Characteristics of Currents in Upward Lightning Flashes Initiated From the Gaisberg Tower

We examined the occurrence characteristics of the lightning observed at the Gaisberg Tower (GBT) in the years 2000 to 2018 and analyzed current waveforms (measured using a 0.25-mΩ shunt) of upward flashes initiated from the tower. During this period, 865 flashes were recorded at the GBT, of which 823 (95%) were upward and 4 (0.5%) were downward. For 18 flashes, the current waveforms were ambiguous and for 20 flashes they were unsuitable for analysis. Of the 823 upward flashes, 651 (79%) were negative, 35 (4.3%) were positive, and 137 (17%) were bipolar. The median initial stage (IS)-current durations in upward negative, positive, and bipolar flashes were 275, 96, and 282 ms, respectively. The median IS-current peaks in these flashes were 1.4, 3.2, and 1.8 kA, respectively. We expanded the traditional classification of bipolar flashes to include five categories. 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).