Anomalous atmospheric circulation characteristics of the sustained torrential rainfall over Guangdong in June

By using the 33-year (1979-2011) 86-station daily precipitation observations of Guangdong Province and the global daily atmospheric data from NCEP-DOE Reanalysis 2, the anomalous circulation characteristics of abnormal rainfall during the pre-flooding season in Guangdong were analyzed. We found whether a torrential rainfall in June would be persistent depends in part on whether there exist an meridional wave train. However, the link between the persistence of severe rainfall and meridional wave train does not exist in April and May. The meridional wave train impacts on the process of the sustained torrential rainfall through changing the atmospheric circulation anomalies at upper, middle and lower levels of the troposphere. Accompanied with an meridional wave train, the meridionality of the westerly circulation enhances at upper and middle levels of the troposphere, which is favorable for the invasion of cold air and upper trough. Southern China is in the airflow spread area between anomalous westerly and southwesterly at 200 hPa which is beneficial to divergence at upper levels. The western Pacific subtropical high is stronger and its west ridge is located more westward than normal. The vertical meridional circulation anomaly in which it is rising in southern China and sinking in the South China Sea is set up. Water vapor transport at low levels from both northern India and the Bay of Bengal, as well as from the tropical western Pacific is enhanced at the same time. Therefore, the stable circulation background and both the dynamics ascending and water vapor conditions in local region for sustained torrential rainfall are provided. In the case of absence of meridional wave train, only 1-day or 2-day torrential rainfall may happen when the trough at 500 hPa is active in southern China, the westerly at 850 hPa is intensive in the northern region of the South China Sea, and dynamical ascending and water vapor conditions in local are satisfied to some extent. So the meridional wave trains may be used to distinguish whether the torrential rainfall is sustained or not. The meridional wave trains associated with sustained anomaly of rainfall in Guangdong are affected by the Rosssby wave from the middle and high latitudes, tropical convection as well as the atmospheric anomaly from the middle and low latitudes in the Pacific.