Why Does Vertical Wind Shear Reduce the Surface Drag of Orographic Gravity Waves?

Orographic gravity waves (OGWs) play an important role in transferring momentum and energy from the Earth's surface to the upper atmosphere. Although previous studies have consistently shown that constant vertical wind shear in the basic flow reduces the surface drag produced by linear hydrostatic OGWs, the underlying dynamical mechanism has remained unclear. Using linear wave theory and an analytical model of hydrostatic OGWs, the surface pressure is decomposed into the component due to the advection of horizontal convergence, <i>p</i>_adv , and that due to the twisting effect of vertical wind shear, <i>p</i>_tws. It is shown that the surface <i>p</i>_tws, although modified by vertical wind shear, remains symmetric and thus contributes no net drag. In contrast, the surface <i>p</i>_adv exhibits a distinct asymmetric response under vertical wind shear, which systematically reduces the drag. Central to this mechanism is a downshear shift of the horizontal divergence field at the surface, which reorganizes the surface pressure distribution. A spectral analysis further reveals that this shift results from changes in the phase spectrum of the surface horizontal divergence under vertical wind shear, while its amplitude spectrum remains unaltered. These findings provide a dynamical explanation for shear-induced reduction in OGWs’ surface drag, and offer physical insights for improving orographic-drag parameterizations in numerical models.