Measured current and close electric field changes associated with the initiation of upward lightning from a tall tower

We examine in detail the simultaneous lightning current waveforms, close electric field changes, and lightning location system data for upward lightning discharges initiated from the Gaisberg Tower (GBT) from 2005 to 2009. Out of 205 upward flashes, most of them (87% or 179/205) were initiated from the tower top without any nearby preceding lightning activity (called “self‐initiated”), whereas 26 upward flashes (13%) were initiated from the tower top with immediately preceding nearby lightning activity (called “nearby‐lightning‐triggered”), including 15 positive ground flashes, one negative ground flashes, and 10 cloud discharges. The possible reasons for self‐initiated upward flashes dominating at the GBT could be the field enhancement due to the Gaisberg Mountain above the surrounding terrain and low altitude of charge region during non‐convective season (September to March), since we note that self‐initiated lightning at the GBT occurred predominantly (79% or 142/179) during non‐convective season. On the other hand the majority (85% or 22/26) of nearby‐lightning‐triggered upward flashes at the GBT occurring during convective season (April to August) and 80 nearby‐lightning‐triggered upward flashes out of 81 upward flashes observed at the ten tall towers in Rapid City in South Dakota of USA occurring during summer seasons, could be due to the result of high altitude of charge region. The triggering flashes were detected to be within 1 and 18 km distance and the time intervals between them and upward lightning initiation are in the range of 0.3 to 90.7 ms.