| In nature,most of the lightning starts from thunderstorm clouds,and initial stage of development and transmission of lightning is also completed in the thunderstorm clouds.In addition,subsequent return stroke of cloud-to-ground(CG)lightning and discharge processes of different types of return strokes are all inspired or initiated by the intracloud discharge.Therefore,understanding the process of intracloud discharge it is of great significance to give sight into the occurrence conditions,formation mechanisms and transmission mechanisms of lightning.Unfortunately,it is difficult to directly conduct optical measurement of the intracloud discharge processes because of the influence of cloud occlusion,detection distance and other factors.Compared with VHF lightning location technology,the 3D location technology of lightning radiation source based on low-frequency signal has the characteristics of strong anti-interference ability,wide detection range and pulse-polarity recognizability,which can effectively make up for the disadvantages of optical detection methods and VHF location technology,and it has become one of the dominating methods in the field of intracloud discharge research.In this dissertation,a 3D lightning location network of low-frequency radiation source was established based on SHAndong Triggering Lightning Experiment(SHATLE),and the data acquisition system was also designed independently.The high-precision 3D imaging of lightning intra-cloud discharge was realized by combining Chan and Double Particle Swarm Optimization(DPSO) based on Tent mapping multi-scale mutation particles.Moreover,the development characteristics and physical mechanism of CG pulse discharge process in artificially triggered lightning,the initial discharge process and the leader propagation with irregular radiation characteristics in natural lightning were studied and analyzed based on the collected lightning 3D location data and synchronous observation of artificially triggered lightning.The main contents and conclusions are as follows:A lightning dual time base data acquisition mode(ABA)was developed to realize the high sampling rate triggered acquisition of single-channel and large-amplitude lightning signal and low sampling rate continuous acquisition of complete signal,which ensured the acquisition efficiency as well as the integrity of lightning data,.By setting FIFO buffer,designing real-time data acquisition software and double time stamp technology,the data acquisition system of lightning detection and location network exhibits high stability and reliability.A location algorithm(Chan-DPSO)based on Chan and dual group PSO of multi-scale mutated particles based on Tent chaotic map was proposed.The influence of Levenberg-Marquarelt(LM)algorithm on the optimal fitting effect of initial values with different accuracy was analyzed.The method combining Chan and DPSO solved the problems of"premature"in the fitting process and the dependence of optimization results on the accuracy of initial solution,which effectively improved the efficiency and accuracy of lightning 3D location.Moreover,Chan-DPSO and Chan-LM algorithm were compared through the simulation experiment,the results indicated that Chan-DPSO possessed higher location accuracy when the time error of each station exceeded 1.6?s.Monte Carlo simulation experiments showed that the horizontal and vertical errors in the dense area of lightning 3D location network were both less than 200 m,and the horizontal location accuracy of the network in the northwest was better than that in the northeast..The processing method of lightning signal and location data was studied,and the high-precision 3D imaging of lightning low-frequency radiation source was realized.The interference waveform that can not be filtered by the filter was eliminated,and the accurate lightning pulse was obtained by the signal processing method of noise pulse recognition.The high-precision 3D imaging of the intracloud discharge processes,such as the initial discharge process of lightning,the K-process,the stepped/dart leader,was realized by setting the chi square threshold and comparing the results of chi square filtering.In addition,the accuracy of lightning location data was evaluated through the comparation with the known artificial trigger lightning"drain rod"position,and the average horizontal error was 104.9m.The lightning radiation sources were clustered and fitted in three dimensions using DBSCAN clustering algorithm,and the fitting curve could represent the characteristics of the winding development of lightning channel and improves the accuracy of speed calculation.Furthermore,the development speed of each branch of lightning and the variation of lightning development speed were also obtained.The discharge characteristics of the pulse to the ground were studied based on the observation results of the channel-base current in artificially triggered lightning from 2005 to 2017.The obtained geometric mean values of the peak current,rising edge,transferred charge and half peak width of the return stroke,initial continuous current pulse,M component were 11.9 kA,0.19 kA,0.4 kA,0.71?s,462.8?s,207?s,0.89 C,0.36 C,0.19 C,14?s,845?s,267?s,respectively.Three types of intracloud discharge processes causing the M component were determined based on the location of lightning radiation source.One of the rare M components was caused by reactivation of the residual charge in the channel after short interruption of the upward return stroke wave.It was observed that the current waveform of return stroke pulse would be affected by the intracloud discharge during the hybrid CG lightning discharge process.The influencing factors were the duration and the current intensity of mixed discharge channel.The initial discharge process of lightning was studied based on the 3D lightning location data of low-frequency radiation source.The obtained results of upward initial leader(UIL)and the downward initial leader(DIL)were as follows:3D velocity were 9.8×104-1.4×106m/s and 1.1×105-1.2×106 m/s,the mean values were 4.2×105 m/s and 5.4×105 m/s,the average values of perturbation were 1.1×10-3and 9.7×10-4,the average curvature values were 2.2×10-3 and 2×10-3.The results illustrated that the winding degree of UIL was greater than that of DIL.In order to study the bifurcation development characteristics of the initial leader,the ratio of the amplitude to the rising edge of the initial discharge pulse was employed to effectively distinguish the radiation sources related to the main development direction of the initial leader and other radiation sources.It was found that the development steps length of the main channel,branch channel and needle leader of the initial leader were about 200 m.Bifurcation may occur at the beginning or intermediate stage of the initial leader development.The process of leader development with irregular radiation characteristics was studied.Sample entropy was introduced to quantify the irregularity of initial leader electric field.And the leaders with irregular radiation characteristics were divided into three categories according to the characteristics of sample entropy,that is,entropy increasing or decreasing in the later stage of the leader development,and entropy being stable in the middle stage.The variation of leader irregularity was related to the leader development characteristics based on the 3D location of lightning low-frequency radiation source.It was found that the bifurcation of the leader would increase the irregularity of the radiation signal,and the ionization degree of the leader channel was also an important factor affecting the irregularity of the radiation signal. |
PDF Full Text Request