Process of A Multiple-Stroke Positive Cloud-to-Ground Lightning and Its Trigger Effect to the Upward Lightning

Positive cloud-to-ground (+CG) lightning transfer positive charge from cloud to the ground and it usually has only one return stroke. The process of multi-stroke +CG lightning is remain poorly understood. It is interesting that there were 35% (6/17) of upward lightning from 325-m meteorology tower in Beijing from 2012-2018 triggered by the multi-stroke +CGs. We investigate the discharge process of an unusual three-stroke +CG lightning flash based on multi-frequency radiation sensors, and the three-dimensional channel development was carefully mapped. Three positive stokes terminated on ground at different positions, being 4-8 km apart, and the locations of striking points on the ground were just below pre-established horizontal IC discharge channels. Three positive strokes were linked with each other through the horizontal negative leader channels inside the cloud. It suggests that downward positive leaders could incept from the decayed negative leader branches, as confirmed by another case captured by a high-speed video. The advancing horizontal negative leaders after each return stroke removed the positive charge from cloud to the ground, and they induced evident electric-field changes over the ground. The horizontal negative leaders after the third return stroke propagated into the stratiform region and passed through the area above meteorology tower, which induced one upward lightning.