Characteristics of upward positive lightning flashes initiated from the Gaisberg Tower

Abstract

We report the measured current characteristics of positive lightning discharges to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. On the basis of the recorded current waveforms, a total of 26 flashes consisting of initial stage only were identified as upward positive discharges initiated by an upward negative leader from GBT. They accounted for 4% (26/652) of the total flashes recorded at the GBT. Nineteen (73%) out of the 26 positive flashes occurred during nonconvective season (September–March). Median values of flash peak current, flash duration, charge transfer, and action integral were determined as 5.2 kA, 82 ms, 58 C, and 0.16 × 103 A2 s, respectively. Current pulses of high repetition rate superimposed on the initial portion of initial continuous current are inferred to be associated with the upward negative stepped leader process. The weighted arithmetic means of leader pulse peak current, leader pulse duration, leader interpulse interval, and leader pulse charge are 3 kA, 31 μs, 32 μs, and 42 mC, respectively. On the basis of an assumed stepped leader speed in the range of 8 × 104 to 4.5 × 105 m/s an upward negative stepped leader channel charge density of 15–87 mC/m, a leader length of 168–945 m, and an average leader step length of 2.4–13.3 m were estimated. The upward negative stepped leader channel charge density and length are significantly larger and smaller than their counterparts in downward negative stepped leaders, respectively, while the upward leader step length is consistent with previous studies. Possible reasons for this are discussed.