中文摘要：

负地闪是闪电危害的主要来源,其对地转移电荷源特征和电荷量不仅对闪电放电机理的研究有重要意义,而且对雷电防护也具有重要的实际应用价值。为了研究具有典型城市下垫面环境的北京地区的闪电活动特征和放电强度,利用北京地区2011年两次雷暴过程的多站GPS（Global Position System）同步闪电地面电场变化定量观测资料,在考虑消除场地和环境因素对电场变化观测资料影响的基础上,基于蒙特卡洛数据处理方法和非线性最小二乘法拟合反演算法,定量研究了负地闪回击中和的电荷源位置和电荷量,并对回击特征与回击转移电荷源之间的关系进行了讨论,得到如下结果：（1）单次回击中和的电荷量为1.1-27.6 C,平均为8.6±5.2 C,不同序数回击转移电荷量的最小值基本不随回击序数的上升而变化,最大值和均值随回击序数的上升而减小;一次负地闪中和的总电荷量随着回击数的增加而增加。（2）负地闪回击数和回击时间间隔都呈对数正态分布,其中负地闪总数70.4%的多回击负地闪其回击间隔平均为99±95 ms,不同序数回击的时间间隔最小值随回击序数的上升基本不变,时间间隔最大值和均值随回击序数的上升而减小。（3）回击转移电荷量的均值随回击间隔的增加呈波动形式的逐渐上升;相邻回击转移电荷源的空间距离均值随回击间隔的增加而增大。

英文摘要：

The characteristics and amount of charge transferred by negative cloud-to-ground（CG） lightning have not only become an important research field with respect to the mechanisms of lightning discharge, but also have important practical applications for lightning protection. In order to study the lightning discharge characteristics and intensity in the Beijing region over a typical urban surface environment, using the slow and fast antenna synchronized by GPS（Global Position System）, quantitative observation data were obtained and analyzed to study the characteristics of negative return strokes. After eliminating the effect of the observation site and environmental factors on the electric field change as much as possible, the magnitude and locations of charge sources neutralized by strokes were fitted by using a Monte Carlo data processing method and nonlinear least-squares fitting algorithm. In addition, the relations between the stroke characteristics and the charge magnitude and locations were also studied. The results were as follows：（1） The charge lowered by a discrete negative return stroke ranged from 1.1 to 27.6 C with an arithmetic mean of 8.6 ±5.2 C. The minimum of the stroke charge did not change significantly with the stroke order, while the maximum and the arithmetic mean of the stroke charge increased with the stroke order. The total charge lowered to the ground by a single negative flash increased with the number of strokes it contained.（2） The percentage of multi-stroke negative CG lightning flashes was 70.4%, and the stroke number and the interstroke interval both fitted a lognormal distribution. The interstroke interval ranged from 3 to 967 ms with an arithmetic mean of 99 ±95 ms. The minimum of the interstroke interval did not change significantly with the stroke order either, while the maximum and the arithmetic mean of the interstroke interval also increased with the stroke order.（3） The average charge lowered by a discrete stroke increased with the interstroke interval pr