Observation And Research Of Upward Leaders For Artificial-triggered Lightning

In this paper, the channel base current, electromagnetic field changes at close ranges, and the optical luminosity of the discharge channel by high speed camera system are measured synchronously with high time resolution during the Guangzhou Artificially Triggering Lightning Experiment(GCOELD). Analysis of two forms of precursor current pulse current may cause, research the discharge characteristics before upward leader initiation and the transmission characteristics of upward leader in the initial stage, the main results of this paper are as follows:Based on the analysis of current data of 25 classical rocket-and-wire triggered lightings, it can be confirmed that the unipolar PP(precursor pulse) is generated from the lightning triggered by copper wire, while the unipolar PP is generated from the lightning triggered by steel wire. As for the unipolar PP, the geometric mean(GM) values of peak current, rise time, duration of waveform and charge transfer are 26 A, 0.33 ?s, 2.3 ?s and 27 ?C, respectively. In contrast, as for the bipolar PP, the corresponding values are 67 A, 0.24?s, 2.1?s and 54?C, respectively. The GM value of peak current for bipolar PP is close to 2.6 times that of the unipolar PP, however, the GM values of duration and rise time are similar to unipolar PP.By using the transmission line model. The simulated waveform is consistent with the observed one. We speculated the current pulses generated by the upward leader initial discharge at the top of the transmission line are unipolar.Before the upward positive or negative leader initiation for artificial-triggered lightning test, the form of corona discharge is the same, the current waveform at the bottom of the channel is not continuous, which occur in a single or group form. Space charge has a certain shielding effect, which will cause the channel base current will appear in a period of “dark period”. The ground electric field is higher, which will cause the lower the height of the trigger triggered lightning, the duration of corona discharge is shorter. By using a simplified upward leader model, simulated the effects of space charge and environmental electric field on the initiation and development of the upward leader, and in consideration of the influence of space charge, the minimum electric field required for different height of upward positive leader initial development.The average 2-D speed of the upward positive leader in negative triggered lightning was 5.5×104 m/s, as the leader developed between 300 m and 600 m above ground. High speed imaging field of view,the average 2-D speed of the upward positive leader was 2.0×105 m/s with the partial speeds ranging from 4.3×104 m/s to 4.9×105 m/s, as the leader developed 600 m above ground. Step length of upward positive leader inception stage is 0.6m~2.7m, the average value of 1.4m. The line charge density of the successful positive lead in the initial phase is 18?C/m~120?C/m, and the average value is 52?C/m, the line charge density of the failed positive lead in the initial phase is 17?C/m~84?C/m, and the average value is 30?C/m. Simultaneous impulsive current waveforms and stepped electric field changes at close range were observed during the initial stage of the leader development, indicating that the leader involved a stepwise propagating mode.As for precursor current pulse of upward negative leader, the geometric mean(GM) values of peak current, rise time, duration of waveform and charge transfer are 38 A, 0.27?s, 14.6?s and 60?C, respectively. Compared to the upward positive leader, the initial of the negative leader is more difficult. The average 2-D speed of the upward negative leader in artificial-triggered lightning was 1.01×105 m/s with the partial speeds ranging from 9.2×104 m/s to 3.07×105 m/s. It is possible that the step formation mechanism of the upward negative leader was similar to that of Long gap discharge test, both involving a bi-directional streamer development ahead of the leader tip, however the cascade feature of upward positive leader may be the occurrence of intermittent stop, because of the shielding effect of space charge, which reduced the intensity of the electric field in the local area of the leader head.