The most powerful lightning discharges in winter thunderstorms in Japan Sea Coast

Using a low frequency lightning location system comprising 9 stations, we have observed and analyzed 374 large and bipolar electric field change waveforms that occurred during the winter of 2012-2013 in Japan Sea Coast. Since the waveforms are different from those produced by any well-studied lightning discharge processes, in this paper, their source discharge events are called large bipolar events (LBEs). LBEs produce very large electric field changes that are even larger than that of positive and negative return strokes, so they seem to be the most powerful lightning discharges in winter thunderstorms. LBEs can be characterized by following features: (1) All have the same polarity as negative return stroke; (2) All exhibit a single bipolar pulse with a pulse width around 15 μs and similar positive and negative cycles; (3) All are located on the land along the Japan Sea coast, indicating they are probably associated with high grounded objects; (4) Most LBEs are temporally isolated within several milliseconds but are frequently followed by intracloud discharges after tens of milliseconds; (5) Most LBEs produce a single well-distinguished ionospheric reflection pulse. It is speculated that LBE is a type of powerful and transient lightning discharge event produced within a compact region of strong electric field formed when the negative charge layer in thundercloud is very close to the top of a tall grounded object.