Numerical Simulation Study On Electrical Activity Of Thunderstorms In Beijing

In order to predict the lightning activity effectively by the Weather Research and Forecasting(WRF)model,the processes of cloud electrification and lightning parameterization are introduced into the Morrison cloud-scale double-moment microphysical scheme in the WRF model version 3.4.1.The electrification process parameterization is SP98 non-inductive charging scheme,including non-inductive graupel-ice and graupel-snow charge separation mechanisms.The lightning process parameterization is a bulk lighting parameterization scheme.Based on these parameterization schemes,the charge and discharge characteristics of an ideal supercell and a squall line occurring in Beijing were simulated.Simulation to the supercell shows that the ideal supercell presents an inverted tripolar charge structure,consisting of a main positive charge region(-40°C to-60°C),an upper negative charge region(near-60°C),and a lower main negative charge region(-10°C to-40°C).The cause of this charge structure is that graupel particles charged positively while ice crystals charged negatively in the region where rime accretion rates(RAR)is above the critical rime accretion rate(RAR_C).On the contrary,graupel particles charge negatively and ice crystals charge positively in the region where RAR below RAR_C.As affected by updrafts and gravitational sedimentation,negatively charged ice crystals occupy the upper negative charge region,positively charged ice crystals and snow particles and some graupel particles occupy the main positive charge region,while negatively charged graupel particles occupy the lower main negative charge region.The maximum net charge density is approximately 2nC/m~3,the maximum positive net charge density is around 1.5nC/m~3,and the maximum negative net charge density is around-1.7nC/m~3.The maximum field strength is between 120 and 160 kV/m,and the lightning frequency is around 1200 fl/5min.On the whole,the simulation results are reasonable.The simulation to the squall line show that graupel particles charge negatively,while ice crystals and snow particles charge positively.As a result,the squall line presents a typical dipolar charge structure,consisting of an positive charge region at the top(-50°C to-20°C)and a negative charge region at the bottom(-20°C to 0°C).The negative charge region is composed of negatively charged graupel particles while the positive charge region is composed of positively charged ice crystals and snow particles.In addition,the maximum net charge density of the squall line is lower than that of the ideal supercell.By comparing the simulated lightning activity in the WRF model with the observed cloud-to-ground(CG)flashes by ADTD Lightning Location Monitoring System,we can see that the temporal characteristics of lightning frequency in the WRF model are largely consistent with the observed flashes rates,and the distribution of lightning density is similar with observed CG lightning density during the mature stage of the squall line.