Simulation Of The Formation Mechanism For The Charge Structure Characteristics Of A Severe Thunderstorm In Lanzhou Area

Affected by special topography and thermal factors,the summer thunderstorms in Lanzhou area have showed some particularities.In order to improve the understanding of the characteristics of thunderstorm,a severe thunderstorm in this area based on the WRF(Weather Research and Forecasting)model has been simulated in this paper.Through the evolution characteristics of charge structure,it is found that the deep positive charge center(LPCC)existed in the mature stage,then the formation mechanism of the LPCC was analyzed based on dynamics and microphysics.The following conclusions are drawn:(1)The simulation results were consistent with the observed.It is a typical summer thunderstorm in the inland plateau,which has small scale,short life and fast moving.The results both simulated and observed has showed the frequency of CG(cloud-to-ground)flashes was low during the thunderstorm,and the positive CG flashes frequency has a jump.The content of the ice-phase particles rapidly increased in a short time was considered to be related to the jump.(2)The characteristics of the charge structure characteristics were not the same in different stages.The structure in the initial was a dipolar one,which has weaker intensity and lower height.The charge structure changed and the evolved into a slightly tilted tripolar one,with the deep upper and middle charge regions.The LPCC has appeared and lasted for a long time in this stage,and the intensity of LPCC was equivalent to the middle negative region.In the stage of thunderstorm dissipation,the charge structure gradually changed to a dipole,the LPCC characteristics obscure and the range of the charge dispersed.(3)The special stratification characteristics affected the microphysical process,thus affecting the charge structure characteristics.Although the surface temperature and ambient humidity were relatively lower,the thunderstorm had special stratification characteristics such as higher cloud bottom height,thinner warm cloud thickness and larger unstable energy,which made the updraft even stronger and more liquid water raised.The particles like graupel and hail could stay longer in the cloud,making in the graupel particles constantly accumulated in the cloud bottom,which was effectively promoted the electrification process in the cloud and thus affected the charge structure.(4)The deep distribution of graupel particles in the cloud bottom was the prerequisite for the formation of the LPCC.Under the situation of moderate liquid water and temperature in the thundercloud,the positive charge carried by non-inductive separated between graupel deep in the cloud bottom and the ice/snow,which played a major role in the formation of the LPCC characteristics.In addition,the inductive collision between graupel particles and cloud droplets could also contribute to the formation of the LPCC.