Research On The Formation Mechanism And Precursor Signals Of Persistent Wet And Dry Cold Events In Southern My Country In Winte

Based on the daily precipitation and temperature data provided by the National Information Center,NCEP/NCAR reanalysis data and daily sea-ice and SST data from the NOAA Center,the spatial and temporal distribution characteristics of the persistent wet and dry cold events in southern China from 1951 to 2019 are studied.The HYSPLIT model is used to track the cold air paths of the two types of events and classify them using the clustering method.The analysis focuses on the causes of circulation anomalies of the persistent wet cold events(PWCEs)and persistent dry cold events(PDCEs)in eastern southern China under different cold air paths,comparing the circulation differences between the two types of events,and then exploring the links between the two types of events with the most cold air paths and SST and Arctic sea ice.(1)Dividing southern China into eastern and western shows that: The single-station frequency of the southern PWCEs and PDCEs in the western part of South China is higher than that in the eastern part,but precipitation and cooling intensity of the eastern events process stronger than those of western events.The study shows that the intensity of the eastern PWCEs(EPWCEs)tends to increase,with a sudden change point in 1983,while the intensity of the eastern PDCEs(EPDCEs)tends to weaken,with a sudden change point in 1986.The cold-air paths of eastern PWCEs and PDCEs invading eastern South China are divided into three types,namely the west,northwest and north types,while the western PWCEs and PDCEs of that are dominated by west path.(2)Studies on the circulation characteristics and pre-cold atmospheric circulation signals of EPWCEs and EPDCEs under different cold air paths show that the tilted ridge and trough pairing over mid-and high-latitude the conditions for the persistent cold air invasion in both types of events.For the EPWCEs,the west type,the ridge area is in Novaya Zemlya,and is associated with the wave train from Barents Sea Ocean,and the deepening of the South Branch trough is associated with the wave train from the North Atlantic.The northwest type,with the ridge east of Novaya Zemlya,is related to the wave energy from the Norwegian Sea.The north type is mainly affected by the blocking circulation,and is associated with the southward offset of the polar vortex,and the wave energy from the North Atlantic.The mid and high latitude circulation characteristics and ground characteristics of EPDCEs are similar to the EPWCEs.The difference in circulation between the two types of events is the difference in the lower tropospheric stratification and water vapor conditions.The existence of the inverse thermosphere and the deepening of the south branch trough to transport water vapor transport are favorable to the occurrence of precipitation,while this condition is not satisfied in EPDCEs.(3)The Barents Sea-Karabakh Sea ice loss 20 days before the EPWCEs of west type,which in turn has an impact on the establishment of the key Ural Mountains ridge,and at the same time,the wave energy of 300 h Pa propagating eastward can deepen the East Asian trough,the trough ridge cooperation leads to the continuous transport of cold air to the south.The deepening of the southern branch trough is associated with the excitation of wave trains propagating along the subtropics by positive SST anomalies in the North Atlantic,providing water vapor for process precipitation,the convergence of warm and cold air in the south and east produces wet and cold events.The Barents Sea-Karabakh Sea ice loss before the EPDCEs of north type for 23 days,and then the polar vortex weakens,and then the AO index overtakes the process for 14 days to shift from positive to negative phase,allowing persistent cold and dry cold events to occur.This conclusion can provide a reference for the extended period forecast of the two types of persistent low temperature events.