Preliminary breakdown of intracloud lightning: Initiation altitude, propagation speed, pulse train characteristics, and step length estimation

Abstract

Using a low‐frequency lightning location system comprising 11 sites, we located preliminary breakdown (PB) processes in 662 intracloud (IC) lightning flashes during the summer of 2013 in Osaka area of Japan. On the basis of three‐dimensional location results, we studied initiation altitude and upward propagation speed of PB processes. PB in most IC flashes has an initiation altitude that ranges from 5 to 10 km with an average of 7.8 km. Vertical speed ranges from 0.5 to 17.8 × 105 m/s with an average of 4.0 × 105 m/s. Vertical speed is closely related with initiation altitude, with IC flashes initiated at higher altitude having lower vertical speed during PB stage. Characteristics of PB pulse trains including pulse rate, pulse amplitude, and pulse width are also analyzed. The relationship between pulse rate and vertical speed has the strongest correlation, suggesting that each PB pulse corresponds to one step of the initial leader during the PB stage. Pulse rate, pulse amplitude, and pulse width all show decreasing trends with increasing initiation altitude and increasing trends with increasing vertical speed. Using a simple model, the step length of the initial leader during the PB stage is estimated. Most of initial leaders have step lengths that range from 40 to 140 m with an average of 113 m. Estimated step length has a strong correlation with initiation altitude, indicating that leaders initiated at higher altitude have longer steps. Based on the results of this study, we speculate that above certain altitude (~12 km), initial leaders in PB stages of IC flashes may only have horizontal propagations. PB processes at very high altitude may also have very weak radiation, so detecting and locating them would be relatively difficult.

Intracloud Lightning Flashes Initiated at High Altitudes and Dominated by Downward Positive Leaders

“…(E‐change waveforms of the three downward +IC flashes analyzed in section are provided in Figures S9–S11.) This is in fact an expected result according to Wu et al (), which showed that the PB of IC flashes initiating at higher altitudes generally produced less frequent PB pulses with smaller amplitudes and smaller pulse widths. Similar results have also been reported by Proctor ().…”

“…Proctor () compared lightning flashes with low origins (1 to 7.4 km) and those with high origins (7.4 to 12 km) and found that channel extension speed was generally lower for high origin flashes. Wu et al () analyzed initial negative leaders in 662 IC flashes and demonstrated that the upward speed of initial negative leaders decreased as the initiation altitude increased. These results made us speculate that, if the initiation altitude is high enough, the initial negative leader of IC flashes would not even develop upward, and results of the current study confirm this speculation.…”

“…The most common morphology is the bilevel structure as first revealed by lightning interferometer observations (Shao & Krehbiel, ) and later confirmed by observations with the Lightning Mapping Array (LMA; e.g., Rison et al, ; Riousset et al, ). It is now clear that normal polarity IC flashes (i.e., positive IC flashes or +IC flashes) with the bilevel structure usually start with initial negative leaders propagating upward with speeds on the order of 10 5 m/s (Behnke et al, ; Wu et al, ).…”

Relationship between thunderstorm electrification and storm kinetics revealed by phased array weather radar

“…BOLT is an LF sensor network that can be used to locate both IC and CG discharges in three dimensions. Each LF sensor detects electromagnetic wave radiations in the LF band associated with lightning discharges [ Ushio et al ., ; Wu et al ., , , ; Yoshida et al ., ]. After the identification of the pulse peaks of the LF records, the pulse peaks are located in three dimensions by using the time of arrival technique.…”

Characteristic pulse trains of preliminary breakdown in four isolated small thunderstorms

“…The range normalization was based on the 1/ r range dependence, where r was the average horizontal distance of the central station to all locations of the flash. Out of totally 2155 flashes, the number of relative errors ≥ 0.2 between the maximum (minimum) horizontal distance of the central station to all locations of each flash and r was 8 (2); note that most of PB heights were found below 10 km by Wu et al [], therefore below this height, the 2‐D location accuracy was estimated to be better than 2 km within areas of all four thunderstorms. So it could not affect the analysis results of PBA or RSA by r we took.…”