93 / 2021-07-18 17:17:36

A Wintertime Heavy Rainfall Event in Southern China Associated with the South Branch Trough of Subtropical Westerlies

heavy rainfall; southern China; south branch trough; subtropical westerlies; East Asian subtropical jet

The south branch trough (SBT) of subtropical westerlies is one of the important weather systems affecting the wintertime heavy rainfall in southern China. In this study, we analyze a wintertime heavy rainfall event in southern China associated with the SBT activity from January 8th to 13th, 2015. The results show that this heavy rainfall event includes two different stages closely related to the variations of the SBT. Due to the stable maintenance of the SBT in the northern Bay of Bengal and its eastward movement, heavy rainfall events occurred in the southwestern and southeastern China, respectively. Despite the common feature that the southerlies appearing in the front of the SBT transported abundant water vapor, the related physical mechanisms triggering heavy rainfall respectively in the southwestern and southeastern China are quite different. The SBT-related precipitation in the southwestern China is closely associated with the topographic forcing, and heavy rainfall events mainly occurred on windward slopes of the mountains there. The heavy rainfall in the southeastern China occurred mainly due to the baroclinic frontogenesis caused by the interaction between the warm and wet airflow in the front of the SBT and the cold air intruding along the eastern edge of the Tibetan Plateau. Besides, the enhancement of the upward motion and frontogenesis within the equator-ward entrance region of the East Asian subtropical jet also favored the occurrence of heavy rainfall in both southwestern and southeastern China. This study can provide a scientific basis for precipitation forecast in South China in winter half year.