Ensemble size for numerical seasonal forecasts

The predictability of 500 hPa height, 850 hPa temperature and precipitation is studied using the “perfect model” approach with ensemble numerical forecasts. The sea surface temperature interannual variability is introduced to provide some source of seasonal predictability. The mean scores of 16 winter forecasts show a large potential in the tropics, a weak one in the midlatitudes, in particular over Europe. The weakness of the scores in the midlatitudes may be partly explained by the underestimation of the amplitude of the seasonal anomalies by the model. The seasonal forecasts are based on 9 individual model integrations starting at slightly different initial conditions. The variation of the scores with the size of the ensemble is estimated empirically. It is shown that, for a perfect seasonal forecast, the size necessary to approach the saturation score is about 3 for the tropical precipitation, 20 for midlatitude height, and 40 for temperature over Europe.