Does increasing model stratospheric resolution improve extended-range forecast skill?

The effect of stratospheric resolution on extended-range forecast skill at high Southern latitudes is explored. Ensemble forecasts are made for two model configurations that differ only in vertical resolution above 100 hPa. An ensemble of twelve 30-day forecasts is made from mid-November for years 1979 to 2008. November is when the Southern Hemisphere stratosphere is most variable, and so this is when impacts on the Southern extratropical troposphere are expected to be greatest. As expected, the high resolution model is associated with better forecast skill in the stratosphere throughout the 30 day integration. Surprisingly, the high resolution model is also associated with significant forecast skill improvement (∼5%) in the troposphere ∼3–4 weeks after the initialization date. The results suggest extended-range forecast skill can be improved in current forecast schemes by increasing model stratospheric resolution, improving representation of stratospheric dynamics and thermodynamics, and improving stratospheric initial conditions.