An experimental study of positive leaders initiating rocket-triggered lightning

Simultaneous, co-located measurements of ambient, electrostatic-field profi les and rocket-triggered lightning phenomenology beneath Florida thundersto rms are reported. Ambient-field conditions that are sufficient to initiate and sustain the propagation of positive lightning leaders are identified. I t is found that lightning can be triggered with grounded triggering wires a pproximately 400 m long when the ambient fields aloft are as small as 13 kV /m (foul-weather polarity). Ambient potential differences between the heigh t of the triggering wires and ground were as small as -3.6 MV (negative wrt . earth) when lightning occurred. 'Precursors,' the first measurable curren t pulses from the triggering wires, were initiated at similar fields aloft but at wire heights only about half as large, where ambient potentials were as small as - 1.3 MV. The mean speed of one 'failed leader' is estimated a t 1.9 X 10(4) m/s over 35 m of propagation. The lengths of 'leader' extensi on and positive-streamer 'fan' during individual impulses of multiple-pulse precursors are estimated to be 0.7 and 2.0 m, respectively. (C) 1999 Elsev ier Science B.V. All rights reserved.