Abstract/Summary

Orographic Gravity Wave Drag (GWD) is known to be affected by vertical wind shear. Linear variation in the magnitude of the wind with height causes GWD to be decreased, whilst negative wind profile curvature (which usually occurs in directional shear, i.e. wind turning with height) causes an increase. Analytical formulae have previously been derived which evaluate this correction to the drag due to shear relative to its value for vertically uniform wind and static stability. The corresponding drag enhancement formulae are tested here for their sensitivity both to the height in the atmosphere at which the associated vertical derivatives of the wind velocity are evaluated and to the anisotropy of the subgrid-scale orography. It is found that whilst the correction is qualitatively robust to changes in calculation height, results show significant quantitative variation between the two heights chosen. Use of an axisymmetric orography profile causes an overestimation of the drag relative to a realistic orography anisotropy. Directional shear effects, which increase the drag, are found to be dominant for a high fraction of the time over a large proportion of the Antarctic region. Inclusion of a simulated boundary layer is found to reduce the magnitude of wave activity seen in idealized simulations. The magnitude of the GWD is thus reduced, in some cases by a factor as large as 3. This highlights the need to apply a corrective factor to the theory to account for the effects of the boundary layer on the surface GWD. This corrective factor is deduced using simulations with and without a boundary layer. Further idealized simulations are carried out to try to identify the optimum height at which to evaluate the shear-induced drag correction. Drag enhancement values are calculated using both linear theory and WRF model output, which are then compared in order to deduce at what height the two methods are closest. It is not possible to identify a single optimum height, however the error between the model derived and analytical drag enhancements is frequently found to be minimised at a level within the boundary layer.