Comparative Study of Upward Leader Inception During the Initial Stage of Negative and Positive Altitude‐Triggered Lightning

Abstract Comparative analysis of positive and negative altitude‐triggered lightning striking a 30‐m tower beneath 35‐m floating wires reveals detailed bidirectional leader processes. It provides the first observational evidence of bidirectional leader behavior in positive altitude‐triggered lightning and highlights the connection process's driving effect on leader initiation/development. In positive triggered lightning, the bidirectional leader system first emerged from the wire bottom with positive polarity, rapidly connected with the tower and generated an impulsive current wave initiating an upward negative leader from the wire top. The wire ends generated different E‐field distortions, but less significant than the different initiation thresholds of positive and negative leaders. Consequently, the positive leader always initiates first for both triggering cases, regardless of background E‐field direction and formation position. Wire end's E‐field distortion alone was insufficient to initiate negative leader. Besides slow charge accumulation from the opposite end's positive leader, impulsive current surge is another effective/indispensable abrupt driver.