Simulation Of The Occurrence Of Positive Cg Flash Under Precipitation Shielding And Reverse Polarity Charge Structure

Due to the rareness of the positive CG flash and the difficulty of observation,computer numerical simulation has its unique advantages for the study of positive flash. Previous studies carried out a lot of speculation and verification to how the positive CG flash comes from. However, there is no corresponding numerical simulation for the relationship precipitation shielding and positive CG flashes in thunderstorm cell. So we simplified this problem by establish a normal tripole charge structure model, including lightning discharge parameterization, and change the altitudes and densities of middle negative and lower positive charge regions of thunderstorm to study the relationship between precipitation shielding and positive CG flashes in thunderstorm cell. The results indicated, when precipitation occurs in thunderstorm cell, the altitudes and charge densities of the middle negative and lower positive charge region will decrease, which will change the distribution of spatial electrical field and potential and then triggered lightning. When the altitudes of the middle negative and lower positive charge region decrease for the reason of downward flow caused by precipitation, the maximum electrical field strength would decrease and its correspond electrical potential would increase, thereby, the electrical potential difference between the point of the maximum electrical field and ground surface will increase, the positive leader is easier to reach the ground to form positive CG flashes. And when the charge densities of the middle negative and lower positive charge region decrease for the reason of the electric charge adhere to precipitation fell out of the cloud, the maximum electrical field strength would decrease and its correspond absolute value of electrical potential would decrease first and then increase. During the absolute value of electrical potential is decreasing,which is negative and its orientation is vertical upward. It is of course impossible to produce a positive CG flash. But when the densities of middle negative and lower positive charge region decrease to a certain degree, the value of the electrical potential would be positive and its orientation is vertical downward. Then the value of the electrical potential would keep increasing. The electrical potential difference between the point of the maximum electrical field and ground surface will increase too; the positive leader is easier to reach the ground to cause positive CG flashes. So, the hypothesis that precipitation shielding can cause positive CG flashes is reasonable in thunderstorm cell.Besides, many observation and experimental results have supported the important role that reverse polarity charge structure played in the progress of the production of positive CG flash in strong thunderstorm. So we established corresponding charge structure model and focus on the research of how lower negative charge structure area effected the production of positive CG flash under the condition of reverse polarity charge structure. Parameters of lower negative charge structure area concerned including charge density,horizontal range,thickness,how far between lower negative charge structure area and others and horizontal offset of it.Control variable method is used to study different influences of different parameters to production of positive CG flash. The simulation results are as follows:1) A certain range of increase of charge density of lower negative charge structure area is beneficial to the production of positive CG flash.2) A certain range of increase of horizontal range of lower negative charge structure area is beneficial to the production of positive CG flash.3) A certain range of increase of thickness of lower negative charge structure area is beneficial to the production of positive CG flash.4) The distance between lower negative charge structure area and others is smaller, the easier of the production of positive CG flash.5) The horizontal offset between lower negative charge structure area and others is smaller, the easier of the production of positive CG flash.