Characteristics Of Cold Front Activity In Eurasia In The Past Three Decades Based On Objective Identification

Based on ERA5 reanalysis data,this paper presents a two-step cold front identification method in Eurasia and performs a series of verifications.These verifications include the comparison of different cold front identification methods and the identification results of different reanalysis data,the evaluation of objective cold front in continuous time,and the comparison of objective cold front and subjective analysis of cold front in long time.Based on the method,the objective cold front data set from 1989 to 2018 is obtained.On this basis,the activity characteristics of cold fronts in extreme precipitation events and cold wave event are revealed from the perspective of synoptic analysis.In addition,the activity characteristics of wintertime cold front in Eurasia and abnormal years and circulation characteristics of cold front activity in winter over North China from 1989 to 2018 are revealed from the perspective of climatology.The main conclusions are summarized as follows:(1)Based on ERA-5 reanalysis data,this paper presents a two-step cold front identification method over Eurasia.In this method,the thermal front parameter(TFP)and cold advection are used to locate the cold frontal zone,and then locates the cold front by finding the warm boundary of the frontal zone.In the comparison of different cold front identification methods,the results of two-step cold front identification method and subjective analysis are more consistent,and the positions of auto-identified cold fronts are more reasonable.The two-step cold front identification method can be used in different reanalysis data(ERA-5,ERA-Interim and CRA-40).The cold front data of the wintertime of 2008/2009 and 2014/2015 are collected,and the performance of the two-step cold front identification method is evaluated from the two aspects of hit rate and difference degree.The hit rate between the objective cold front and the subjective analysis cold front is 0.94,difference degree is 0.25,indicating the reliability of the two-step cold front identification method.(2)From the extreme weather cases,it is found that the coordination of the cold front and circulation systems in the middle and lower troposphere in wintertime is conducive to the production of severe weather such as extreme precipitation and strong cooling near the cold front,especially behind the cold front.In the case of extreme precipitation,the objective cold front is located at the front of the surface cold high.The precipitation area presents a zonal distribution,the direction is parallel to the cold front,and the precipitation is mainly located behind the cold front.The surface system and wind field near the cold front are conducive to the formation of precipitation.In a cold wave event,the cold front is located at the front of the surface high,and the ground temperature drops sharply behind the cold front.The 500 h Pa trough over the cold front deepened and moved rapidly eastward.The 850 h Pa area was densely packed with isotherms.Under the influence of northerly winds,the cold front area moved eastward and southward caused cooling.The accelerated movement of the 500 h Pa trough may cause the cold front to move rapidly to the south and the cold air to move south rapidly,causing a drastic cooling in a large area of China.(3)In the wintertime of Eurasia,there are several large value centers of the frequency and intensity of the cold front,including the Baltic Sea,the Mediterranean Sea,the African region,the east of the Ural Mountains and the coastal area of East Asia.The maximum value of the frequency and intensity exists in the coastal area of East Asia,and it is greater in winter than in autumn.In autumn and winter,the frequency and intensity of the cold front in the northern part of Eurasia mainly show a decreasing trend,while in winter,these in the southern part of Eurasia mainly show a significant increasing trend.(4)North China is the key region of cold front activity with the highest frequency and intensity in China,and the index of cold front activity intensity in North China has obvious inter-annual variation.In the years of strong cold front activity in North China,there is an obvious positive EU teleconnection wave train over the North Atlantic to Eurasia,and the negative geopotential height over North China is anomalous,which represents the strengthening of the East Asian trough and the stronger of the Siberian High.The West SiberiaBaikal-Mongolia region is a key area of cold air activity that affects the intensity of the cold front in North China.When the northern marginal sea area of Eurasia is abnormally warm,the cold air activity in the key area of cold air activity and North China will increase.The SST anomaly in the northern Atlantic Ocean can stimulate the Rossby waves in the middle and upper troposphere and spread eastward to the key areas of cold air activity in Western Siberia,enhances cold air activity,thus affecting the intensity of cold front activity in North China.