中文摘要：

三维雷电观测系统LMA（Lightning Mapping Array）是最近发展起来的基于GPS时钟同步的闪电VHF辐射源到达时间差（TOA）定位技术，能以很高的时间分辨率（50as）和空间定位精度（50—100m）展现闪电放电发展过程的三维时空分布，揭示雷暴中电荷结构及其与放电过程的关系。文中利用三维雷电VHF辐射源观测资料分析了负地闪预击穿过程的时空分布特征，讨论了云下部正电荷区对负地闪发生的影响，其结果表明在首次回击之前存在长时间预击穿过程的负地闪中，预击穿过程是云中部负电荷区与下部正电荷区之间的一种云内放电过程，闪电起始于云中部负电荷区，然后向下发展传输，进入正电荷区后闪电通道在云下部正电荷区水平发展，其放电特征与反极性云闪放电一致，云内放电过程最后阶段的K型击穿激发了地闪的梯级先导，梯级先导穿过云下部正电荷区向下发展传输。云下部正电荷的存在是导致负地闪首次回击之前存在长时间云内预击穿过程的主要原因。

英文摘要：

A new lightning locating technology, called Lightning Mapping Array （LMA）, has been developed. The system takes the advantage of GPS technology to measure the times of arrival （TOA） of lightning impulsive VHF radiation events at each remote loca- tion. The temporal and 3-D spatial development processes of lightning are described by measurements of the system with high time resolution （50 ns） and space precision （50 - 100 m）. The charge structures in thunderstorms and the relationship between the charge structure and lightning discharge processes are revealed. The temporal and spatial characteristics of preliminary breakdown process in negative cloud-to-ground（CG） lightning discharges are analyzed based on the data of lightning VHF radiation events. The effect of the positive charge region in the lower part of thunderstorms on the occurrence of negative CG lightning discharge is discussed. The results indicate that the preliminary breakdown process with longer lasting time of negative CG lightning discharges is an intracloud discharge process between the negative and positive charge regions located in the lower and middle parts of thunderstorms respectively. It initiated from the negative charge region in the middle part and propagated downward. After entered the positive charge region in the lower part, the lightning channel developed horizontally. The characteristics of preliminary breakdown process are consistent with that of inverted intracloud lightning discharges. The stepped leaders were initiated by the K type breakdown which occurred in the last stage of the preliminary breakdown process and penetrated downward through the positive charge region. The existence of positive charge region in the lower part of thunderstorms is the reason why there is a preliminary breakdown process with longer lasting time in thunderstorms before the first return stroke of negative CG lightning discharges take place.