Fig. 2

Difference in the resolved (green), parametrized (red) and total (resolved+parametrized, black) orographic torques between horizontal resolutions of 16 and 150 km. The torques are integrated over 10° latitude bands and averaged over a set of 24 h forecasts performed during December 2015 with models used for operational weather prediction at a the UK Met Office and b the European Centre for Medium-Range Weather Forecasts (ECMWF) b. The sign of the differences is such that the resolved westward torques between 30 and 60°N (which act as a drag on the westerly flow) are larger at 16 km compared to 150 km resolution, while the parametrized torques are smaller. The reduction in parametrized orographic torque greatly exceeds the increase in resolved orographic torque, leading to a smaller total orographic torque at higher resolution. The differences in the total (resolved+parametrized) orographic torques are, depending on latitude, between 20% and 40% of the total orographic torque in the UK Met Office model at 150 km resolution. See ref. ⁶ for further details of the methodology