A Study Of The Positive Leader Transmission Process Based On High-speed Optics And Lightning Localization

The development and propagation of the leader is an important basis for the research of lightning physics and the fundamental to reveal the discharge mechanism.The study of the physical processes of the leader essentially how lightning begins and develops.The leader according to the polarity can be divided into the positive leader and negative leader,and the positive leader'light is too weak to detect on very high frequency(VHF).Therefore,there are few researches on the positive leader at present,Its propagation characteristics need to be further discussed.This study using simultaneous VHF interferometer map,channel-base current,fast electric field change and high-speed video images,characteristics of a natural positive cloud-to-ground lightning and a rare triggered bipolar lightning flash with two reversals of the polarity in discharge channel obtained by the SHAndong Triggering Lightning Experiment(SHATLE)are analyzed.This paper focuses on the initial of the positive cloud-to-ground lightning in the cloud and the propagation features of the downward positive leader,the development characteristics of the leaders before and after polarity reversal were studied and the mechanism of polarity in bipolar was discussed.The main findings are as follows:On June 29,2017,during a thunderstorm in Binzhou region,Shandong province.The total number of natural positive cloud-to-ground lightning occurred was 197times,accounting for 26.8%of the cloud-to-ground lightning.And these positive cloud-to-ground lightnings occupied during the initiation and extinction stages of the whole thunderstorm process.The peak currents of positive and negative cloud-to-ground were 325.2k A and-57.3k A,respectively.One case of natural positive cloud-to-ground occurred in the development stage of thunderstorm,9.7km away from the main observation point,with a duration of about 166ms.There was only a single return stroke,and the discharge time in the and the discharge time before the return stroke was about 155.75ms.At the bidirectional starting point in the cloud about 5.83km from the ground,the negative leader developed in the direction of increasing elevation angle in the form of multiple branches.After about 40ms,the positive leader developed in the direction of decreasing elevation angle.The speed of the downward positive leader increased from 2.78×10~4m/s in the preliminary breakdown process in the cloud to about1.32×10~5m/s near the ground,which increased by about 4.8 times over time.We find that the step pulse in the fast field before the return was caused by the discharge of negative polarity channel in the cloud.After the downward positive leader hitted the ground,it triggers a positive return stoke with a 2D speed of about 8.67×10~7m/s and the peak current of about 56kA.About 0.65ms after the return stroke,there was an M component process along the original negative polarity channel,with a duration of about 0.44ms,a 2D transmission speed of about 2.84×10~6m/s,and positive charge was transferred to the ground.1.The bipolar flash only contained the initial stage of about 315ms without return strokes.The polarity of net charge transferred to ground was first negative,then positive and finally negative,with transferred charge of about-40.0C,+13.3C and-1.0C,and peak current of about-3.8k A,1.6k A,and-2.25k A,respectively.According to the VHF interferometer map,the upward positive leader(UPL)initiated from the top of the wire and negative charges started to be transmitted to the ground.The UPL developed with no branches at 2D speed of3.7×10~4 m/s until it arrived about 1.39km above the ground.Multiple positive branches developed in the form of small-scale and dense recoil leaders in the cloud in a short time,which indicated the positive branch channels were active,transmitting negative charge to the ground.A negative leader initiated on a previously ionized positive branch channel and further developed into the virgin air horizontally at 2D propagation speed of1.59×10~5m/s,simultaneous transmitting positive charge to the ground.A negative pulse occurred in the fast electric changes after the negative leader was initiated for 28.82ms.About40ms after the negative pulse in the fast electric changes,the current polarity changed rapidly from negative to positive.Subsequently,the negative leader gradually faded out about 65.84ms after the first polarity reversal,the unextinguished positive branch channels in the cloud continues to develop,transmitting negative charge to the ground,and the channel-base current slowly changed to negative,producing the second polarity reversal.We also found a recoil leader generated on a previously ionized positive branch channel connected to the trunk channel,causing the initial continuous current pulse.A new negative developing to virgin air,instead of a recoil leader developing along the existed positive channel,was inferred to be closely related to the potential distribution of heavily branched positive leaders.2.The formation mechanism of upward bipolar lightning was reveal.Based on the analysis,the positive branch channel persistently transmitted negative charge to the ground during the whole discharge process.When the area of positive charge accumulation at the tail end of the negative leader connected to the trunk channel through a negative breakdown and transmitted positive charge to the ground,the net charge transferred to the ground was dominantly positive and the channel current polarity reversal occurred.With the extinction of the negative leader,the supply of positive charge ceased,so the current polarity reversed for the second time.Stable development of the positive leader and occurrence of the negative leader were inferred to play an important role in the two current polarity reversals.