Triggered upward flashes: Analysis of positive cloud-to-ground waveforms

Upward flashes are a subject of study and research since 1939 [1]. The recent increasing number of tall buildings, wind turbines and telecommunication towers has also increased the need of a better understanding of these lightning flashes, particularly, how they are triggered. Upward flashes may be self-triggered or triggered by some previous electrical activity (intracloud or cloud-to-ground flashes). In Brazil, upward flashes are a recent study subject. The first upward flash was observed in January 2012. All cases observed since then were preceded by some lightning flash. The same situation was observed in upward flashes recorded in Rapid City during 2011-2013 which are included in our dataset. To investigate upward flashes we use high-speed cameras (1,000 up to 100,000 frames per second), electric field sensors and lightning location system data. In this paper, waveforms of fast electric field sensors of positive cloud-to-ground flashes which triggered upward lightning flashes were analyzed and compared with high speed cameras observations.