Characteristics Of Lightning Activity And Convection In Central Tibetan Plateau And Their Correlations

This study focused on the lightning activities and convections over the Tibetan Plateau.Firstly,a systematic evaluation on the performance of the World Wide Lightning Location Network(WWLLN)over the Tibetan Plateau was conducted,referring to the data from the Cloud-to-Ground Lightning Location System(CGLLS)developed by the State Grid Corporation of China for 2013–15 and lightning data from the satellitebased Tropical Rainfall Measuring Mission(TRMM)Lightning Imaging Sensor(LIS)for 2014–15.Using the WWLLN and CGLLS data,the temporal and spatial distribution characteristics of lightning activity in the entire plateau and in Nagqu,the area with high incidence of lightning in the hinterland of the plateau,were analyzed respectively.Then,based on the 2014–15 Nagqu Doppler radar data and CGLLS CG flash data,a precipitation clouds feature dataset in Nagqu was established.Deep-convection and thunderstorm samples from May to September were identified according to the dataset.Finally,the temporal and spatial distribution and structural characteristics of deep convection in Nagqu area were revealed.The quantitative relationship between CG flash frequency and radar echo parameters was also established based on thunderstorm samples.The primary conclusions are as follows:The detection performance of the WWLLN over the Tibetan Plateau: 1)WWLLN detected 9.97 % of CGLLS flash in the midsouthern Tibetan Plateau(MSTP)region,and the average location separation of the return stroke is about 9.97 km compared with CGLLS.WWLLN detected 2.62 % of LIS flash over the whole Tibetan Plateau,with an average deviation of 10.93 km from LIS flash location.2)The detection efficiency(DE)of the WWLLN rose markedly with increasing stroke peak current,and the mean stroke peak currents of positive and negative cloud-toground(CG)lightning detected by theWWLLN in the MSTP region were 62.43 and-56.74 kA,respectively.The duration,area,and radiance of the LIS flashes that were also detected by the WWLLN were 1.27,2.65,and 4.38 times those not detected by the WWLLN.3)The DE of the WWLLN in the MSTP region was 9.37% for CG lightning and 2.58% for total lightning.Over the Tibetan Plateau,the DE of the WWLLN for total lightning was 2.03%.In the MSTP region,the CG flash data made up 71.98% of all WWLLN flash data.Based on the above mentioned results,the ratio of intracloud(IC)lightning to CG lightning in the MSTP region was estimated to be 4.05.Characteristics of plateau lightning activity: 1)Lightning activity detected by WWLLN over the plateau showed a decreasing trend from southeast to northwest.The eastern part of Nagqu area was the maximum area of lightning density.The seasonal movement characteristics of the plateau lightning were obvious.In May,the lightning activity moved westwards,and retreated eastwards after September.However,high-lightning activity centers in some specific area(e.g.Shigatse and Nagqu)had always existed,and there had been no apparent movement with the season.2)The proportion of positive CG flashes in the Nagqu area was 3.47%.The area with the strongest total and negative CG lightning activity was located in Dangxiong County,southwest of Nagqu,and the positive CG flash density decreased from southeast to northwest.The diurnal variation of CG flash was obvious,which mainly occured from afternoon to midnight,and the peak time of flash frequency is 18: 00 pm.The monthly frequency exhibited a distinct double-peak feature,with the most frequent lightning flashes in June,August and September.Establishment of precipitation feature data set in Nagqu region: Based on the Doppler weather radar data of Nagqu,the precipitation cloud monomer was identified by clustering method,and the parameter information of monomer was extracted.The precipitation feature data set,containing 461710 precipitation cloud monomer,was established from 2014 to 15.The data set includes the time,location,area,reflectivity of each layer,vertically integrated liquid water,ice precipitation content,lightning frequency,lightning peak current intensity and other information of precipitation cloud monomer.The spatial-temporal distribution and structural characteristics of deep convection(20 dBZ echo top height greater than 14km)and intense-deep convection(40 dBZ echo top height greater than 14 km)in Nagqu region were as follows: 1)Deep convection mainly occured from mid-afternoon to midnight,with a distinct unimodal feature,and the peak occurred at 17:00 pm.In the monthly distribution,deep convection occured most frequently in July and August,and the proportion of intense-deep convection to deep convection was inversely related to the frequency of deep convection.2)There were two high frequency centers for deep convection activity,one of which was located in the mountainous area southeast of Nagqu radar station,with the maximum frequency exceeding 180 between May and September 2013 – 2014,and the other one located in the hilly area north of the radar station.There were significant differences in spatial distribution between deep convection and intense-deep convection.The spatial distribution of intense-deep convection accounted for close relationship with the topography,where intense-deep convection accounted for a relatively high level in flat and open terrain.3)The deep convection over the plateau had small horizontal scale and high vertical extension height.The average horizontal and vertical scales of intense-deep convection were higher than those of deep convection.The echo top height of deep convection had obvious daily variation characteristics,and the peak time of echo top height was 18:00 pm.Relatively speaking,the deep convections which occered in the regions that the ratio of intense-deep convection was high,had larger areas and stronger echoes in the lower layers.The relationship between lightning activity and convective cloud parameters: 1)the spatial distribution of lightning activity and deep convection was in good agreement.The peak daily frequency of lightning frequency was at 18: 00 p.m.,which was delayed by the peak time of the deep convection frequency,but coincide with the peak time of the echo top height of the deep convective.This indicates that the lightning activity has a certain role in indicating the intensity of deep convection.2)The peak frequency of the maximum radar reflectivity near the position of the CG lightning in the Nagqu area was concentrated at 34~41 dBZ,and the peak values of the 20 dBZ and 30 dBZ maximum echo tops were concentrated at 11~15 km and 8.5~12 km,respectively.3)The radar echo parameters processed by interval segmentation had obvious correlation with the average CG flash frequency.The maximum radar reflectivity and the volume of strong reflectivity above 0 °C had the best correlation with the average CG flash frequency,and the correlation coefficients reached 0.83 and 0.82 respectively.4)The maximum grid value and the cumulative value of the ice precipitation content after logarithm were both linearly correlated with the average CG flash frequency,which indicated that the ice precipitation particles played an important role in the electrification process of thunderstorm over the plateau.The cumulative vertically integrated liquid(VIL)water after logarithm also had a good correlation with the average CG flash frequency,and the correlation coefficient was 0.90.