| Lightning location system can ascertain number lightning and its position, time and discharge parameters in observation areas, which is very useful to study lightning disaster location, trouble searching, lightning-parametric statistics and of lightning activity law reaseach. Currently the CG (cloud to ground) location system can only provide the position、time and amplitude of CG lightning current, and can not provide the intra-cloud lightning information. Developing three dimensional total lightning location technology which is able to probe CG lightning return stroke, IC (intra-cloud) lightning and special intra-cloud lightning Narrow Bipolar Event (NBE) is of great significance in thunderstorm activity distinguishing, spatial distribution of lightning discharge, lightning-parametric statistics of total lightning warning.VLF/LF three dimensional total lightning location technology based on time-of-arrival (TOA) technology is studied in this thesis. Nine stations VLF/LF three dimensional total lightning location system (CTLLS) have been built in Chongqing area since2009, which have been used to observe three dimensional lightning discharge for three years. Main results are showed as follows:Precise location is closely related to the performance of hardware and software technology of location system. Self-adjusting bipolar floating trigger level mode, multi-threaded and multi-task management mode were used in our system. The timing accuracy of the GPS module achieves to50ns. Then the key problems including trigger settings continuous high-precision time synchronization and real-time data acquisition in complex lightning multi-burst enviorment have been effectively solved. Owing the fuzzy solution existing in two dimensional location with three stations as well as three dimensional location with four stations, three-dimensional location algorithm with five stations is adopted which can transform nonlinear equations into linear matrix equation, and unique solution is achieved. Due to the puzzle of exact match in redundant multi-station precision location, waveform time window cross-correlation time delay acquiring method and Levenberg-Marquardt (L-M) optimal least square algorithm are put forward, i.e., it is according to waveform characteristic but not the absolute time difference of some pulse to calculate waveform time delay among the stations. As a result, problem of waveform inconsistency stemming from propagation characteristic and other factors is solved, and precise geographic information and time information are obtained.Numerical examination of positional error and reasonable station distribution are vital to improve location system precision. Random number Monte Carlo three dimensional lightning location error inspection method is proposed in this thesis, which can execute numerical calculation of location error in different network stations arrangement inside and outside the location network station. In three-dimensional location network based on TOA, it shows that both radial distance error and vertical height error are in linear relationship with square of distance named r of radiation source, but in inverse ratio with square of physical dimension named D of station network. Azimuth angle error increases proportionately with the distance of radiation source r but inversely with physical dimension D. So it is effective to optimize location precision in monitoring area by setting reasonable network long-short baselines arrangement and distance of station baselines.Waveform characteristic of lightning electric field radiation is a basic issue of recognition of CG, IC, NBE parametric statistics of total lightning. Statistical location results of CTLLS display that trailing edge of first lightning return stroke exist a obvious secondary peak structure, which has equivalent amplitude with primary peak, and much anomalistic noise is added on trailing edge of waveform. Subsequent lightning return strokes come up with obvious isolate secondary peak structure, which is half of primary peak in amplitude and with smooth trailing edge of waveform. It is a distinguishing characteristic of lightning return strokes radiation signal that secondary peak ratio is bigger on an average of0.68. Among those lightning return strokes, more than88%return strokes are with secondary peak ratio bigger than0.5, and nearly70%with ratio of forward Signal-to-Noise Ratio (SNR) to rearward SNR of VLF radiation signal bigger than50. The rise time, fall time and impulse width of CG lightning return stroke averages3.98μs,26.45μs and30.44μs respectively, and its rise time increases with lightning distance extending. However, radiation waveform of intra-cloud lightning is with smoother rising edge but rapider trailing edge, whose rise time and fall time is8.43μs and9.71μs respectively, and its SNR, forward SNR and rearward SNR values58,137and32, differencing from CG lightning return stroke signal apparently. NBE radiant intensity, which is often stronger than that of CG lightning return stroke, is with SNR bigger than20dB. Waveform of positive polarity NBE is with rise time, fall time and impulse width averaging3.09μs,2.16μs and5.26μs, while negative polarity NBE is3.97μs,1.8μs and5.77μs respectively, namely that positive polarity NBE is with rapider rising edge but slower trailing edge. Impulse width time of NBE waveform is less than one fifth of CG lightning return stroke, but rearward inverse peak ratio of NBE is twice of that. Due to digital waveform observation of lightning electric field radiation waveform, recognition method and quantitative parameters of radiated electric field waveform are carried out in CG lightning, intra-cloud lightning and NBE.Combinating with CTLLS and Radar reflectivity, typical summer thunderstorm in Chongqing area is observed. According to the observation results, most active time of lightning strong convective weather in Chongqing is mainly concentrated in June to August. The convective activity at night is more frequently. Active area of transit thunderstorm is large but the intensity of convection is not frequent, and convective activity of locally generated thunderstorms is very frequent, the local thunderstorms show small regional growth and decline.The percentage of the thunder intra-cloud flash, cloud-to-ground lightning and NBE is66%,31%and3%respectively in Chongqing area, the positive and negative cloud flash ratio is3.0while the ratio of the intra-cloud flash to cloud-to-ground lightning is2.2. Among the cloud-to-ground lightning, the negative ground lightning flash covers an area of94%of the total. The height of about all the lightning is20km or less for the80%is below15km. The negative intra-cloud flash lay mainly in8to11km while the positive ones mainly concentrated in7to9km and11to14km. In addition, the discharge height of negative NBE is7to10km and the discharge height of negative NBE is12to16km. the discharge height of negative NBE is obviously higher than the discharge height of negative NBE.75%of the positive itra-cloud lightning current is concentrated in10to25kA, lightning current amplitude of negative intra-cloud flash is significantly higher than those of the positive, and lightning current amplitude of nearly50%of the negative cloud flash is over30kA. It displays the important application value.of ground-base VLF/LF three dimension total lightning location system in real-time monitoring of thunderstorm, lightning activity parameters and thunderstorm charge structure research. |
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