Characteristics of the Preliminary Breakdown in Inverted-polarity Intracloud Lightning Flashes

Abstract

The intracloud (IC) flashes can be classified into normal and inverted-polarity types according to their initial leader propagation directions. Due to the rare occurrence of inverted-polarity IC flashes with downward preliminary breakdown (PB) processes, the corresponding PB characteristics are much less understood than those in normal IC flashes.Based on the lightning data observed by a low-frequency lightning mapping system FALMA (fast antenna lightning mapping array) deployed in Ningxia, the characteristics of the PB process in 312 inverted-polarity IC flashes are statistically analyzed. The parameters of PB waveforms show that the arithmetical mean (AM) of the PB duration is 9.8 ms. The pulse shape is characterized by rise time, half-peak width, fall time, and pulse width, respectively, with AM values of 7.3 μs, 4.5 μs, 5.6 μs, and 24.7 μs. The pulse rate and pulse interval are 5.7 ms-1 and 169.2 μs.The statistical results for PB channels show that the inverted-polarity IC flashes are usually initiated at the AM altitude about 6.9 km, obviously higher than the initiation altitude of normal-polarity IC flashes. The difference indicates that there could be an inverted dipolar charge structure in the thunderclouds of Ningxia. The superposition of PB locations on the radar reflectivity suggests that these inverted IC flashes tend to be initiated in the region with radar echoes weaker than normal IC flashes (19.3 versus 27.8 dBZ). The vertical length and speed of PB channels are 2.0 km and 2.8×105 m·s-1.Furthermore, PB parameters show significant correlations with the initiation altitude. Specifically, both vertical speed and pulse rate decrease with the initiation altitude, and the correlation coefficients are -0.44 and -0.53, respectively. However, both PB durations and vertical distances show positive correlations with the initiation altitude, with the coefficients of 0.64 and 0.46, respectively.In general, the PB characteristics of inverted IC flashes present both similarities and differences to the PB processes in other flash types. It is believed more lightning observations in the northwest inland of China can facilitate the interpretation of these similarities and differences.

Keywords

• inverted-polarity intracloud flashes
• preliminary breakdown