The Study Of Characteristic Pulse Trains Of Preliminary Breakdown Based On VLF/LF Multi-station Lightning Detection System

Preliminary Breakdown Pulses(PBPs)or Initial Breakdown Pulses(IBPs),which is characterized by a train of microsecond-scale bipolar pulses produced during the stage of preliminary breakdown,have attracted great attention from academia as its great significance to reveal the mechanism of lightning excitation for the close relationship with the preliminary breakdown process.In this paper,a VLF/LF multi-station lightning detection system based on GPS time synchronization technology is used to observe the characteristics of PBPs in two different types of thunderstorms in Hefei,China.The main conclusions are summarized as follows:1.According to whether there are obvious PBPs in lightning flashes,the negative cloud-to-ground(CG)flash and intracloud(IC)flash is divided into PBCG-type flash,CG-type flash,PB-type flash and IC flash.PBA,RSA,PB-RS ratio and PB-RS interval are defined for PBCG-type flash.2.The characteristics of PBPs in four isolated small thunderstorms are reported.The results show that PBPs exist during the early stage of thunderstorm and partially exist during the active stage,but do not exist during the final stage.The negative CG flashes have been reduced when PB-type flashes occur during the active stage,which is very consistent with the LPCC model.However,at the early stage it is not the case,so a modified LPCC model is proposed to explain this phenomenon well.On the basis of 22 PB-type flashes out of totally 2155 flashes,it indicates that the number of PB-type flashes is very small.It is found that PBCG-type flashes and CG-type flashes are always in the form of relatively separate in the time-frequency distribution statistically.The percentage of PBCG-type flashes in four thunderstorms is calculated and shows that it is positively correlated with PBA and PB-RS ratio in three thunderstorms.Further analysis reveals that the changes of MNCC make it not always positively correlated with PBA or PB-RS ratio,suggesting that the changes of MNCC makes it not always positively correlated with the PBA or PB-RS ratio,and that this may be one of the reasons for the low percentage of PBCG-type flashes.The PB-RS interval in the four thunderstorms is calculated,with the minimum value of 21.4ms,which is significantly different from that of other studies.3.The characteristics of PBPs in one mesoscale multi-cell storm are reported.The results show that PB-type flashes are observed at the early and active stage while there are no PB-type flashes at the final stage.There is no reduction in negative CG flashes when PB-type flashes occur at the active stage,which is different from the result in the small thunderstorm,but can also be explained by the modified LPCC model.Out of totally 1000 flashes,the recorded 14 cases of PB-type flashes is only 1.4%.Unlike the results in the small thunderstorms,it shows that PBCG-type flashes and CG-type flashes have a high degree of coincidence in the time-frequency distribution.The PB-RS interval in this thunderstorm is calculated,with the minimum value of 3ms,which is similar to that of others.4.Comparison of the characteristics of PBPs in two types of thunderstorms.The results show that there are PB-type flashes during the early stage but no reduction in negative CG flashes(if there has a negative CG flash),which is consistent with the modified LPCC model,speculating that there exist a large LPCC with a very short duration during early stage in most thunderstorms.The possibility of occurrence of PB-type flashes in both types of thunderstorms is very low and is considered to be true in all thunderstorms.The merging and splitting processes are observed in the two types of thunderstorms.It is found that PB-type flashes are always observed during the stage of distinct cell merging or splitting.The minimum values of PB-RS intervals in the two types of thunderstorms are 21.4 and 3ms,respectively.It is found that the minimum value of PB-RS intervals is determined by the relationship between PBPs and return strokes,and the difference between them makes the difference between PB-RS intervals.After further analysis,it is found that the different relations between PBPs and return strokes are mainly from the uncertain relations between PBA and RSA.The conceptual model of the relations between PBPs and return strokes is proposed,and the possible existence of the physical meaning will be discussed.