The Parameterization of Drag Induced by Stratified Flow over Anisotropic Orography

A new parameterization of drag arising from the flow over unresolved topography (UT) in a general circulation model (GCM) is presented. It is comprised of three principle components: a parameterization of the source spectrum and drag associated with freely propagating hydrostatic gravity waves in the absence of rotation, a parameterization of the drag associated with low-level wave breaking and a parameterization of low-level drag associated with upstream blocking and lee-vortex dynamics. Novel features of the scheme include: a new procedure for defining the UT in each GCM grid cell which takes account of the GCM resolution and includes only the scales represented by the parameterization scheme, a new method of representing the azimuthal distribution of vertical momentum flux by two waves whose direction and magnitude systematically vary with the flow direction and with the anisotropy of the UT in each GCM grid cell, and a new application of form drag in the lowest levels which can change the direction of the low-level flow so that it is more parallel to unresolved two-dimensional topographic ridges.

The new scheme is tested in the Canadian Centre for Climate Modelling and Analysis (CCCma) third generation atmospheric GCM at horizontal resolutions of T47 and T63. 5-year seasonal means of present-day climate show that the new scheme improves mean sea-level pressures (or mass distribution) and improves the tropospheric circulation when compared to the gravity-wave drag scheme used currently in the GCM. The benefits are most pronounced during northern hemisphere winter. It is also found that representing the azimuthal distribution of the momentum flux of the freely propagating gravity-wave field with two waves rather than just one allows 30--50\% more gravity-wave momentum flux up into the middle atmosphere, depending on the season. The additional momentum flux into the middle atmosphere is expected to have a beneficial impact on GCMs that employ a more realistic representation of the stratosphere.