Research On Wide-area Lightning Detection Technology Based On Time Compensation Algorith

Long-range lightning detection has great value for studying statistically the temporal and spatial characteristics of wide area lightning as well as developing lightning monitoring and early warning.However,the accurate detection performance of ground-based long-range lightning locating systems（LLS）is limited by the propagation effects on lightning electromagnetic waves.In order to improve the performance of the long-range LLS,the propagation effect on very low frequency（VLF）ground wave is evaluated and a propagation correction method is presented based on the long-range detection network.Then the lightning detection data is used to estimate the peak currents and the ground equivalent conductivity in several regions in China.The main results are as follows:（1）Due to the propagation effects from ground finite conductivity and the curvature of the earth,the arrival time of the ground wave peak is increasingly delayed for longer propagation paths.A propagation correction method is presented to improve the location accuracy of long-range lightning detection network by compensating the peak time of lightning-radiated VLF ground waves.The improvement of the relative location accuracy and location offsets are evaluated by comparing with the Advanced Direction Time Lightning Detection System（ADTD）datasets.It shows that the mean relative accuracy is improved from 7.74 km to 4.32km,and the median relative accuracy is improved from 7.28 km to 2.46 km.The mean westwards offset of the total lightning location data drops from 2.05 km to 0.93 km,and the mean southwards offset drops from 1.19 km to 0.63 km.The existence of terrain over the propagation paths compromises the positioning performance of the long-range network.It shows that removing the sensor which is under the effect of the terrain and using the terrain envelope method can both further improve the location accuracy.（2）The time reversal（TR）technique makes use of the spatiotemporal consistency of homologous signals,and can accurately achieve the grouping of homologous signals.With the increase of the number of waveforms received by the stations,homologous signal grouping technology can still maintain excellent performance,and the increase of the number of homologous waveforms can provides some robustness to this technology.In general,relative location accuracy of TR method and Time of Arrival（TOA）method is not much different.But because of the robustness of TR method,the waveform with significant time error can be automatically removed in TR back propagation simulations,making the relative positioning location of TR method for this part of cloud-to-ground（CG）lightning slightly higher than that of TOA method.（3）The signal strength is proportional to the distance dependence of D^-1.32.The peak current of CG far away from the LLS will be seriously underestimated if not take the attenuation of ground wave into account.After modeling the attenuation of the radiation field amplitude,the correlation coefficient between the peak-current values and the range normalized signal strengths（RNSS）for each sensor is increased.（4）Based on the measured propagation delay of ground waves,the estimated ground equivalent conductivity in China in the range of 10^-4-10^-2 S/m,the average value is close to10^-3 S/m.The ground equivalent conductivity in different terrain areas is different,for it in Tibetan Plateau is in the range of 10^-4-10^-3 S/m,while in Loess Plateau is in the range of 10^-3-5×10^-3 S/m,and in North China Plain and Yangtze Plain,Middle and Lower is in the range of5×10^-3-10^-2 S/m.