Trigger Mechanism of Upward Lightning by Positive Cloud-to-Ground Lightning in Beijing

Inhalt:
We conclude the observation of tower-initiated upward lightning observation in Beijing in the last decade, using two high-speed cameras targeting lightning on a 325-m meteorology tower and using the Beijing lightning network (BLNET) to record the broadband electric field and mapping the radiation produced by progressing lightning leaders. We captured 21 upward lightning (UL), which have synchronized high-speed video and multiband electric field detection, and they are all triggered by a nearby positive cloud-to-ground (CG) lightning flash. The UL can be triggered by a stroke current wave immediately after a positive return stroke (RS) if a previous negative leader has passed overhead the tower, or an approaching negative leader can trigger it during the long continuing current after +RS. The parent CG lightning with a structure of upper positive leaders and lower negative leaders is favorable for the inception of UL. In contrast, the lower positive leader in a normal +CG lightning can occupy the lower charge layer to hinder the sustained propagation of an upward leader. In such kind of parent +CG lightning, the positive leader initiated from the lower horizontal negative leader to produce a +RS or subsequent +RS, which explains why 38% of the parent +CG lightning have more than one termination.