Low-frequency Characteristics And Causes Of Large-scale Persistent Low Temperature Events In Winter In China

Based on the daily temperature data of 654 stations provided by the National Meteorological Information Center and NCEP / NCAR reanalysis data from 1980 to 2017,the large-scale persistent low temperature events(referred to as low temperature events)in winter in China are classified,and the atmospheric low-frequency characteristics and differences of various low temperature events are compared and studied.The main results are as follows:Low temperature events can be divided into national type,northeast type and eastern type.The winter temperature has a significant period of 10?30 days.In the lower troposphere,the three types of events are mainly affected by the low-frequency anomaly of Siberian high(LASH).In terms of intensity,LASH is the strongest in the national type and the weakest in the northeast type;In terms of coverage and location,among the national type of evnts,LASH is the largest,and among the northeast and east events,LASH is to the east.In addition,the west anomaly of Aleutian Low is also the main low-frequency system affecting northeast type.In the middle troposphere,both national and eastern types are characterized by a horizontal trough to vertical low-frequency circulation;In the national type,the East Asian trough tilts to the southwest,while the East Asian trough shows an inverted "?" anomaly in eastern type,and the latitudinal-scale of the western horizontal trough is larger,but the average position is more to the east.Northeast type of events are mainly affected by the low-frequency anomaly of northeast cold vortex.In the upper troposphere,all three types of events are characterized by weak temperate jets and strong subtropical jets at low-frequencies,but the location of the jets varies significantly.The three types of events differ in the time of appearance of the upstream aura lowfrequency signal in the 500 hPa height field,with the national type at-12 days,the northeastern type at-9 days,and the eastern type at-10 days.The low-frequency downward transmission of the top tropospheric potential vorticity anomaly has an impact on the low-level temperature.In the national and eastern types,lowfrequency potential vorticity positive anomalies are downward transmitted from the stratosphere to the troposphere,and there is also downward transmission of positive weatherscale potential vorticity anomalies in the national type;in the middle and upper troposphere,all three types show the characteristics of low-frequency high potential vorticity,which is beneficial to the maintenance of their low-level cold anomalies through potential vorticity inversion.The scale decomposition of the 500 hPa temperature advection and its low-frequency characteristics show that there are strong low-frequency temperature cold advection in all three types of events,in which the advection of the 10?30 d low-frequency flow field to the mean temperature field and the advection of the mean flow filed to the 10?30 d low-frequency temperature field play an important role in the 10?30 d low-frequency temperature advection for all three types of events,while in the national type,the 10?30 d low-frequency flow field to the 10?30 d low-frequency temperature advection and the 30?90 d low-frequency flow field to the 30?90 d low-frequency temperature advection,their 10?30 d low-frequency components also have a more important role?...