Sub-seasonal Forecast over East Asian Subtropical Region by Combination of Dynamical and Statistical Method

Sub-seasonal forecast is one important focus for filling the gap between medium-range weather forecast and seasonal prediction in present seamless forecast systems. Despite recent interest and development in sub-seasonal prediction, it is still unclear how skillful forecasting systems are in East Asian subtropical region beyond two weeks. Whether internal mode with sub-seasonal variability, influences of initial conditions or external forcing relate to sub-seasonal prediction are either unclear. In this presentation, the sub-seasonal predictability over East Asian subtropical area is discussed by evaluation of sub-seasonal prediction performance basing on dynamical and statistical method. Results show that the climatological intra-seasonal oscillation (CISO) of Asian summer monsoon is reasonably well captured in the sub-seasonal numerical model. Statistically significant forecast skill of 2-meter air temperature (T2m) is achieved for all lead times up to day 26-32 over East Asian subtropical area. The sub-seasonal variability of T2m anomaly over East Asia is found to be linked to an eastward propagating extratropical Rossby wave from the North Atlantic across Europe to East Asia and the Madden-Julian Oscillation (MJO). Meanwhile, the El Nino-Southern Oscillation (ENSO) is also likely to influence the forecast skill of T2m at the sub-seasonal timescale over East Asian subtropical area. In comparison with the dynamical model, the sub-seasonal forecast skill of precipitation may be increased by 1 week over East Asian subtropical area basing on dynamical-statistical method. In addition, a statistical method basing on MJO is also used for sub-seasonal forecast of pentad precipitation, which provides a potential to guide the forecast of Meiyu and heavy rainfall processes over East Asian subtropical area on sub-seasonal timescale.