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.