Five sets of regional ensemble forecasts with lead times of 36 h over two months from 24 June 2008 to 24 August 2008 from the Beijing 2008 Olympics Research and Development Project(B08RDP) are evaluated and analyzed.This is firstly done by means of standard probabilistic verification scores,including root-mean-square error(RMSE),ensemble spread,talagrand diagrams,reliability,and ROC(Relative Operating Characteristic) curves.Then,to improve the forecast quality,a combined decaying averaging bias correction scheme(BC) is applied to the ensemble forecasts of B08RDP to reduce the bias in the ensemble mean and to adjust the improper spread of ensembles with sufficient performance evaluation.The BC scheme is designed based on the original Kalman filter.It contains the first moment bias correction,mainly for correcting the bias in the ensemble mean to improve the reliability of the ensemble forecasts,and the second moment bias correction mainly for adjusting the ensemble spread to make the ensemble forecasts fully representative of the uncertainties in the observations.Lastly,the BC scheme's capacity is evaluated and discussed by means of the verification scores mentioned above.Temperatures at 850 hPa are corrected and verified in this study,wherein ECMWF reanalysis data are used as the reference for the verification.
The results show that,among the five sets of regional ensemble forecasts in B08RDP,the regional ensemble forecasts from NCEP possess the best forecast quality,with minimal bias,the most appropriate spread,and the best performance in terms of reliability,resolution and talagrand distributions.Meanwhile,the regional ensemble forecast from CAMS demonstrates the worst forecast quality,due to its largest forecast bias.On the whole,a relatively small spread is a common problem for several of the ensemble forecasts,except those from NCEP.In general,the combined bias correction scheme is proven to be efficient in reducing the RMSE of the ensemble mean,and in generating a more appropriate ensemble spread,for the five sets of ensemble forecasts,revealing its ability to improve the quality of ensemble forecasts,especially for ensemble forecasts of an already low quality.