Atmospheric Circulation Patterns Associated With Persistent Wet-freezing Events Over South China

The characteristics of atmospheric circulation patterns associated with persistent wetfreezing events(PWFEs)over South China during 1980-2010 are investigated by using reanalysis data and observation dataset of daily temperature and precipitation.The results reveal that there are at least two types of large-scale atmospheric circulation patterns of PWFEs: a single blocking high type and a double blocking high type.The main conclusions are as follows:(1)At 500 hPa,the single blocking high type is characterized by the development of a blocking high to the Lake Baikal,with a negative anomaly over mid-latitude Asia and an East Asian Trough which is to the east of its normal.In particularly,PWFEs responsible for this type are very likely to occur under positive phase of the Arctic Oscillation(AO).In the upper troposphere(200 hPa),subtropical jet over the west of Qinghai-Tibet Plateau is intensified.The single blocking high type continuously steers cold air from Northeast China to South China in the lower troposphere(850 hPa).And the route of transporting vapor mainly includes low-level southwest path and southerly low-level path,originating from Bay of Bengal and South China Sea,respectively.Correspondingly,the lower level layer air is likely to capped with a stable inversion.(2)At 500 hPa,the double blocking high type is characterized by the appearance of two blocking highs near the Ural Mountains and the Sea of Okhotsk,with an intensified trough between them and a stronger negative anomaly over most of the mid-latitude region.Meanwhile,the positive anomalies dominate in the Arctic,and also PWFEs connected with the latter one are more likely than not occur under negative phase of the AO.Subtropical jet over mid-latitude East Asia is strengthened,locating in the north of the former type's.At the 850 hPa level,the north wind linked to the anomalous anticyclone over Siberia move southward from Lake Baikal into South China,and southwest path is the mainly transportation path of water vapor.Additionally,there is no obvious evidence that inversion layer takes place during event period.The present results also emphasize that planetary Rossby wave energy dispersion plays an important role in persistent large-scale circulation anomalies during PWFEs.A cold surge event,which is occurred on January 21st-25th 2016,characterizes by short duration and rapid decay.The comparative analysis of the event on large-scale circulation is investigated.The results are as follows:(1)there is strong planetary Rossby wave energy dispersion near blocking high region,leading to a rapid decay of the blocking high,while the blocking high is sustainable during the period of 2008-1-13 due to planetary wave energy dispersion from the upstream.(2)The Siberia high in January 2016 is stronger than that of 2008-1-13 with a colder air masses,resulting in less precipitation.(3)The Southern branch trough is weaker than that of 2008-1-13,causing less water vapor transportation from the Bay of Bengal.(4)The situation of the western Pacific subtropical high is more south and east than that in 2008,leading to less water vapor transportation from South China Sea.(5)The path of cold air masses during the cold surge event is the same as the double blocking high type.