Can MCGE Outperform the ECMWF Ensemble?

In this study, Multi-Center Grand Ensembles (MCGEs) were constructed by combining five operational medium-range ensemble forecasts: CMC, ECMWF, JMA, NCEP, and UKMO with equal weights and no bias correction. The forecast performance of the MCGEs relative to the ECMWF ensemble, having the best forecast performance in the world, was investigated using the seasonal Root Mean Square Error (RMSE) and the Ranked Probability Score (RPS) for 500 hPa geopotential height over the Northern Hemisphere (20 degrees N-90 degrees N) from December 2006 to November 2007. It was found in the deterministic and probabilistic verifications that the MCGEs generally outperformed the ECMWF ensemble at least in the medium forecast range (day 6-9) for all seasons. The improvements in the RMSE and the RPS are several percentage points. These are almost comparable with the rate of improvement in a single-center ensemble forecast during the latest few years.