Daily Forecast Skill of Multi-Center Grand Ensemble

In this study, we investigate the daily forecast skill of Multi-Center Grand Ensemble (MCGE), consisting of three operational medium-range ensemble forecast data by the Japan Meteorological Agency (JMA), the National Centers for Environmental Prediction (NCEP), and the Canadian Meteorological Center (CMC). The skill is evaluated by comparison among the daily RMSE of ensemble mean forecasts for 500 hPa geopotential height over the Northern Hemisphere (20 degrees N-90 degrees N) from August 2005 to February 2006. It is found that MCGE with the same ensemble size as that of the JMA ensemble is more skillful than JMA ensemble for about 75% in frequency both in autumn and winter. Reduction of error with MCGE has little dependence on the atmospheric flow. The RMSE of MCGE can be reduced up to about 20% whether the atmospheric field is easily-predictable or not. Even for the case that MCGE is not more skillful than JMA, the RMSE is increased at most 10%. We argue that the major benefit of MCGE is to avoid the poorest forecast.