Three-dimensionalNumerical Simulation Of Discharge Activity In Thunderstorm

The lightning parameterization proposed by Mansell has been improved in threshold value for initiation and the redistribution of induced charge on channel, and then it is introduced into a three-dimensional dynamic-electrification coupled model in order to improve the simulation capacity to electrical activity of the cloud model. By the simulation of a thunderstorm during Severe Thunderstorm Electrification and Precipitation Study (STEPS), we tested the simulation capability of the lightning parameterization. And through the simulation of an ideal thunderstorm in mid-latitude region we analyzed the interaction of the lightning and the electrical environment of thunderstorm, the fractal feature of lightning, and the effect of the initial breakdown threshold and the propagation threshold on characteristic of lightning. The relationship between the occurrence of+CG lightning and the tilted triple charge structure of thunderstorm is researched by establishing various three-dimensional models of triple charge structure of thunderstorm with the lightning parameterization scheme.The results show:(1) The net negative thunderstorm charge is not a necessary condition of-CG occur, and there is no obvious relationship between the lightning type and net charge of thunderstorm. The type and polarity of lightning is controlled by potential of the initiation point, and the initiation points of negative CG flash are all with a high negative potential. Lightning discharge have some effect on the formation of complex charge distribution.(2) The lightning branches is few around the initiation point, when lightning propagate to the charge region of high charge density the lightning branches will increase rapidly. The fractal dimension of CG flash is less than IC flash.(3) The variation of threshold value for lightning initiation will effect the first flash time, flash number, and the ratio of IC flash and CG flash. The lower of propagation threshold value of lightning will change some IC flash to CG flash, effect the lightning type slightly. It will also make the propagation scope of lightning more wide, neutralize more charge per lightning, and reduce the total lightning number.(4) The horizontal movement of the upper positive charge region can’t make the flash type convert from cloud lightning to+CG flash, but will reduce the maximum field strength in the cloud, that makes the lightning more difficult to occur. When the middle negative charge region is moved in horizontal direction, the maximum field strength in the cloud will be strengthened, it will result in more lightning discharges. Meanwhile the potential difference between initial point and ground reduced gradually. Due to this, although the downward propagation of the positive leader will not be blocked by the lower positive charge region, the reduction of the potential difference between the initial point and ground makes the positive leader more difficult to propagate to the ground. The tilt of charge structure will not make the thundercloud inclined to produce+CG.(5) In the triple charge structures, when the charge density and scope of upper positive charge region are very large, it will make a big positive potential difference between initial point and ground, so the positive leader is possible to propagate to ground.