Preliminary breakdown pulses of cloud-to-ground lightning in winter thunderstorms in Japan

Preliminary breakdown pulses (PBP) of 26 positive and 104 negative cloud-to-ground (CG) lightning flashes observed in winter thunderstorms in Hokuriku region of Japan are analyzed. Pulses in PBP train are mainly bipolar. Zero-crossing time of each pulse is about 7 us and total duration of each pulse train is about 1 ms. PBPs in negative CG lightning are classified as “BIL type” (59 cases) and “BL type” (45 cases). “BIL type” PBP contains an intermediate stage with little electric field changes. The time interval between PBP and the first return stroke (PBP-RS interval) is on average 5.4 ms for “BIL type” PBP, while that for “BL type” PBP is only 1.3 ms. Ratio of peak amplitude of PBP to the first return stroke (PBP-RS ratio) is on average 0.47 and 0.44, respectively, for “BIL type” and “BL type” PBPs, but the ratio for “BIL type” PBP has a much wider distribution. It is speculated that the intermediate stage in “BIL type” PBP is caused by horizontal propagation of leader channel. PBPs in positive CG lightning are classified as +PBP (11 cases) and −PBP (15 cases) according to their initial polarities. +PBP and −PBP have similar distributions of both PBP-RS interval and PBP-RS ratio, but their value of PBP-RS interval is much larger and their value of PBP-RS ratio is much smaller than corresponding values of PBPs in negative CG lightning. It is speculated that different initial polarities of +PBP and −PBP in positive CG lightning are caused by different directions of channel propagation.