Numerical Simulation Study On The Initiation Of Multiple Upward Leaders In Isolated Building

To explore the initiation and propagation of upward leader in the cloud-to-ground lightning attachment process,on the basis of the existing multiple upward leaders three-dimensional model and considering optical observations and environmental electric field,the background electric field module is implanted and the upward and downward leader modules are improved.Based on the new model,sensitivity tests were carried out to find the macro factors affecting the inception of multiple upward leaders.Base on the qualitative results,the reason of single leader and multiple upward leaders inception on isolated building are explained,including the causes of four common scenarios when the lightning terminated on a structure.The main conclusions are as follows:(1)The new model simulates a variety of downward leaders,and the number of lightning branches is greatly reduced.The new model solves the problem that the old model greatly overestimates the influence of the negative leader on the environmental electric field.Simulation results show that the upward leader length and the striking distance are positively correlated with the height of the building.(2)The factors affecting the initiation of multiple upward leaders include the geometric structure of the building,the intensity of lightning,the spatial position of lightning relative to the building,the length and propagation direction of the first positive leader,and the occurrence time of the final jump.The height of the building are positively correlated with the frequency of triggering multiple upward leaders.The greater the lightning intensity,the more likely to start multiple upward leaders.The closer the top corner is to lightning,the faster its electric field grows.Due to the increase of the average weighted distance difference between the flash and the four top corners of building,the time difference appears at the initiation of multiple leaders.In addition,the longer the upward leader length is and the closer it is to the building,the stronger the inhibition effect will be.(3)The simulated results of flash terminating on a structure are divided into four scenarios,and each simulated discharge process was divided into two stages.In the first stage,if the weighted distance difference between lightning and the top corners of the building exceeds a critical value,it can directly judge whether the building starts with multiple upward leaders or a single one.Otherwise,it is necessary to analyze the competitive relationship between positive and negative leaders in the electric field of the top corner is much greater than that of the upward leader,multiple upward leaders initiate almost simultaneously.On the contrary,only a upward leader initiates on the building.When both positive and negative leaders are not absolutely dominant,if the final jump occurs slowly,the multiple upward leaders initiate with a obvious delay.Otherwise,only a short upward leader appears on the building.