Abstract

The present work shows high-speed videos of two upward flashes that started with positive upward leaders and, instead of being followed by negative subsequent return strokes, they were followed by positive subsequent return strokes. In both cases, after the positive leaders developed, recoil leaders appeared in their decayed branches as would be usual in negative upward lightning flashes. However, in these flashes the negative end of a recoil leader connected to a positive leader of an intracloud flash nearby. The connection initiated a downward positive leader that re-ionized the decayed channel of the upward flash all the way to the tower giving origin to a positive subsequent return stroke. This work shows that recoil leaders do play an important role in the occurrence of bipolar upward flashes and their interaction with intracloud flashes can provide explanations for all types of bipolar upward flashes initiated by upward positive leaders.