Radiation field pulses associated with the initiation of positive cloud to ground lightning flashes

Seventy one electric field pulse trains that occurred during millisecond-scale time intervals before positive cloud to ground lightning flashes were analysed. These pulses are bipolar in nature and somewhat similar in pulse characteristics to the breakdown pulses preceding negative cloud to ground lightning. However, in the case of these positive flashes, the pulse characteristics of the pulse trains are confined in a much wider range of values than those of the pulse trains associated with negative return strokes. The leading edge of the pulses of the most commonly observed pulse trains that precede positive return strokes are relatively smooth, thus, different from their counterparts associated with negative flashes, in which case a few narrow pulses are superimposed on the rising edge of the bipolar pulses. Considering the initial polarity of pulses, four types of bipolar pulse trains preceding positive return strokes were identified. For each type of pulse trains, statistics of pulse characteristics were given. In contrast, in the case of negative ground flashes, the bipolar pulse trains were almost always composed of pulses of the same polarity as that of the succeeding return stroke. The possible causes of the observation of several types of pulse trains and the significantly diversified pulse characteristics of the breakdown pulse trains of positive flashes were discussed. The frequency spectrum of the electric fields of the most common type of pulse trains was compared with the spectrum of the breakdown pulses of negative flashes and those of negative return strokes. This spectrum of the preliminary breakdown pulse trains of positive ground flashes is comparable with that of the preliminary breakdown pulse trains of negative ground flashes. (C) 2004 Elsevier Ltd. All rights reserved.