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Atmospheric gravity waves (GWs) play an important role in atmospheric dynamics but accurately representing them in general circulation models (GCMs) is challenging. This is especially true for orographic GWs generated by wind flow over small mountainous islands in the Southern Ocean. Currently, these islands lie in the “grey zone” of global model resolution, where they are neither fully resolved nor fully parameterised. It is expected that as GCMs approach the spatial resolution of current high-resolution local-area models, small-island GW sources may be resolved without the need for parameterisations. But how realistic are the resolved GWs in these high-resolution simulations compared to observations? Here, we test a high-resolution (1.5 km horizontal grid, 118 vertical levels) local-area configuration of the Met Office Unified Model over the mountainous island of South Georgia (54∘ S, 36∘ W), running without GW parameterisations. The island's orography is well resolved in the model, and real-time boundary conditions are used for two time periods during July 2013 and June–July 2015. We compare simulated GWs in the model to coincident 3-D satellite observations from the Atmospheric Infrared Sounder (AIRS) on board Aqua. By carefully sampling the model using the AIRS resolution and measurement footprints (denoted as model sampled as AIRS hereafter), we present the first like-for-like comparison of simulated and observed 3-D GW amplitudes, wavelengths and directional GW momentum flux (GWMF) over the island using a 3-D S-transform method. We find that the timing, magnitude and direction of simulated GWMF over South Georgia are in good general agreement with observations, once the AIRS sampling and resolution are applied to the model. Area-averaged zonal GWMF during these 2 months is westward at around 5.3 and 5.6 mPa in AIRS and model sampled as AIRS datasets respectively, but values directly over the island can exceed 50 mPa. However, up to 35 % of the total GWMF in A

New observations at South Georgia (an extremely intense source of gravity waves) are used to evaluate model simulations of gravity waves and wakes, paving the way for improved understanding of these important phenomena. Gravity waves (GWs) play an important role in many atmospheric processes. However, the observations-based understanding of GWs is limited, and representing them in numerical models is difficult. Recent studies show that small islands can be intense sources of GWs, with climatologically significant effects on the atmospheric circulation. South Georgia, in the South Atlantic, is a notable source of such “small island” waves. GWs are usually too small scale to be resolved by current models, so their effects are represented approximately using resolved model fields (“parametrization”). However, the “small island” waves are not well represented by such parametrizations and the explicit representation of GWs in very high resolution models is still in its infancy. Steep islands such as South Georgia are also known to generate low-level wakes, affecting the flow hundreds of kilometers downwind. These wakes are also poorly represented in models. We present results from the South Georgia Wave Experiment (SG-WEX) for 05 July 2015. Analysis of GWs from satellite observations are augmented by radiosonde observations made from South Georgia. Simulations were also made using high-resolution configurations of the Met Office Unified Model (UM). Comparison with observations indicates that the UM performs well for this case, with realistic representation of GW patterns and low-level wakes. Examination of a longer period simulation suggests that the wakes generally are well represented by the model. The realism of these simulations suggests they can be used to develop parametrizations for use at coarser model resolutions.

Redistribution of snow by the wind can drive spatial and temporal variations in snow accumulation that may affect the reconstruction of paleoclimate records from ice cores. In this paper we investigate how spatial variations in snow accumulation along a 13 km transect across Lyddan Ice Rise, Antarctica, are related to wind-borne snow redistribution. Lyddan Ice Rise is an approximately two-dimensional ridge which rises about 130 m above the surrounding ice shelves. Local slopes on its flanks never exceed 0.04. Despite this very smooth profile, there is a pronounced gradient in snow accumulation across the feature. Accumulation is highest on the ice shelf to the east ( climatologically upwind) of the ice rise and decreases moving westward, with the lowest accumulation seen to the west ( climatologically downwind) of the ice rise crest. Superimposed on this broad-scale gradient are large ( 20-30%), localized variations in accumulation on a scale of around 1 km that appear to be associated with local variations in surface slope of less than 0.01. The broad-scale accumulation gradient is consistent with estimates of wind-borne redistribution of snow made using wind speed observations from three automatic weather stations. The small-scale variability in accumulation is reproduced quite well using a snow transport model driven by surface winds obtained from an airflow model, providing that both the wind shear and static stability of the upwind flow are taken into account. We conclude that great care needs to be exercised in selecting ice core sites in order to avoid the possibility of blowing snow transport confounding climate reconstructions.