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

Recent studies have demonstrated how global circulation models may suffer from missing drag in the southern hemisphere stratosphere, and that this causes systematic errors in the behaviour of the models. The extent to which this is due to missing momentum flux associated with mountain waves generated by unresolved islands is currently not clear, but recent work has suggested that some of the small mountainous islands in the South Atlantic may make significant contributions to the stratospheric gravity wave momentum flux.

In this study the mountain waves and wakes generated by the island of South Georgia in the South Atlantic are examined in a series of high-resolution numerical simulations. Long duration (~1 month) NWP forecasts with a horizontal grid spacing of 1.5 km have been conducted with the Met Office Unified Model (UM), the dynamical core of which has recently been upgraded to ENDGame, providing a more accurate treatment of gravity waves.

The simulations are used to quantify the gravity-wave momentum fluxes generated by the island and the low-level drag associated with flow splitting. The characteristics of the drag such as the occurrence of high and low drag states, its dependence on wind direction and its predictability will be considered. The extent to which orographic drag parametrization schemes can reproduce the resolved drag will be investigated by comparison with coarse-resolution simulations in which South Georgia is very poorly resolved.

Finally, a new research project will be introduced: the South Georgia Wave Experiment (SG-WEX). SG-WEX aims to observe and model the gravity waves above South Georgia, through the troposphere, stratosphere and mesosphere. Observations made during SG-WEX will be used to verify future model simulations and in turn these will be used to help interpret the observations.