Characteristics Of Large-scale Extreme Low Temperature Events In China And Analysis Of Typical Cases

Based on the NCEP/NCAR daily reanalysis data and the temperature data collected at conventional observation stations of National Meteorological Information center,characteristics of the large-scale extreme cold events in China was analyzed.Based on the three conditions of intensity,scope and time,the 38 large-scale extreme cold temperature events(LECES)were defined.The inter-generational change showed a warming trend,the number of extremely cold days showed a decreasing trend,and the inter-generational mutation occurred in the mid-1980 s.The large-scale extreme cold temperature events(LECES)were divided into five types: Countrywide type,Eastern type,southern-northern type,northwestern-Southern type and northeastern-northern type,according to the different region.The circulation backgrounds of different styles of the large-scale extreme cold temperature events were also analyzed.The results showed that: The countrywide LECES has a precursor signal about 15 days,blocking situation over Ural Mountain is obvious.The Upper-level jet stream at 300 h Pa shows zonal direction in positive and negative,which strengthens the circulation system of East Asian monsoon and Siberian high,and cold air accumulated in northern Asia.An extraordinarily strong anticyclone covers Siberia at 850 h Pa,and most of China is controlled by northerly winds.Such a spatial configuration and evolution of circulation system in the mid and lower troposphere affect China and cause extreme cold temperature events.Besides,compared with the countrywide LECES,the other types of LECES have the precursor signal about five days before the event,with short duration and weak intensity.And the anomaly area of 500 h Pa height field and sea level pressure field determines the type of LECES.By analyzing the typical extreme cold events from January 26 to February 4,2018,the results showed that: the generation and evolution of polar vortex reflects the early characteristics of extreme cold events.Before the extreme cold events occurs,polar vortex begins to shrink and gradually present an ellipse shape.At the same time,it tilts westward,and the polar vortex is abnormally strong,which strengthens the cold air southward.At the same time,a strong high ridge forms around the first 15 days of extreme weather,and develops into a stable and strong Siberian high in the Ural Mountains in the first 7 days.The high pressure continues to strengthen and moves eastward in northern Eurasia,and cold air gathers at this point.As the East Asian trough turns vertical,it invades China along the middle path of cold wave.