Analysis of the Spread–Skill Relations Using the ECMWF Ensemble Prediction System over Europe

Abstract The Ensemble Prediction System (EPS) of the European Centre for Medium-Range Weather Forecasts (ECMWF) was used to analyze various aspects of the ensemble-spread forecast-skill relation. It was shown that synoptic-scale upper-air spread measures can be used as first estimators of local forecast skill, although the relation was weaker than expected. The synoptic-scale spread measures were calculated based on upper-air fields (Z500 and T850) over western Europe for the period June 1997 to December 2000. The spread–skill relations for the operational ECMWF EPS were tested using several different spread definitions including a neural network-based measure. It was shown that spreads based on upper-air root-mean-square (rms) measures showed a strong seasonal cycle unlike anomaly correlation (AC)-based measures. The deseasonalized spread–skill correlations for the upper-air fields were found to be useful even for longer lead times (168–240 h). Roughly 68%–83% of small or large spread was linked to the c...