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Results are presented from extended annual-cycle integrations of the Meteorological Office 11-layer atmospheric general circulation model. an 8-year control integration produces a good simulation in the southern hemisphere in both summer and winter and also in the northern hemisphere in summer. In northern winter, however, there is excessive westerly near-surface flow polewards of about 40°N. It is argued that this indicates a lack of orographic forcing of the flow by the major mountain ranges. Two solutions which have been proposed for this problem are investigated. the first is envelope orography and the second is a parametrization of the effects of orographic gravity-wave drag. Each was included separately in 3 1/2 year integrations in parallel with the control. Both produce substantial changes in the model's climatology, but envelope orography does not improve the westerly problem over the European area and it also degrades the summer circulation. the gravity-wave drag scheme, however, gives results which in general are in very good agreement with the observations in both hemispheres and seasons. Comparison of the zonally averaged torques from the three experiments demonstrates that with envelope orography the northern hemisphere near-surface westerlies are reduced because of a dramatic increase in the mountain pressure torque. In the experiment with the gravity-wave scheme, however, the reduction is due to the gravity-wave drag and the mountain torque is similar in magnitude to that in the control. A study of the variability of the 500mb height fields on synoptic timescales demonstrates the improvement in the positions of the northern hemisphere storm tracks in this integration. Some remaining errors in the simulations are identified, particularly in the stratosphere. Finally, it is argued that despite these results it may be too early to come to a final conclusion as to the role of gravity-wave drag in the general circulation and also as to the relative importance of dynamical and radiative processes in the polar winter stratosphere.
Slingo et al. (Wed,) studied this question.