Tracks And Characteristics Analysis Of Strong Cold Air Invading Northern China During Winter Half Year

This paper defines the drop of average temperature of greater than or equal to 10? at least once within 24h in a cooling process during the winter half years as the standard for strong cold air strike based on NCEP/NCAR reanalysis data and uniform ground air temperature datasets of basic reference stations in China.The improved three-dimensional wind speed trajectory inverse method is used for trace analysis of strong cold air intrusion tracks that occurred in North China in 1970-2013.Given the differences in strong air cold intrusion tracks and activity characteristics in different regions,this paper firstly divides strong cold air that intrudes into North China,and then analyzes strong cold intrusion tracks in different regions.This paper focuses on regional and sustainable climate characteristics of strong cold air from different tracks,preliminarily analyzes the mechanism of flowing of the cold air masses out of the polar regions,and discusses atmospheric circulation characteristics of strong cold air from various tracks.Finally,this paper evaluates the model simulated deviations from differences in cooling mechanisms in Northwest China and other regions based on historical CMIP5 simulation results.Main conclusions are as follows:(1)In 43 winter half-years,strong cold air intruded into North China for 1,138 times in total according to strong cold air standards.Strong cold air intruded into Inner Mongolia for 469 times in total,including 303 northwest tracks,118 northward tracks,32 westward tracks and 16 strange tracks.Strong cold air intruded into Northeast China for 418 times in total,including 230 northwest tracks,101 northward tracks,75 westward tracks and 12 strange tracks.Strong cold air intruded into Northwest China for 251 times in total,including 143 westward tracks,88 northwest tracks and 20 strange tracks.(2)In general,strong cold air intruding into North China is mainly from 40?120°E of the Arctic Circle.Through comparison of circulation fields of the cold air masses from various tracks on the same day when cold air masses flow out of the polar regions,it is discovered that main differences mainly exist to the east and west of 80°E.When AO is located at the negative phase,the smaller(the larger)the AO index,the lower(the higher)the cold air mass to the west of 80°E;when NAO is low,the cold air mass from the Arctic Ocean easily flow out of the polar regions to the west of 80°E;when NAO is high,as high-pressure activities occur in high latitudes of Siberia,the cold air masses easily flow out of the polar regions to the east of 80°E.In addition,distribution of the source regions to the east of 130°E is related to cyclonic activities around the Sea of Okhotsk.(3)59%of strong cold air is caused due to that the cold air masses in high attitudes of the Northern Hemisphere flow southward under the effect of the East Asian winter monsoon,and intrudes into Inner Mongolia and North China from the northwest direction and Northwest China from the westward direction.In consideration of weak intrusion in the southward direction,North China is mainly affected.The conditions for the outbreak of 7%of strong cold air are that the temperature is low in high attitudes of Siberia,warm eastward anticyclone produced in the Central Asia intrudes into Northeast China in the westward direction.Due to that cold air masses are weak,North China is mainly affected.27%of strong cold air is affected by warm anticyclone in the Urals,and intrudes into Inner Mongolia and Northeast China in the northward direction and Northwest China in the westward direction,which easily leads to unusual low temperature in East China.3%of strong cold air is affected by warm anticyclone to the east of Europe and changes of negative phase of the AO,and intrudes into Inner Mongolia in the westward direction.Strong cold air easily leads to unusual low temperature in Northwest China and cold damage for continuous 3d or longer in South China.During 1970-2013,the frequency of 59%and 7%of strong cold air was obviously decreased,but the frequency of 27%and 3%of strong cold air was slightly increased.It reveals that strong cold air leads to no obvious decrease in unusual low temperature and decrease(increase)of the frequency of strong cold air in the north(south)with climate warming.(4)Through comparison of the spreading of strong cold air into the downstream areas from different tracks,it is summarized based on the peak value that,within 1 day before outbreak,cold advection occurs in various regions and temperature departure begins to decline,and cold air is relatively thin when its forward edge moves to a low attitude.In addition,warm departure in these regions leads to dominated lifting movement in various areas and increase of cloud coverage.Non-adiabatic process results in warming(latent heat release and radiation cooling weakening)and weakens vertical warming and cooling of cold advection.During the outbreak,cold advection will lead to sharp temperature drop.In addition,cold anticyclone arrives at the ground,and sinking movement is prevailing.Radiation cooling is enhanced in clear sky,thus maximizing the temperature drop with an offset of downward warming effects.Within 1 day after the outbreak,the temperature departure reaches the minimum in various areas,and the cold maintenance mechanism mainly refers to warming effect of non-adiabatic process and vertical process of cooling of cold advection.Horizontal advection,non-adiabatic process,temperature departure and temperature variation are positively correlated,and correlation decreases gradually from north to south.The correlation coefficient between vertical transmission and temperature change is negative in North China but positive in South China,because landform change as the cold air masses intrude southwards will improve the spatial variation of thermal and dynamic processes and lead into various changes of heat budget.In addition,the results of correlation analysis indicate that vertical transmission calculated based on coarse resolution is physically consistent with others.(5)After the outbreak of strong cold air in Northwest China,non-adiabatic process is a main cooling process,and cold advection lasts for a short time.This is different from the cooling mechanism where strong cold air intruding into other regions of North China becomes colder mainly due to cold advection.According to relevant analysis,vertical transmission is obviously and positively correlated with regional temperature,and non-adiabatic process is negatively correlated with regional temperature.However,the contrary is the case in other regions.The particular cooling mechanism for strong cold air intruding into Northwest China brings about the poorest effect for CMIP5 simulation of strong cold air activity in Northwest China.Model simulation effects of strong cold air activities in Northeast China are the best,in which the decline trends of cold air frequency during 1970s-mid 1980s are simulated by CanESM2,and the frequency trends of increasing first and remaining stable later on during early 1990s to 2005 are simulated by MIROC-ESM.