Fig. 4

Almost 1 day of skill is lost when the turbulent orographic form drag parametrization¹⁷ is switched off in global 10-day weather forecasts performed with the Integrated Forecasting System of the European Centre for Medium-Range Weather Forecasts: in the control experiment (black), forecasts 7 days ahead have an accuracy of 80% in terms of anomaly correlation of the geopotential height at 500 hPa, while in the experiment where the form drag scheme was switched off (red) this level of accuracy is lost after 6 days. In other words, in the new experiment the degree of accuracy of the forecasts only matches the degree of accuracy of numerical weather prediction systems of 10 years ago.²³ The simulations have been performed at a horizontal resolution of 32 km, for the period December–February 2016–2017