Research On Pre-breakdown In Cloud And Cloud To Ground Lightning Process Based On VLF/LF Lightning Location Network Observation

Lightning detection technology is one of the most important and indispensable technologies for studying lightning physics.In the past few decades,in order to obtain two-dimensional and three-dimensional images of lightning,lightning detection networks have been established in various parts of the world.The construction of the lightning detection network provides the necessary technical means for scholars around the world in studying the physical process of lightning.Based on the lightning detection network established by Nanjing University of Information Science and Technology in Nanjing and Kunming,this paper uses multi-synchronous observation data to study the characteristics of cloud flash and ground flash pre-strike processes in Nanjing;ground flash strike pulses based on different frequencies Waveform data,combined with the four methods of difference in time of arrival,we analyzed the differences in positioning results between Nanjing and Kunming.The main research results obtained are as follows:(1)Using the observation data of Nanjing Lightning Detection Network in July and August2018,combined with the’BIL’model,statistical analysis was performed on the characteristics of the pre-breakdown pulses in the 110 positive and 110 negative ground flashes within the cloud in Nanjing.The results show that the arithmetic averages of the total duration of the positive ground flash and the negative ground flash pre-breakdown are 17.64 ms and 14 ms,respectively,and the weighted average values of the duration of the single ground pulse and the pulse interval of the negative ground flash are 15.2μs and 169.5μs respectively.,The arithmetic average of the ratio of the maximum amplitude of the positive and negative ground flash pre-breakdown process to the peak value of the first returning magnetic field is 17.9%and32.7%.The time interval is 64.1 ms and 51.3 ms.(2)The Nanjing low frequency(VLF/LF)lightning magnetic field detection network was used to locate the pre-breakdown process of 60 cloud flashes in the afternoon of August 3,2018in Nanjing.Based on the 3D lightning location results,the relationship between the initial height of the pre-breakdown process in the cloud and the upward propagation velocity was studied.The results show that the initial height of the cloud flash pre-breakdown process ranges from1.75km to 10.2km,with an average height of 5.74 km.The vertical speed range is from0.25?10~5 m/s to12.34?10~5 m/s,and the average vertical speed is2.86?10~5 m/s.The vertical velocity is closely related to the pre-breakdown starting height.The higher the starting height,the pre-breakdown process will propagate upward at a lower speed,and the lower starting height,the pre-breakdown process will propagate upward at a higher speed.(3)Based on the observation data of the Nanjing Low Frequency(VLF/LF)Lightning Positioning Network and Kunming Low Frequency(VLF/LF)Lightning Positioning Network,a positive ground flashover process that occurred in Nanjing on August 3,2018 and June 2018were selected A negative ground flashover occurred in Kunming on the 8th.For the original time-domain waveforms of these two ground flashbacks,the Butterworth filter was used to arbitrarily extract the time-domain waveforms in the frequency range of 10 k Hz～50 k Hz、10k Hz～100 k Hz、10 k Hz～200 k Hz、10 k Hz～300 k Hz、10 k Hz～400 k Hz、10 k Hz～500 k Hz.Then,based on the ground flashback time-domain waveforms in the above different frequency bands,the four most commonly used TOA(time of arrival)time difference positioning methods,cross-correlation,peak-by-peak method,10%peak method and 50%peak method,are used for comparative analysis.The difference of the positioning results obtained by these four different time differences.The results show that the positioning results in each frequency band in Kunming area are greater than the reference point in Nanjing area.In each frequency band,the positioning results of the cross-correlation method are basically unchanged from the reference point,and the positioning results in different frequency bands of the peak-finding method are different from the reference point.Larger.