations and that an IEC is needed prior to the first IB pulse in a flash. IEC durations averaged 0.23 ms for cloud‐to‐ground (CG) flashes (range 0.08–0.54 ms) and averaged 2.7 ms for normal intracloud (IC) flashes (range 0.04–9.8 ms). IEC point dipole moments averaged 26 C m for CG flashes (range 4–86 C m) and averaged −140 C m for normal IC flashes (range −8 to −650 C m). IEC current moments averaged 120 kA m for CG flashes (range 41–410 kA m) and averaged −91 kA m for normal IC flashes (range −2 to −630 kA m). E‐change data support the suggestion that weak narrow bipolar event type events initiate some lightning flashes, but 41 of the 75 flashes had no detectable initiating pulse > 0.04 V m−1 range normalized to 100 km. Two flashes had two IECs; the second IEC of each flash initiated a new lightning channel that propagated in a new direction and at a higher altitude than the original development after the first IEC.

Initial Breakdown Pulse Parameters in Intracloud and Cloud‐to‐Ground Lightning Flashes

“…We analyzed the IB stage of flashes from a storm on 1 August 2010, which we call the “main” storm. (The main storm is the same as Storm1 in Chapman et al, ). The main storm had 46 normal IC flashes and 11 negative CG flashes.…”

“…As mentioned above, it is unknown if all normal IC flashes have an IB stage with classic IB pulses near the beginning of the flash, so we have investigated all 46 normal IC flashes in the main storm for IB pulses. Chapman et al () previously studied all the flashes in this storm for IECs (Initial E‐Changes) preceding the IB stage and found that all flashes (negative CG, normal IC, and inverted IC) in the storm had an IEC. (We note that Chapman et al, listed 48 normal IC flashes in this storm, but this count included two normal IC flashes that started twice and had two IECs, making four of their 48 normal IC flashes; herein we have not included the second IC flash of the 2 two‐start IC flashes, so our count is 46 normal IC flashes.…”

“…Recent research (Chapman et al, ; Marshall, Stolzenburg, et al, ) indicates that both CG and IC lightning flashes typically develop in a three‐step process with (1) an initiation event (often not seen with an E‐change sensor) followed by (2) a relatively small and slow electric field change (the “initial E‐change”) with a duration of 0.08–5.7 ms that is followed by (3) the first IB pulse of the flash. Thus, the IB pulses are not the first events in a lightning flash, but they do occur close to the beginning of each flash.…”

Initial Breakdown Pulses Accompanied by VHF Pulses During Negative Cloud‐to‐Ground Lightning Flashes

“…(d) The developing in‐cloud discharge was propagating generally downward based on the Position By Fast Antenna geolocation method for IB pulses (Karunarathne et al, ). (e) Except for initiation activity (e.g., the initiating event and IEC) in the millisecond immediately preceding the first IB pulse, we required that there was an electrically quiet period of many milliseconds before the first IB pulse in each flash (Chapman et al, ; Marshall, Stolzenburg, et al, ; Marshall et al, ). Starting with the first IB pulse, we investigated 2‐ms durations of FA and LogRF recordings in detail for all 20 selected flashes.…”

“…The IB pulses can be detected hundreds of kilometers from the parent thunderstorm (Kolmašová et al, ; Kotovsky et al, ) and are one indication of lightning initiation. Another indication of lightning initiation is an initial E‐change (IEC) that occurs immediately before the first IB pulse and can be detected only at short distances (typically within 7 km) from the developing discharge (Chapman et al, ; Marshall, Stolzenburg, et al, ; Marshall et al, ). Based on very high frequency (VHF) interferometric measurements, Rison et al () hypothesized that many or all lightning flashes are initiated by a phenomenon called fast positive breakdown, which they explained as a system of positive streamers developing in a locally intense electric field region.…”