Initial Stage of Lightning Discharges Initiated by NBE and IBP

Lightning initial stage is an important and fundamental physical process. How lightning initiates is a hot topics in the research of lightning physics. Initial breakdown is observed by many researchers, and the first pulse of IBP is regarded as the starting marker of intracloud lightning or cloud-to-ground lighting. Lightning discharge characteristics and laws during initial stage in a thunderstorm progress are researched based on Low-frequency E-field Detection Array (LFEDA). In 212 intracloud lightnings and cloud-to-ground lightnings of short range, 32 lightnings are initiated by NBE, accounting for 15%, and 180 lightnings are initiated by IBP, accounting for 85%. As an initial NBE (INBE), it is more isolated and larger than the first pulse of initial IBP (FIBP). The average time interval from INBE to subsequent first pulse is 7 ms, with a mid-value of 3.9 ms. The amplitude ratio between INBE and the subsequent pulse is 3.5. The above values are larger than those of FIBP with the corresponding values of 0.6 ms, 0.2 ms and 0.8. Most of the positive INBE and FIBP corresponds to an upward discharge in the initial stage while negative correspond to a downward one. The average height of INBE is 9.9 km, which is less than isolated NBE of 13.6 km. There is also a sensible difference in discharge height between isolated positive NBE and the negative one. The isolated negative NBE is much higher than the positive one. It may occur between upper positive charge region and shielding layer, which is related to the middle and upper atmosphere discharge. The average height of FIBP is 8.1 km. There are no obvious differences in discharge height between INBE and FIBP. The average speed during the first 15 ms of the lightning initiated by NBE is 3.1×10⁵ m·s^-1, with a mid-value of 3.0×10⁵ m·s^-1, which decreases with the initial altitude. The maximum and minimum average speed is 7×10⁵ m·s^-1 and 1.2×10⁵ m·s^-1, respectively. To better reveal the difference in INBE and FIBP, an estimated velocity is calculated based on the hypothesis of low-frequency pulse corresponding to a channel extending. The estimated speed of INBE is 4.7×10⁷ m·s^-1, which is faster than FIBP with a velocity of 1.5×10⁷ m·s^-1. The shorter rising time for INBE also indicates a faster velocity. There are no apparent differences in rising time, half-peak width and pulse width for INBE, NBE in process and isolated NBE. It is difficult to distinguish INBE from other NBE by pulse property.