Mountain waves and wakes generated by South Georgia: implications for drag parametrization

High-resolution simulations of flows over South Georgia (South Atlantic) are used to increase understanding of the likely influence of small isolated mountainous islands on the large-scale flow and to ascertain the extent to which parametrization schemes can account for the missing drag in models where such islands are only partially resolved. Long-duration (1 month) austral winter forecasts with a horizontal grid spacing of 1.5 km are used to quantify the mountain-wave momentum fluxes generated by the island and the low-level drag associated with flow-blocking dynamics. The characteristics of the drag, such as the occurrence of high and low drag states, its dependence on wind direction and the spectral contributions to the mountain-wave momentum flux, are considered. Flow splitting and low-level wave breaking are shown to be responsible for wake regions that extend for hundreds of kilometres from the island. Regions of deceleration are also evident in the stratosphere, due to mountain-wave dissipation. The extent to which an orographic drag parametrization scheme can reproduce the drag is investigated by comparison with coarse-resolution (15 km grid spacing) simulations in which the orography is poorly resolved and a large proportion of the drag is parametrized. It is demonstrated that the total of the resolved plus parametrized drag in these simulations closely resembles that at high resolution, although it is underpredicted during instances of high drag. Simple modifications to the scheme, which enhance the drag when the low-level flow is approximately normal to the major axis of the subgrid orography, are shown to rectify this. The study demonstrates that, at least for relatively simple isolated mountain ranges, the drag and mountain-wave momentum fluxes can be predicted in a deterministic sense by a well-tuned parametrization scheme of suitable complexity, although inclusion of stochastic effects might lead to yet further improvements.