Activation of Abundant Recoil Leaders and Their Promotion Effect on the Negative-End Breakdown in an Intracloud Lighting Flash

A unique intracloud (IC) lightning flash with multiple positive leader branches spreading out from the same point is studied using simultaneous high-speed images, electromagnetic (EM) field changes, and radiation source mapping. A channel of the positive leader protruded from the cloud and induced a bidirectional luminous segment at tens of meters ahead of the leader tip. They led to a small-scale attachment where multiple channels successively emerged and propagated toward diverse directions. The propagation mode of the positive leader experienced a noticeable transition from stable and smooth channel extension to active development accompanied with frequent recoil leaders, attribute to the arrival of an impulsive current wave induced by the in-cloud activity. We find solid evidence that long recoil leader activated the negative-end breakdown with a regular EM pulse train. The back-and-forth interaction from the opposite ends of the associated channel is revealed. Plain Language Summary Recoil leader propagates along the preconditioned channel that is originally formed by positive breakdown. It acts as a critical manner of lightning channel reactivation and maintenance, and is closely related to the lightning channel decaying and cutoff. Its occurrence results in various lightning discharge processes such as subsequent return stroke, M component, and K event. The knowledge of recoil leader is of great significance to understanding the nature of lightning discharge and propagation. In this paper, by conducting simultaneous high-speed video observation, electromagnetic field detection, and lightning radiation source mapping, we reveal that the fruitful production of recoil leaders can be facilitated by the arrival of an in-cloud-activity-induced strong current wave to the positive leader channel extremity. In turn, some long recoil leaders would also activate the opposite-end negative breakdown. Back and forth re-illumination of dimmed channel is discovered during the interaction between the opposite ends. These findings provide new insight into the recoil leader formation and its effect on other breakdown processes.