Numerical Simulation On Favorable Electrical Environment Characteristics For The Initiation Of Upward Lightning And Discharge Effects Of Upward Lightning

Upward lightning is classified into two categories according to whether or not they are triggered by prior lightning discharge activities near the structure,one is other-triggered upward flashes,and the other is the self-initiated upward flashes.At present,the understanding of the relationship between the characteristics of upward lightning initiation,propagation and thundercloud electrical environment parameters is not clear,and there is no effective evaluation for the discharge effects of upward lightning.In this paper,an improved upward lightning discharge parameterization scheme is embedded into an existing two-dimensional thundercloud electrification and lightning model.A large number of upward lightning flashes obtained by adjusting the initial disturbance of the model were analyzed for exploring the favorable electrical environment characteristics for the initiation of upward lightning,and estimating the influence of upward lightning discharges on the space potential,electric field,electrostatic energy and charge structure.In addition,a new other-triggered upward lightning model was established for discussing the possibility of triggering subsequent upward flashes during lightning propagation.The main results are as follows:1.Other-triggered upward flashes:Other-triggered upward flashes under various charge backgrounds are simulated by thundercloud model.The preceding intracloud lightning(IC)or upward lightning(UL)discharges neutralize charge in the upper charge region,and the original space potential and electric field are greatly changed.The effect of the bottom charge region on the space potential and electric field is greatly enhanced,which causes the range and intensity of the potential well at the bottom of thundercloud enhanced,and the ambient electric field at the tip of the building jumps above the initiation threshold of other-triggered upward flashes.In addition,other-triggered upward flashes triggered during the propagation of the preceding IC are successfully simulated by the other-triggered upward lightning model.2.Self-initiated upward flashes:Counting all the self-initiated upward flashes that propagating into the bottom charge region,it is found that there is a good correlation between the weighted average of the lower charge heights(H)and the average of the lower charge density(D),and the correlation under the dipole charge structure is obviously better than that under the tri-pole structure.As D increases,the critical H increases.Comparing the ratios of the charge amount in the bottom charge region to the charge amount in the upper charge region for all self-initiated upward leaders and aborted upward leaders,it is found that the average of the ratio of the charge amount in the bottom charge region to the charge amount in the upper charge region for self-initiated upward leaders is 40%,and the height of the bottom charge region is low.While the charge amount in the bottom charge region is much smaller than that in the upper charge region for the aborted upward leaders,the average value is about20%.Although the height of the bottom charge region is very low in some cases,due to the charge amount is much small,therefore the bottom potential well is so weak,which is not strong enough to maintain the sustained development of the upward leader.3.Discharge effects of upward lightning:The estimates of total induced charge in discharge channels for one upward lightning range from 0.67 C to 118.8 C,the average value is 19.0 C,and the ratio of induced charge to the bottom charge is in the range of 5.9%to47.3%,the average value is 14.7%,the average value of the space electrostatic energy consumed by one upward lightning is 1.06·10~9 J.The direct effect of lightning discharge is to implant heteropolar charge in the 0channels.Subsequently,these heteropolar charges are gradually neutralized by the turbulent exchange,gravity sedimentation,advection and electrification processes,and the total charge amount in the lightning action region is gradually reduced.The above process is very slow relative to the discharge duration,which usually lasts for several tens of seconds.After that,with the decreasing of the heteropolar charge in the channels,the separation of the water particles and electrification process,the total charge amount in the lightning action region will gradually rise to the original level and form a new space charge structure.In addition,the previously destroyed space potential and electric field were reconstructed to provide suitable conditions for subsequent discharges.