Charge Transferred to Ground during Different Stages of Upward Lightning Flashes from the Gaisberg Tower

We examined current waveforms for 823 upward flashes occurring in 2000 to 2018 initiated from the Gaisberg Tower located in Salzburg, Austria. Current was measured at the tower-top using a 0.25 mΩ shunt. Of the upward flashes, 651 (79%) were negative (transferred negative charge to ground), 35 (4.3%) were positive (transferred positive charge to ground), and 137 (17%) were bipolar (transferred both negative and positive charge to ground during different stages of the flash). For positive flashes, the median initial stage (IS) current duration, peak current, and charge transfer were 96 ms, 3.2 kA, and 20 C, respectively. For bipolar flashes, the values of these parameters were 282 ms, 1.8 kA, and 43 C, respectively. Of the 137 bipolar flashes, 45% were of Type 1S (single reversal of current polarity during IS), 47% of Type 1M (multiple reversals of current polarity during IS), 5.1% of Type 2 [different polarities of current during IS and return stroke (RS)], 1.5% of Type 3 (RSs of different polarities), and 0.73% (one flash) of Type 4 (different polarities of RS and the following continuing current). The median absolute value of charge transfer during the IS for Type 1S, 1M, 2, and 3 flashes were 43 C, 43 C, 93 C, and 39 C, respectively. The charge transferred during the IS of the Type 4 flash was 57 C. In this study, we will additionally examine the charge transferred by upward negative flashes in our dataset and compare the charge transferred during the IS of flashes with that during the post-IS period in which one or more returns strokes may occur. Also, we will analyze in detail the charge transfer characteristics of flashes with and without return strokes.