Temporal and Spatial Characteristics of Preliminary Breakdown Pulses in Intracloud Lightning Flashes

We have studied the temporal and spatial characteristics of preliminary breakdown (PB) pulses in 32 normal intracloud flashes recorded by a fast antenna lightning mapping array system called Fast Antenna Lightning Mapping Array. It was found that PB processes started with a stage of preliminary isolated pulses (PIPs) and followed by classic PB pulse (PBP) clusters. A typical PIP stage consists of two or three PIPs and on average lasts about 129 μs. A typical PBP cluster consists of several narrow pulses (NPs) and one classic PBP and on average lasts about 193 μs. During one PIP stage, the pulses occurring later usually have a lower altitude, indicating a kind of backward extension relative to the overall upward progression of PB process. On average, the horizontal and vertical extensions involved in a PIP stage are 60 and 216 m, respectively. In each PBP cluster, PBP is always located at the lowest height and behind its preceding NPs, while these NP locations could have various patterns in term of extension. On average, the overall horizontal and vertical extensions involved in a typical PBP cluster are 161 and 515 m, respectively. For both the PIP stage and PBP cluster, their vertical extensions are apparently longer than their horizontal extensions, reflecting a predominantly vertical channel of PB process. Based on the temporal and spatial characteristics of PB pulses, we have proposed a model to interpret how the PIPs, NPs, and PBPs are produced.