Characteristics of Discharge Initiation Processes of Tropical Lightning Flashes

: We have examined the initiation discharge process before the first classic initial breakdown (IB) pulse within the reversal distance of 64 tropical lightning flashes, 4 positive cloud-to-ground (+CG), 20 negative cloud-to-ground (–CG) and 40 intracloud (IC) flashes, using a broadband fast electric field change antenna (FA), a broadband magnetic field change antenna (BA) and a very high-frequency antenna (VA). The FA data found that some +CG, – CG and IC flashes were initiated with and some without a negative bipolar event (NBE). However, the VA data showed that an impulsive initiation event always initiated lightning flashes. The initiating event was usually followed by one or more enhancing events that increased the local electric field before the first classic IB pulse occurred during the initial electric field change (IEC) period. More VA pulses were detected than FA pulses during the IEC period. These findings indicate that lightning initiation emits VHF pulses or fast positive breakdowns (FPBs) and is accompanied by several negative breakdowns of IB pulses. The FA data clearly showed a negative slope of the IEC in the +CG flashes The IB pulse of the +CG flashes moved negative charge upward, similar to IC flashes. The arithmetic mean (AM) IEC duration for the +CG, –CG, and IC flashes was 0.24 ms, 0.29 ms, and 0.53 ms, respectively.