地闪三维随机模型的改进及多上行先导的数值模拟研究

为了探讨地闪连接过程中多上行先导的起始与发展，考虑到光学观测事实及雷暴电场环境，本研究在已有多上行先导三维随机参数化方案的基础上，植入背景电场模块并改进上、下行先导模块。基于新模型开展大量敏感性试验，结果如下:（1）新模型模拟的下行先导形态多样、分支数大幅度减少，能更好地再现多样的地闪连接过程。（2）在孤立建筑物情况下，上行先导长度、闪击距离以及触发多上行先导的次数与建筑物高度成正相关。（3）随着建筑物增高，同一次地闪中首个始发的上行先导与后续始发的上行先导间的起始时间差整体呈现上升趋势。初步研究表明:建筑物高度对触发多上行先导的影响显著。此外，受到建筑物高度及正、负先导发展情况的影响，低矮建筑物触发的多上行先导几乎同时起始，而高建筑物上允许首个始发的上行先导优先发展一定长度后再始发后续上行先导。 

Improvement of the three-dimensional stochastic cloud-to-ground lightning model and numerical simulation of multiple upward leaders

To explore the initiation and propagation of upward leader (UL) in the cloud-to-ground lightning attachment process, on the basis of the existing multiple upward leaders (MULs) three-dimensional model and considering optical observations and environmental electric field, the background electric field module is implanted and the UL and downward leader (DL) modules are improved. Sensitivity experiments are then carried out using the improved model. The results are as follows: (1) The new model can simulate a variety of DL, and the number of its branches is greatly reduced. Also, the model can well reproduce various cloud-to-ground lightning attachment processes. (2) Statistical analysis of simulation results shows the UL length, striking distance and the frequency of MULs originating from a structure are positively correlated with the structure height. (3) In addition, as the height of the building increases, the initial time difference between the MULs that start successively in the same lightning attachment process shows an overall upward trend. Preliminary research results indicate that the structure height has a significant impact on the initiation of MULs. And due to the influence of the structure height and the positive and negative leaders, MULs triggered by a low structure start almost at the same time, while on the tall structure, the first UL can propagate a certain distance before initiating subsequent UL.