Discharge initiation processes of tropical lightning flashes

We have examined the discharge initiation process before the first classic initial breakdown (IB) pulse within the reversal distance of tropical lightning flashes, positive cloud-to-ground (+CG), negative cloud-to-ground (–CG) and 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). From the FA data it was 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 the lightning flashes were always initiated by an impulsive initiation event. 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.