Temporal And Spatial Variation Of Lightning Activity In China And Its Causes

Lightning,as a spectacular and extremely long and intense discharge process in nature,is generally related to thunderstorms and severe convective weather.It is not only an important meteorological disaster,but also a major natural source of nitrogen oxide in the atmosphere.Lightning is also the main inducement of natural forest fires.As the largest carbon pool in terrestrial ecosystem,forest plays an important role in realizing the goal of"carbon peaking and carbon neutrality"in China.This paper mainly studies the temporal and spatial variation characteristics of lightning activity and its causes in China,in order to provide a theoretical basis for lightning disaster protection and the impact of lightning on carbon and nitrogen cycle.Firstly,an 11-year cloud-to-ground（CG）lightning dataset of the China National Lightning Detection Network（CNLDN）is used to investigate the lightning temporal and spatial distribution,the difference between land and sea,and the trend over China’s land and offshore areas.And then,from a macro perspective,the empirical orthogonal function EOF method is used to analyze the spatial mode distribution characteristics and climate causes of lightning in China’s land area.Finally,the characteristics of lightning activity and vertical motion in a convective thunderstorm case are analyzed by using the latest high-resolution phased array weather radar.The possible reason for the differential lightning activity of positive CG（PCG）and negative CG（NCG）lightning is discussed from the view of cloud dynamics and microphysical mechanisms.The main conclusions of this paper are as follows:（1）Lightning activity in China’s land area generally decreases from south to north and from east to west.The mean CG lightning density of China’s land area is 0.9 fl km^-2 yr^-1,while the mean CG lightning density in Guangdong,Hainan,Fujian,Zhejiang and other coastal provinces in southeast China exceeds 10 fl km^-2 yr^-1.The maximum of 13.0 fl km^-2 yr^-1 is found in Dongguan City in Guangdong Province.About 90%of CG lightning occurs from May to September.The lightning activity in spring,summer and autumn seems consistent throughout the daytime cycle,with the lightning activity being relatively weak in the morning and most active in the afternoon.In contrast,winter lightning appears to be more involved in the nighttime and early morning,produced by nocturnal thunderstorms.The CG lightning activity in the tropical and subtropical areas within 15-25°N is mainly concentrated in the late spring and early summer,with two weak peaks in May and August,while lightning in mid-latitudes within 25-55°N mainly occurs in the summer（June-July-August,JJA）.During the last 11 years,the CG lightning activity shows a weak but clear decline trend by-0.0392 fl km^-2 yr^-1.The NCG lightning activity shows a similar decreasing trend,while the PCG lightning activity shows an opposite increasing trend.The regional lightning trend appears a diversity of variation across China’s land areas,some regions with increasing while others with decreasing trends.（2）Lightning activity is concentrated in offshore areas 100 km from land,which is predominant in the southern offshore area and less in the eastern offshore area.In the offshore areas,the lightning density is about 0.9 fl km^-2yr^-1and the maximum is 6.7 fl km^-2yr^-1,which occurs in the coastal regions of Guangdong Province（within 100 km from the land,located in the offshore areas of 113.7°E and 22.6°N）.The seasonal variation of the offshore lightning activity is also apparent,with less in autumn and winter,active in spring and peak in summer.Sea-land circulation patterns and tropical cyclones in the offshore region may be the main reasons for the formation of several high lightning density centers on the south coast of China.Unlike the monthly variation of land lightning,the monthly variation of offshore lightning in China shows a bimodal pattern,with the most active offshore lightning in June and August.As far as the diurnal variation is concerned,the PCG and NCG lightning activity in the offshore area of China presents a bimodal distribution,with strong lightning activity in the late afternoon and early morning.The CG and NCG in offshore areas show a weak upward trend,while the PCG shows a weak downward trend.（3）From the perspective of climatic causes,the EOF decomposition shows that,the contribution rate of the first modal variance of the anomaly in China’s land area is 25.2%.The contribution rate of the first modal variance in summer is 32.9%,and that of the second modal variance is 14.3%.Summer lightning accounts for about 70%of annual lightning,and the first modal spatial distribution of summer is mostly consistent with that of the first modal of the year.Lightning activity is well correlated with the El Ni?o ONI index,indicating that the El Ni?o-South Oscillation（ENSO）phenomenon is the climate driver of the subsequent lightning activity in China’s land area.The PC1 time series of the first mode of the summer anomaly variation is regressed to the circulation situation field and the sea surface temperature（SST）field.When the SST in the East Pacific Ocean and the Indian Ocean warms abnormally and the SST of the Northwest Pacific Ocean cools,cyclonic circulation is aroused in the Yellow Sea,the East China Sea and the tropical Western Pacific Ocean of China,corresponding to weaker the lightning activity in the North and Northeast China.As the water vapor continues to flow southward to the South China region,that is,the outer area of the high pressure,and the upward movement is strengthened,so that the lightning activity in the southern region is strengthened.The PC2time series of the second mode of the summer anomaly variation is regressed to the circulation situation field and the SST field.When the North Pacific,northeast Pacific and equatorial Pacific warms abnormally,there is a positive anomaly in the geopotential height from the upper to the lower level in East China and the East China Sea.The water vapor from the Northwest Pacific and the South China Sea diverges here,and there is a significant abnormal subsidence movement in the corresponding area.As a result,there is less lightning activity in East China,especially in Jiangsu and Zhejiang.（4）To reveal the possible causes for the different trends of PCG and NCG,the relationship between lightning frequency variation and the dynamic field is studied from the thunderstorm scale.A three-dimensional wind field retrieval method based on three-dimensional variational data assimilation（3DVAR）is developed,taking advantages of phased array weather radar’s fast all-space scanning technique.Then the fine evolution of the three-dimensional wind field,especially the vertical velocity,was obtained for a local strong convection system in Foshan,Guangdong province.In this case study,it was found that the total lightning frequency reached a peak of 96 flashes/（2 min）.The PCG was dominant in the development stage of convection,and the peak proportion of PCG was 64%.In the stage of vigorous convection,the continuous development of convective cell resulted in active lightning,showing an upward trend,and dominated by NCG.Compared with the array radar echo,it was found that the lightning activity was mainly concentrated in the strong echo region above 30d BZ,and the NCG occured more intensively in the strong echo area,while the PCG was relatively scattered in the edge of the strong echo,that is,stratus cloud area.The time trend of lightning frequency is prior to or at the same time of the strong echo volume of 30 d BZ and 35 d BZ（V30d BZ and V35d BZ）,and the exponential fitting degree R² between them are 0.82 and 0.65,respectively.The strong updraft resulted in a higher PCG ratio based on the temporal evolution of lightning frequency and the maximum updraft velocity.The NCG became more active when the updraft was weaker.Furthermore,the parametric fitting relationship between lightning frequency,maximum updraft velocity and strong echo volume was established respectively.It reveals the lightning activity regularity and the mechanism of lightning occurrence from the point of view of cloud dynamics and provides theoretical basis for lightning data assimilation.