Integrated Circulation Anomalies Of The Impacting Large-scale Synoptic System Of Extreme Low-Temperature Events In East Asia

Extreme low-temperature events have occurred frequently in China in recent years,resulting in a great impact.In this paper,the threshold of extreme minimum-temperature events is determined by the percentile method.By using the surface observation station data during 1970-2016,the frequency,intensity and site frequency of extreme minimum-temperature events in spring,autumn,and winter are calculated,and the time and space distribution of frequency and intensity is analyzed.Then,choosing two typical extreme cold wave events,i.e.,the extended snow storm in 2008 and the strong cold wave in 2016,by using NCEP/NCAR reanalysis data,quantitative characterization index of the Siberian high,blocking event,and the stratosphere polar vortex are calculated to analyze the integrated circulation anomalies during extreme cold events.Finally,this study uses the WRF model to simulate the strong cold wave in 2016,to evaluate the model simulation ability of predicting the integrated circulation anomalies of the large-scale synoptic systems.The main conclusions are as follows:(1)Under the background of global warming,the frequency,intensity and site frequency of extreme minimum temperature events in spring,autumn,and winter tend to decrease.The frequency,site frequency,and intensity of extreme minimum-temperature event in winter increased in recent years,showing characteristics of decadal variation.The distribution of the intensity of extreme minimum-temperature events is greatly influenced by latitude and topography rather than the season,which is characterized that the intensity is strong in the north and weak in the south and there are different manifestations in special terrain areas.The frequency and intensity of extreme minimum-temperature events in the three seasons are analyzed by the EOF analysis.The first models all show negative values in most areas of China but negative values in the Tibetan Platau,while the corresponding time coefficient turns around 1990,reflecting the trend of warming.(2)The quantitative index analysis show that during the extreme minimum temperature event in 2016,Siberian high is 16.8hPa stronger,blocking frequency is 60% higher,and polar vortex is 296 gpm stronger than the 1970-2016 climatological mean.While during the event in 2008,Siberian high is 8.7hPa stronger,blocking frequency is 50% higher,and polar vortex is 190 gpm stronger than the climatological mean.This means that the anomalies of the three large scale systems in 2016 are more obvious than that in 2008.As for the integrated circulation anomalies of such three systems during the two events,results show that polar vortex is stronger during the event in 2016 than the climatology and there is strong cold air in the Arctic.Explosive warming in Arctic and the development of mid-latitude blocking send heavy cold air to the Siberia and airflow at 500 hPa leads the cold air to China,which makes temperature in most areas of China decrease sharply in a short time.While during the event in 2008,the anomalies of stratosphere polar vortex propagate downward and affect the troposphere.Then blocking sustains for a long time,as well as the Siberian high.The cold air moves to the south and the east constantly,maintaining a low temperature in southern China for a long time,and leading to cryogenic freezing rain and snow weather,with the help of southern warm and humid air.(3)The WRF model is used to simulate the typical cold wave process in 2016.The result show that the WRF model can simulate the integrated circulation anomalies of large scale systems well.In other words,WRF model can reproduce the stronger polar vortex,blocking high,and the Siberian high during the extreme low-temperature event.As for the quantitative index,results show that the simulation of the position index of the polar vortex is good and the simulation of polar vortex intensity index is smaller than that of the NCEP/NCAR.There are some problems with the center position and strength of the blocking index,but the model simulation can describe the start and end time of the blocking process.The simulation of the Siberia high pressure index is smaller than that of the NCEP/NCAR,while it can show the trend of strength and weakness in the course of the cold wave.