Sub-seasonal Variations and Predictions of Precipitation over the Asian Monsoon Area with BCC_DERF2.0 in Spring-Summer Transition Season

The sub-seasonal characteristics and prediction of rainfall over the Asian Monsoon Area during spring-summer transitional season (April-May-June) are investigated using a full set of hindcasts generated by the Dynamic Extended Range Forecast operational system version 2.0 (DERF2.0) of Beijing Climate Center, China Meteorological Administration. The onset and development of Asian summer monsoon and the seasonal migration of rain belt  over East Asia can be well depicted by the model hindcasts at various leads. However, there exist considerable differences between model results and observations, and model biases depend not only on lead time, but also on the stage of monsoon evolution. In general, forecast skill drops with  increasing lead time, but rises again after lead time becomes longer than 30 days, possibly associated with the effect of slowly-varying forcing or  atmospheric variability. An abrupt turning point of bias development appears around mid-May, when bias growths of wind and precipitation exhibit significant changes over the northwestern Pacific and South Asia, especially over the Bay of Bengal and the South China Sea. This abrupt bias change is  reasonably captured by the first two modes of multivariate empirical orthogonal function analysis, which reveals several important features associated  with the bias change. This analysis may provide useful information for further improving model performance in sub-seasonal rainfall prediction.