The Effect Of The Southern Branch Trough On Snowstorms In Southwestern Tibet In Winter

Based on the NCEP/NCAR reanalysis data and the objective identification method of the South Branch Trough（SBT）,this paper identifies the SBT 500hPa Upper in winter half year of 1981-2015（November to April of the next year）,and mainly analyzes the climate characteristics of the SBT in winter（January）and its impact on the circulation of the Qinghai Tibet Plateau（hereinafter referred to as the plateau）.Taking 80°E as the boundary,the SBT in winter is divided into eastern type and western type,and the structural characteristics of the two types are compared and analyzed.Based on the station precipitation data,the impact of the SBT on the precipitation in Tibet is analyzed,and three snowstorms in southwest Tibet under the background of the SBT are diagnosed and analyzed.The results are as follows:（1）In winter,the daily height field of the SBT is abnormally negative anomaly in the whole layer on the south side of the plateau,which indicates that the weather system of the SBT is deep in winter.In the southern part of the plateau in front of the SBT and on the plateau,the atmospheric upward motion is abnormally enhanced,the southwest airflow makes the water vapor and heat abnormally enhanced,and the western part of the plateau behind the trough is abnormally downward motion.The activity of the SBT strengthened the ascending movement of the southern part of the plateau and above,and then transported water vapor and heat to plateau.（2）In winter,there are two frequency centers in the east and west of the SBT,so the SBT can be divided into eastern type and western type based on 80°E.Comparing the structural characteristics of the two types of SBT,it is found that:dynamic structure,the ascending movement in front western type of the SBT is stronger than eastern type of the SBT;thermal structure,theθ_sein the ascending movement area of eastern type of the SBT is significantly higher than western type of the SBT,indicating that its thermal condition is better;water vapor condition,the water vapor of eastern type of the SBT is mainly the West Pacific subtropical heat The western type of water vapor mainly comes from the Arabian Sea,accompanied by the supplement of water vapor from the bay of Bengal.The water vapor transport and convergence in eastern type of the SBT are obviously larger than those in the western type.（3）In winter,the influence of the SBT on the precipitation in Tibet is obvious,especially in the southwest of Tibet,the snowstorm is mainly caused by the SBT.The diagnostic analysis of three snowstorms in southwest Tibet under the background of SBT shows that 500 hPa,700 hPa SBT and high and low level jet are the main influence systems of the three snowstorms,and the water vapor is mainly transported from the Arabian Sea to Tibet.The snowstorm occurs in the north side of the center of the high value of water vapor flux and the convergence center of water vapor flux.Under the conservation of moist potential vorticity,the absolute value of MPV1 is obviously larger than MPV2,MPV1<0 and MPV2>0 in the middle troposphere over the blizzard area is one of the important criteria for judging the occurrence of blizzard;the positive value of vertical helicity is maintained near500 hPa over the blizzard area,and its central position and size are closely related to the temperature The snowstorm area and precipitation intensity correspond well;The distribution of negative value center of moist helicity at 500 hPa corresponds well with that of snowstorm center.During heavy snowfall.