Fine Observation And Study On Initial Stage Of Upward Positive Leader Of Artificially Triggered Lightning

Based on the advantages of artificially triggered lightning,we carry out positioning observation and current measurement on the initial stage of upward positive leader by using continuous interferometer(CINTF),Low Frequency E-field Detection Array(LFEDA)and lightning current measurement system.This study improves fine detection capability of artificially triggered lightning,and gains a deeper understanding of initial stage of upward positive leader.The main work and conclusions are as follows.1.In order to analyze the development characteristics of initial stage of upward positive leader by using CINTF and LFEDA positioning systems together,study on related data processing methods is carried out.(1)A new lightning low frequency discharge three-dimensional positioning algorithm,namely time of arrival-time reversal(TOA-TR)algorithm,is established.And a window sliding correlation pulse matching is implemented.Fine positioning capability of LFEDA is improved when the signal is weak and the number of matched stations is small.The analysis results of a cloud to ground lightning flash show that,the number of positioning points obtained by using the new algorithm is greatly improved.And reliable three-dimensional positioning results can be obtained by using only four-station data.Based on the artificially triggered lightning data obtained in 2017,the performance of LFEDA by using the new algorithm is evaluated.The results show that the detection efficiencies for seven triggered lightning flashes and 45 return strokes are 100%and 95.6%,respectively.The average plane error of the return strokes is 153m.The positioning accuracy is close to that of traditional method.(2)The compensation method of CINTF positioning result is studied.The error calibration of CINTF positioning result of initial stage of upward positive leader of close triggered lightning is realized.And the accuracy of elevation is improved.When the elevation angles of initial stage of upward positive leader of triggered lightning are 40°,50°and 60°respectively,the angle errors can be reduced by about 4.3°,4.0°and 3.4°respectively after calibration.And the distance errors can be reduced by about 11 m,14 m and 20 m after calibration.2.Fine observation of 14 triggered lightning in Conghua,Guangzhou in 2019 was carried out and further understanding of initial stage of upward positive leader is obtained.(1)The discharge process including a self-sustaining precursor current pulse cluster(SPCP)process and an initial continuous current(ICC)process is defined as an initial process.64.3%of the triggered lightning has only one initial process during the initial stage,and precursor current pulse(PCP),PCP cluster,SPCP and ICC are the typical forms of current waveform.35.7%of the triggered lightning has multiple initial processes,and there are two or three PCP-SPCP-ICC processes on the current.The duration of short self-sustaining development that occurs in the initial stage is usually less than 10 ms.And the current of ICC is very weak,which will not cause wire fusing.(2)During the initial process,the pulse duration and transfer charge of PCP are linearly increasing.When they reach a certain level,PCP cluster and SPCP will appear.In this study,the average values of pulse duration and transfer charge of PCP corresponding to the appearance of PCP cluster are 3.5?s and 19.5?C,respectively,and the average values corresponding to the appearance of SPCP are 4.7?s and 27.2?C,respectively.(3)PCP is produced by a weak upward positive breakdown and a subsequent strong downward negative breakdown near the tip of the rising rocket,which is in the form of an isolated pulse.PCP cluster and SPCP are signs that the upward positive leader starts to attempt self-sustaining development.Compared with the nearest PCP cluster,SPCP contains more pulses,and the critical value range between them is 6-7,and the total duration is longer,the critical value range is 137.8-198.1?s.The average pulse duration and average pulse transfer charge of SPCP are also larger,with the average values of 8.1?s and 51.8?C respectively.In contrast,the average values of PCP cluster are 4.8?s and 34.3?C respectively.After PCP cluster,self-sustaining development ends immediately.But after SPCP enters the ICC process,self-sustaining development may fail,or it may continue to develop steadily for a long time.(4)During the successful self-sustaining stage,the upward positive leader first develops upward.The average speeds at the height of 1100-1700 m and 1700-2300 m are about 1×10~5m/s and 1.6×10~5 m/s respectively.The speed is increasing.After the leader enters the charge region in cloud,it develops horizontally.The development speed increases to about 6×10~5 m/s.