Extratropical Prediction Skill of the Subseasonal-to-Seasonal (S2S) Prediction Models

The deterministic prediction skill of the 10 operational models participating in the subseasonal-to-seasonal (S2S) prediction project is assessed for both the extratropical stratosphere and troposphere. Based on the mean squared skill score of 50- and 500-hPa geopotential height forecasts, the overall prediction skill is on average 16 days in the stratosphere and 9 days in the troposphere. The high-top models with a fully resolved stratosphere typically have a higher prediction skill than the low-top models. Among them, the European Centre for Medium-Range Weather Forecasts model shows the best performance in both hemispheres. The decomposition of model errors reveals that eddy errors are more important than zonal-mean errors in both the stratosphere and troposphere. While the errors in the stratosphere are dominated by planetary-scale eddies, those in the troposphere are equally influenced by planetary- and synoptic-scale eddies. This result indicates that subseasonal-to-seasonal prediction could be improved by better representing planetary-scale wave activities in the model.