In-Cloud Discharge of Positive Cloud-To-Ground Lightning and Its Influence on the Initiation of Tower-Initiated Upward Lightning

We conducted a comprehensive observation of tower-initiated upward lightning (UL) during the summer season from 2012 to 2020, using high-speed video and the Beijing Broadband Lightning Network (BLNET). A total of 25 UL flashes were observed, and 21 of them were triggered by nearby positive cloud-to-ground (CG) lightning. It is interesting to note that eight (38%) of the parent +CG lightning flashes have more than one return stroke (RS). The distance between the UL and the grounding point of its parent +CG lightning was 4-38 km, where nine (43%) of the UL were triggered less than 2 ms after the positive RS, and 11 (52%) of them were triggered after a long time delay of 5-187 ms. Based on high-speed video and mapping results, the short time delay was associated with the reactivation of the decayed established leader above the tower. The long time delay was associated with the negative leader extending to the region above the tower during the long continuing current. The positive E-field change during RS or continuing current period favored the inception of an upward leader. Although both preliminary breakdown (PB) and RS in a +CG flash produced positive E-field changes, an upward positive leader induced by PB aborted its sustained propagation after +RS.