Lightning-initiated Upward Lightning Flashes Observed at Tokyo Skytree in April 2017

Lightning observation has been carried out at Tokyo Skytree, which is a 634-m high freestanding broadcasting tower located on flat terrain at 1 m ASL. 25 downward and 40 upward flashes were observed at the tower from 2012 to 2017. This paper focuses on upward negative flashes initiated from the top of the tower observed in April 2017. Of the 3 confirmed upward flashes, all current waveforms were recorded by Rogowski coils installed on the tower at a height of 497 m. One out of 3 upward flashes was a positive flash initiated by a negative upward leader, and the rest were negative flashes initiated by positive upward leaders. For each event, we conducted simultaneous observations of the lightning current waveform, EM waveform, and optical images. Here, we report our recent observation results of lightning events.