Analysis of Two Bipolar Flashes Recorded in Austria

Bipolar flashes account for a small percentage of total Cloud-to-Ground (CG) flashes. For that reason, studies are rare compared to positive and negative CG flashes. Bipolar flashes analyzed in this paper are characterized by different polarities of the strokes grouped to a flash. For the present publication, data of two bipolar CG flashes are analyzed. The flashes were recorded in Austria in July 2015 and August 2018. To record the data, a high-speed Video camera and an electrical Field Recording System (VFRS) were used. To complete the data set the recorded VFRS data are also correlated with Lightning Location System (LLS) data. This data set allows an analysis and parametrization of each step of the bipolar flashes. In addition, a comparison with parameters from the literature is performed, to describe the basics of bipolar lightning and compare the results with national and international analyses. One of the two analyzed cases shows special characteristics concerning the polarity reversal from negative to positive return stroke peak current for two strokes following the same channel. The second flash shows a typical formation process of a bipolar flash concerning the attachment of CG stroke channels to two different areas in the cloud. The analyzed flashes were categorized as bipolar of Type 3 flashes according to Rakov [1] (polarity reversal between return strokes).