Ensembles using multiple models and analyses

The performance of the European Centre for Medium‐Range Weather Forecasts (ECMWF) Ensemble Prediction System (EC EPS) is compared with that of a number of alternative configurations which incorporate information from additional analyses or an additional model or both. Each configuration is approximately equivalent in size, resolution and computational cost and could therefore in principle provide an alternative operational EPS. A multi‐centre ensemble system (MC EPS) is constructed by replacing half of the EC EPS with integrations of the Met Office (UKMO) model perturbed about the UKMO analysis. Two additional configurations are constructed using the ECMWF model only, but adding information available from four other operational analyses. A ‘consensus’ ensemble (CONS EPS) is generated by adding the operational EC EPS perturbations to the mean of the available analyses (the consensus analysis); a multi‐analysis ensemble (MA EPS) is generated by combining smaller ensembles of ECMWF‐model integrations perturbed about each of the available operational analyses. The different systems are compared over a large sample of 60 cases covering the period October 1998 to July 1999.

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