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The Global Environmental Multiscale (GEM) model is the Canadian atmospheric model used for meteorological forecasting at all scales. A limited-area version now also exists. It is a gridpoint model with an implicit semi-Lagrangian iterative space-time integration scheme. In the ''horizontal,'' the equations are written in spherical coordinates with the traditional shallow atmosphere approximations and are discretized on an Arakawa C grid. In the ''vertical,'' the equations were originally defined using a hydrostatic-pressure coordinate and discretized on a regular (unstaggered) grid, a configuration found to be particularly susceptible to noise. Among the possible alternatives, the Charney-Phillips grid, with its unique characteristics, and, as the vertical coordinate, log-hydrostatic pressure are adopted. In this paper, an attempt is made to justify these two choices on theoretical grounds. The resulting equations and their vertical discretization are described and the solution method of what is forming the new dynamical core of GEM is presented, focusing on these two aspects.

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An integrated forecasting and data assimilation system has been and is continuing to be developed by the Meteorological Research Branch (MRB) in partnership with the Canadian Meteorological Centre (CMC) of Environment Canada. Part II of this two-part paper presents the objective and subjective evaluations of the intercomparison process that led to the operational implementation of the new Global Environmental Multiscale model. The results of a ''proof of concept'' experiment and those of a meso-␥-scale simulation further demonstrate the validity and versatility of this 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, Lott et Miller (1997), permet de représenter la composante non-résolue de l'effet de résistance sur les vents de surface due aux montagnes. Le terme de blocage ainsi que le paramétrage de la résistance sur l'écoulement due aux ondes de gravité déferlantes forment maintenant un paramétrage unifié des effets orographiques sous-maille devenu opérationnel dans le modèle GEM global le 11 décembre 2001. 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. RÉSUMÉ L'impact d'un blocage orographique sous-maille sur le modèle canadien GEM (Global Environnemental Multi-échelle) est étudié. Le paramétrage, basé sur une formulation deDes résultats provenant de tests faits avec différentes configurations du modèle sont présentés, démontrant l'impact du terme de blocage sur l'écoulement à grande échelle ainsi que l'amélioration des prévisions à courtes et moyennes échéances, particulièrement en hiver. Il est démontré qu'au jour cinq de l'intégration du modèle, l'influence de la force de blocage appliquée près de la surface est en fait ressentie sur toute la profondeur de la troposphère ainsi que sur la basse stratosphère. Un mécanisme basé sur les perturbations du flux d'Eliassen-Palm induites par le forçage près de la surface est proposé afin d'expliquer la propagation verticale du signal induit par la force de blocage. ATMOSPHERE-OCEAN 41 (2) 2003, 155-170