For an anisotropic topographic feature in a large-scale flow, the
orientation of the topography with respect to the flow will affect
the vorticity production that results from the topography–flow
interaction. This in turn affects the amount of form drag that the
ambient flow experiences. Numerical simulations and perturbation
theory are used to explore these effects of change in topographic
orientation. The flow is modelled as a quasi-geostrophic homogeneous
fluid on an f-plane. The topography is taken to be
a hill of limited extent, with an elliptical cross-section in the
horizontal. It is shown that, as a result of a basic asymmetry of
the quasi-geostrophic flow, the strength of the form drag depends
not only on the magnitude of the angle that the topographic axis
makes with the oncoming stream, but also on the sign of this angle.
For sufficiently low topography, it is found that a positive angle
of attack leads to a stronger form drag than that for the
corresponding negative angle. For strong topography, this relation
is reversed, with the negative angle then resulting in the stronger
form drag.