A new orographic gravity-wave drag (GWD) parameterization is derived for implementation in the Canadian Climate Centre GCM. The new scheme is comprised of three principle components. The first component is a parameterization of the freely propagating gravity wave field forced by the unresolved topography (UT) in the GCM, and its interaction with the mean flow. Two waves are employed to represent the gravity-wave field launched by the UT in each GCM grid cell. The properties of each wave are derived from the linear-theory spectral density of horizontal momentum-flux for stratified flow over an elliptical obstacle (Phillips 1984). In this way, anisotropy in the gravity-wave field is represented by the direction and magnitude of momentum flux associated with each wave. The second component of the new scheme is a parameterization of drag associated with low-level wave breaking and downslope flows. The final component of the new scheme employs form drag to parameterize the drag resulting from near-surface-flow dynamics (e.g. blocking and lee-vortices). This drag is applied independently to the components of the low-level wind which are parallel and anti-parallel to two-dimensional ridge-like structure in the UT in each GCM grid cell. Consequently, in addition to retarding the flow, the form drag in the new scheme has the potential to deflect the flow along prominent unresolved topographic ridges. The Climate resulting from the implementation of the new scheme is compared with the climate obtained with the current GWD scheme (McFarlane 1987). The comparison indicates an improved tropospheric circulation with reduced anomalies from the observed climate.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics