Improvement of the three-dimensional stochastic cloud-to-ground lightning model and numerical simulation of multiple upward leaders
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摘要: 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。Abstract: 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.
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Key words:
- Numerical simulation /
- Multiple upward leaders /
- Structure height
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图 1 一次地闪模拟示意 （模拟域顶端红点代表下行先导随机起始位置，下行先导末端黑色段代表最后一跳）
Figure 1. Simulation of cloud-to-ground lightning （the red dot at the top represents the random initial position of the DL，and the black segment at the end of the DL represents the final jump）
图 2 模型模拟结果的对比 （a. 旧，b. 新）
Figure 2. Cloud-to-ground lightning simulated by the old （a） and new （b） models
图 3 单建筑物中地闪情况模拟 （闪电击中建筑物：a. 建筑物始发单个上行先导，下行先导竖直向下发展并伴随两个明显主分支；b. 建筑物始发多个上行先导，下行先导的一个主分支倾斜向下发展。闪电击地：c. 建筑物始发单个上行先导，闪电竖直向下发展；d. 建筑物始发多个上行先导，闪电竖直向下发展）
Figure 3. Simulation of cloud-to-ground lightning in a single structure （Lightning strikes the structure： a. a single UL initiates from the structure，and the DL with two main branches goes straight down；b. MULs initiate from the structure，and one of the DL branches slopes down. Lightning strikes the ground： c. a single UL initiates from the structure，and the lightning channel goes straight down；d. MULs initiate from the structure，and the lightning channel goes straight down）
图 4 不同建筑物高度下触发上行先导频次 （蓝色柱表示触发单个上行先导次数，橙色柱表示触发多个上行先导次数，绿色柱表示触发上行先导总次数）
Figure 4. Frequencies of initiating UL from structures with different heights （blue columns represent the frequency of initiating only one UL，orange columns represent the frequency of initiating MULs，and green columns represent the total frequency of initiating UL）
图 6 首个始发与后续始发的上行先导间起始时间差随建筑物高度的变化
Figure 6. Variation of the initial time difference between FUL and SUL with structure height
图 5 不同建筑物高度下闪击距离 （a） 及上行先导长度 （b） 的统计结果
Figure 5. Statistical results of striking distance （a） and UL length （b） for different structure heights
图 7 与首个始发和后续始发的上行先导间起始时间差相关的个例 （a. 建筑物高200 m，c. 建筑物高600 m，b、d分别为a、c所示个例对应的建筑物顶角电场强度随下行先导延伸的变化情况，NO1—NO4分别表示建筑物的4个顶角）
Figure 7. Two cases related to initial time difference between FUL and SUL （a. 200 m，c. 600 m，b，d are the electric field changes of top corners with the DL steps corresponding to the cases shown in a and c respectively，NO1—NO4 are the four top corners of the structure）
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