Leader Polarity-Reversal Feature and Charge Structure of Three Upward Bipolar Lightning Flashes

We have analyzed three upward bipolar lightning flashes (UBLFs) that occurred in Japanese winter thunderstorms. Leader polarity-reversal processes in three flashes share the same features. During the several tens of milliseconds (lightning A 56ms, lightning B 21ms, lightning C 67ms) before the initiation of the polarity-reversal leader, one branch of the preceding leader in bipolar flashes was nearly decayed while other branches of the preceding leader were still in propagation. Then the polarity-reversal leader will be initiated at the end of the decayed branch of the preceding leader. Initial sources of the polarity-reversal leader are characterized by relatively strong very high frequency power (average value in lightning A, B, and C 24, 18, and 14dBW) and obvious upward progression. Two of the three upward lightning (lightning A and B) occurred at a normal dipolar charge structure, while the remaining one (lightning C) occurred at an inverted dipolar charge structure. Based on the common features of the polarity-reversal leader and charge structure, we have proposed a scenario to interpret the process of upward bipolar lightning flashes.