Evaluation Of Ensemble Forecasts By PECA

Mozheng Wei and Zoltan Toth

NCEP Environmental Modeling Center

A method called Perturbation vs. Error Correlation Analysis (PECA), which evaluates the ensemble perturbations instead of the forecasts themselves by measuring their ability to explain forecast error variance, is used to evaluate ensemble forecasts from NCEP and ECMWF. Ensemble perturbations from NCEP and ECMWF were found to perform similarly. The error variance explained by either ensemble increases with the number of members and the lead time. The dynamically conditioned NCEP and ECMWF perturbations outperform both randomly chosen perturbations and differences between lagged forecasts ("NMC" method). Thus ensemble forecasts potentially could be used to construct flow dependent short-range forecast error covariance matrices for use in data assimilation schemes.