Assessing the Extended Range Forecast Error of NCEP/CFS in the Summer of East Asia

Regional and persistent heavy rain in the southern parts of China can lead to severe flooding, and abnormality of key circulation system in East Asia is the most important inducement for the abnormal precipitation in target area. Furthermore, the low frequency oscillation (LFO) has special impact on the extended range forecast of general circulation. By use of fully coupled retrospective forecasts covering a 24 year period (1981—2004) provided by NCEP Climate Forecast System (CFS), the 1 to 30 days predictive skill and errors between forecast and observations (NCEP/DOE Atmospheric Model Intercomparison Project (AMIP) Ⅱ Reanalysis) are diagnosed and the predictive skill of three typical cases of persistent heavy rain, in the Yangtze Huaihe River Valley, the southern parts to the Yangtze River and South China are examined respectively. The influence of LFO on extended range forecast skill is revealed.The forecast skill of 200 hPa, 500 hPa and 850 hPa heights is obtained, by calculating the anomaly correlation coefficient (ACC) between predicted and observed fields over a typical region of the East Asia during the summer. Based on the 24 year climatology, the ACC skill maintains greater than 0.6 at a lead time of less than 6 days, which is the average level of current numerical weather prediction models. The ACC drops rapidly with increasing lead time but when it comes to 10—19 days, the ACC is relatively stable and ranges from 0.05 to 0.15. The errors also keep stable at the lead time of more than 10 days with the predicted fields lower than the observations in areas north of 60°N and higher than the observations in areas between 40°N and 60°N.By use of 30—60 days component data which is obtained by Butterworth Band pass Filter, the forecast exhibits more skillful with the ACC to observed 30—60 days component data of about 0.15 at lead time of 15 days.From the typical cases, it's found that the low frequency precipitation with quasi two weeks and 30—60 days fluctuation is an important characteristic of the persistent heavy rain, and the CFS exhibits some useful extended range forecasts skill to the low frequency oscillations of major circulation system which is useful for estimating the main region and intensity of persistent heavy rain. This may provide valuable information for utilizing the numerical products and modeling extended range forecasts.