Upward bipolar lightning flashes observed at the Gaisberg Tower

We analyze current records for 21 natural upward-initiated bipolar lightning flashes observed to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. Thirteen (62%) of 21 bipolar flashes occurred in non-convective season (September- March) and twelve (57%) of them occurred in seasonal transmission periods of March, August, and November in Austria. Thirteen (62%) of them belong to Type 1 associated with a polarity reversal during the initial stage current, based on the classification as suggested by Rakov and Uman [1]. We also find that the initial polarity reversal from negative to positive occurs more often (16 of 21) than that from positive to negative within a bipolar flash, in agreement with observations in other studies. The geometric mean total absolute charge transfer is 99.5 C with a relatively short total duration of 320 ms.