Characteristics of Currents in Upward Flashes Transferring Negative Charge to Ground

Abstract:

We analyzed current waveforms of 651 upward negative flashes initiated from the Gaisberg Tower located 1287 m above sea-level on a mountain in Salzburg, Austria, between 2000 and 2018. 425 of 651 (65%) flashes occurred during the non-convective season (September–March) while 226 (35%) occurred during the convective season (April–August). The median charge transferred by flashes (whose current measurements were suitable for computing charge transfer) in the two seasons were 33.5 C (N = 423) and 32.5 C (N = 216), respectively. 68% of the negative upward flashes consisted of the initial stage (IS) only while the other 32% comprised of return strokes following the IS. There were 916 return strokes in 207 flashes with an average multiplicity (strokes/flash) of 4.4 and the multiplicity ranged from 1-18. Of these 207 flashes, unsaturated current measurements for the return stroke stage were available for 195 flashes with 864 return strokes. For these 195 flashes, the median duration and charge transferred were 446 ms and 33.9 C, respectively. Note that the median charge transferred during the IS of these flashes was 27.3 C. For 444 flashes without return strokes, the median duration and charge transferred were 271 ms and 32.0 C, respectively. The median interstroke interval, peak current, and 10-to-90% current risetime for strokes were 20 ms, 9.9 kA, and 1.3 µs, respectively. The total charge transferred during return strokes (including, in some cases, continuing currents and M-components occurring after the return stroke pulses) ranged from 0.12 C to 83.3 C with a median of 0.65 C. The charge transferred between the start-time of a return stroke pulse and the time when the return-stroke current reached 10% of its peak value ranged from 51 mC to 5.0 C with a median of 0.42 C, and the charge transferred within 1 ms of the return-stroke start-time ranged from 0.12 to 5.4 C with a median of 0.59 C.