The subgrid‐scale orographic blocking parametrization of the GEM Model

The impact of the physical parametrization called subgrid‐scale orographic blocking, recently introduced in the physics of the Canadian Global Environmental Mutiscale (GEM) model is described. It is based on a formulation by Lott and Miller (1997) and represents the unresolved component of the drag on low‐level winds that are blocked at the flanks of mountains. The blocking term plus the gravity‐wave drag are now part of a unified parametrization of the subgrid orographic drag that became operational in the global GEM model on 11 December 2001. Results from tests made with various configurations of the model are shown, illustrating how the blocking term impacts the large‐scale flow and improves both the short‐ and the medium‐range forecasts, especially in winter. It is shown that at day 5 of the model integrations, the influence of the blocking force applied near the surface is felt by the entire tropospheric and the lower‐stratospheric circulation. A mechanism based on perturbations of the Eliassen‐Palm flux caused by the low‐level forcing is proposed to explain the vertical propagation of the signal generated by the blocking term.