A Comparative Study On Space-time Distribution Characteristics Of Lightning Flashes In Beijing And Chengdu Cities

The spatial and temporal distribution characteristics of lightning activity were analyzed from June to September in Beijing and Chengdu based on the data of ADTD;And the influence of the bottom mat environment characteristics on the ground flash activity were discussed according to the data of underlying surface types,altitude,topography and surface temperature;Finally,with several observational data(the ERA Interim Daily reanalysis data,MICAPS,Satellite data,Radar data),the characteristics of positive and negative thunderstorms in Beijing and Chengdu are analyzed.The main results are as follows:(1)From June to September,the average cloud-ground lightning current strength in Beijing was 35.66 kA,with a positive ground lightning ratio of 10.4%,and the average cloud-ground lightning current strength in Chengdu was 25.27 kA,with a positive ground lightning ratio of 5.5%.The electric current intensity and the proportion of positive CG lightning in Beijing is obviously higher than that in Chengdu.The daily changes in the lightning flashes and negative flashes in Beijing from June to September have obvious double peaks and double valleys.The two peaks appeared at 2-3 o'clock and 10-11 o'clock respectively.The diurnal variation of positive CG lightning shows a single peak,and the peak appeared at 17-19 o'clock.The diurnal variations of CG lightning,negative CG lightning,and positive CG lightning in Chengdu from June to September are consistent with each other,showing the type of single-peak single-valley distinctively,and the peak appeared at 0-3o'clock and the valley appeared at 11-12 o'clock.(2)The spatial distribution characteristics of CG lightning density in Beijing are as follows: The high incidence areas of CG lightning are mainly distributed in Shunyi,Miyun,Pinggu,Mentougou,Fangshan and Yanqing,and whose bottom mats are dominated by the hilly bottom mat.The spatial distribution of CG lightning density in Chengdu is characterized by a decrease in CG lightning density from southeast tonorthwest,and a high incidence of CG lightning mainly in Chengdu,Shuangliu,Longquanyi,and Jintang.The spatial distribution of the local urban heat island effect is closely related to the spatial distribution of the ground flash.The increase in the local urban heat island effect at night is one of the important reasons for the active activity of the CG lightning.The lightning density in Beijing and Chengdu was negatively correlated with altitude,and the correlation coefficients were-0.64 and-0.99,respectively.(3)The relative humidity between 700 hPa to 1000 hPa during the occurrence of a thunderstorm in Beijing is lower than that of a negative thunderstorm in Beijing and a negative thunderstorm in Chengdu.Comparing positive thunderstorms and negative thunderstorms,positive thunderstorms often occur in the lower middle and deeper layers of the troposphere which has a relatively dry environment.When a thunderstorm occurs in Beijing,the distance between 0? and-20? is 3002 m,and when a thunderstorm occurs in Chengdu,the distance between 0? and-20? is2421 m.When a thunderstorm occurs in Beijing,the cold cloud effect is stronger than that of when a thunderstorm occurs in Chengdu.Cape values and CIN values play an important role in controlling the polarity of ground flash.The proportion of positive flashes is closely related to the height of convective cloud tops,which are mainly distributed in the region where the cloud top temperature in the middle of convective clouds is less than-60?.