Comparison of preliminary breakdown pulses observed in Sweden and in Sri Lanka

In this paper we have analysed breakdown pulse trains preceding the first return stroke in 47 negative cloud-to-ground (CG) lightning flashes observed in Sri Lanka (tropics) and that in 41 negative CG flashes recorded in Sweden (temperate regions). In the data obtained in Sri Lanka, breakdown pulses could be detected only in nine flashes. In other flashes either these pulses are absent or, if they are present, their amplitudes should be below the background noise level. From the nine flashes with breakdown pulse trains, we obtained the following results. The ratio between the maximum breakdown pulse amplitude and the return stroke amplitude (BP/RS ratio), as a percentage, is 16.5%. The time duration between the most active part of the pulse train and the return stroke (BP/RS separation), is 11.9 ms and the ratio between the noise amplitude and the return stroke amplitude (N/RS ratio), as a percentage, for all 47 flashes, is 5.0%. In the flashes observed in Sweden, breakdown pulses are detectable in all records. In this data, the BP/RS ratio, as a percentage, is 101.4%, the BP/RS separation is 13.8 ms and the N/RS ratio, as a percentage, is 3.7%. All the above values are arithmetic means. Most of the pulses of breakdown pulse trains observed in both countries are bipolar in nature with the initial polarity the same as that of the succeeding return stroke. (C) 1998 Elsevier Science Ltd. All rights reserved.