2012 Volume 132 Issue 6 Pages 542-547
In order to further understand the initiating mechanism of positive cloud-to-ground (PCG) flashes, the characteristics of initial breakdown pulse trains are investigated by using electric field data of Guangdong province. Considering the difference in initial polarity between the initial breakdown pulses and the first return stoke (RS), three types of bipolar pulse trains, including the same polarity (type I), the opposite polarity (type II) and the composite polarity (type III), are identified. The percentages of type I, type II and type III are about 84%, 10% and 6%, respectively. The statistical results show that PCG flashes including initial breakdown pulse trains, which occur frequently when PCG flashes are not active, are about 12% of all the PCG flashes. The width of individual pulse (Tpw) and the interval of adjacent pulses (Tpi) are about 25μs and 271μs, respectively. The mean values of pulse train duration (Td) and the interval (Tp-ri) between initial breakdown pulse train and RS are 8.88ms and 98.48ms, respectively. The average value of the ratio (Rp-rr) between the maximum amplitude of pulse train and the amplitude of the first stroke pulse is 0.16. The ratio is smaller in Guangzhou province than in Japan and in Brazil. Comparing of the parameters of type I and type II, it can be found that Tpw, Td and Tpi of type II are less than those of type I, however, the values of Tp-ri and Rp-rr of type II are larger than those of type I. The possible causes of the observating several types of pulse trains and the significantly diversified pulse characteristics are discussed.