Title: Estimating the Effects of Vertical Wind Shear on Orographic Gravity Wave Drag Over Antractica

Abstract:

Orographic gravity waves are caused by the flow of air over mountains. The drag these waves exerts is known to be impacted by the variation of the wind profile with height. This effect is investigated using reanalysis data, focusing particularly on the relative importance of linear shear and wind turning. These are predicted to reduce and enhance the drag respectively. Two levels at which the wind derivatives are evaluated are selected to test the sensitivity of the calculation: a height just above the parametrized boundary layer top (BLH) and one of order the standard deviation of the subgrid-scale orography elevation (SDH). A climatology of the Richardson number (Ri) for the decade 2006-2015 suggests that the Antarctic region has a high incidence of low Ri values, implying high shear conditions. Shear estimated at the SDH has a stronger impact on the drag than that estimated at the BLH. Predicted drag enhancement is more widespread than drag reduction as terms associated with wind turning dominate the drag correction for a large fraction of time. A comparison of climatologies of the drag corrections for horizontally elliptical (i.e. anisotropic) and axisymmetric mountains always results in drag enhancement over Antarctica, with a maximum during the JJA season. This shows qualitative robustness to both calculation height and orography anisotropy. The enhancement is smaller when elliptical orography is used because the shear vector is predominantly aligned along mountain ridges rather than across them in this case.