Bipolar lightning flashes with multiple ground terminations including tall towers

Three large-scale (tens of kilometers in horizontal extent) multiple-stroke cloud-to-ground (CG) flashes were examined in detail. The flashes occurred in the same summer thunderstorm in Florida and were bipolar in that some of the strokes (leader/return-stroke sequences) in a given flash transported to ground positive charge, while others transported negative charge. Positive strokes tended to have higher NLDN-reported peak currents and all terminated essentially on the ground surface, each time forming a new channel. Negative strokes tended to terminate on tall towers and all that did so produced characteristic wideband E-field signatures that exhibited oscillations after the initial peak. Additionally, we examined a negative flash composed of seven strokes all of which terminated on tall tower. There were a total of three tall (451 to 497 m) towers involved, with one of them (497-m tall) being strike object for each of the four flashes examined here. The maximum GLM group energy for positive strokes was one to two orders of magnitude higher than for negative strokes. The oscillating E-field signatures are indicative of bouncing current waves that are manifestations of the transient response of a tall object to the lightning-caused excitation at or near its top (e.g., Rakov, 2001); it is due to impedance discontinuities at the lightning attachment point and at the ground. The period of E-field oscillations can be linked to the tower height and is influenced by the presence of guy wires. The oscillations were detectable both in the ground wave and in the first skywave.