Observations of narrow bipolar events initiating regular lightning flashes

With a 3-D LF lightning location system called BOLT, 827 narrow bipolar events (NBEs) were detected during the summer of 2013. 103 NBEs were found to initiate regular lightning flashes including 5 negative cloud-to-ground (CG) flashes and 2 positive CG flashes. These “initiator NBEs” were all positive polarity and were always followed by positive pulse trains propagating upward, behaving like preliminary breakdown pulses of positive polarity. Initiator NBEs are probably produced at the upper edge of the main negative charge region and subsequent positive pulses propagate upward to the upper positive charge region. A major difference between initiator NBEs and other regular NBEs is that initiator NBEs have much lower source altitudes. Most of initiator NBEs and regular NBEs are respectively lower and higher than 10 km.