Causes and Dynamic-statistical Forecast of the Summer Rainfall Anomaly over China in 2011

The large-scale rainfall over China in summer of 2011 is reviewed, the prediction results of dynamic-statistical objective quantitative (DSOQ) and dynamic-statistical diagnostic (DSD) methods are evaluated. Compared to the DSOQ, the anomaly correlation coefficient (ACC) and predictive score (PS) of which are 0.12 and 70, the DSD method has obvious advantage in predicting skill by increasing the ACC and PS to 0.25 and 75, respectively. Taking the middle and lower reaches of the Yangtze (MLRY) as an experiment region, the differences in predictive factors of these two methods are compared and the advantages of DSD method are analyzed. The probable causes of summer rainfall anomaly distribution in 2011 and the relevant circulation anomaly characteristics are also discussed, such as the blocking-high (BH) anomaly in middle-high latitudes and the western Pacific subtropical high (WPSH) anomaly in low latitudes. The results indicate that the abundant rainfall in June over the middle and lower reaches of the Yangtze and the uneven distribution of June-July-August (JJA) rainfall are the direct causes for the southerly rainbelt in the summer of 2011. And this is related to the BH activities, the intra-seasonal oscillations (ISO) of WPSH and the monthly different configurations between them. In June, the atmospheric circulations reveal two trough areas and one ridge area at the middle-high latitudes. The intensity and western boundary of the WPSH are normal, while the latitude of the ridge line is northerly. The southward cold air behind the trough converges with the northward warm wet airflow over MLRY, which causes substantial precipitation in this area. Compared with June, the atmospheric circulations in middle-high latitudes change into the two ridge areas and one trough area in July, the WPSH becomes weaker and its western boundary moves eastward, and the ridge line is more northward. These situations lead to stronger cold air than the warm wet airflow, resulting in drought of the whole areas. In August, the strength of the trough and ridge weaken relative comparing to July, which makes the convergence of the southward cold air and the northward warm wet airflow over East China. The situations above lead to a large amount of precipitation in East China. Besides, the variation of the BH and the ISO of WPSH is affected by the interactions among the East Asian circulation systems (EACS), and the external forcing of sea surface temperature (SST) and snow cover. The interactions and configurations among EACS are key effective factors of summer climate. Thus, by predicting the seasonal and monthly key circulation factors (e.g., BH and WPSH, etc.) to revise the summer precipitation prediction would be a feasible way for the improvements of the dynamic-statistical prediction skill.