Abstract

Abstract. 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 AIRS is actually found upwind of the island compared to only 17 % in the model sampled as AIRS, suggesting that non-orographic GWs observed by AIRS may be underestimated in our model configuration. Meridional GWMF results show a small northward bias (∼20 %) in the model sampled as AIRS that may correspond to a southward wind bias compared to coincident radiosonde measurements. Finally, we present one example of large-amplitude (T′≈15–20 K at 45 km altitude) GWs at short horizontal wavelengths (λH≈30–40 km) directly over the island in AIRS measurements that show excellent agreement with the model sampled as AIRS. This suggests that orographic GWs in the full-resolution model with T′≈45 K and λH≈30–40 km can occur in reality. Our study demonstrates that not only can high-resolution local-area models simulate realistic stratospheric GWs over small mountainous islands but the application of satellite sampling and resolution to these models can also be a highly effective method for their validation.

Highlights

  • Atmospheric gravity waves (GWs) are a key dynamical component of the Earth’s atmosphere

  • Up to 35 % of the total GW momentum flux (GWMF) in Atmospheric Infrared Sounder (AIRS) is found upwind of the island compared to only 17 % in the model sampled as AIRS, suggesting that non-orographic GWs observed by AIRS may be underestimated in our model configuration

  • Meridional GWMF results show a small northward bias (∼ 20 %) in the model sampled as AIRS that may correspond to a southward wind bias compared to coincident radiosonde measurements

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Summary

Introduction

Atmospheric gravity waves (GWs) are a key dynamical component of the Earth’s atmosphere. Accurately representing GWs in global circulation models (GCMs) used for numerical weather and climate forecasting has proved challenging (Alexander et al, 2010; Plougonven et al, 2020) One reason for this is that a large fraction of GWs and their sources lie at physical scales that are below the spatial resolution of GCMs. The momentum forcing of these subgrid waves on the background flow must instead be simulated by parameterisations. By applying the vertical resolution, horizontal sampling and retrieval noise of the AIRS (Atmospheric Infrared Sounder) measurements to the model, we are able to make a direct like-for-like comparison of observed and simulated GW amplitudes, wavelengths and directional momentum fluxes over the island. Three atmospheric datasets over South Georgia are analysed in this study: (1) modelling simulations in a local-area domain centred on the island, (2) 3-D satellite observations from AIRS/Aqua and (3) radiosonde observations launched from the British Antarctic Survey (BAS) base at King Edward Point (KEP). Note that the June–July radiosondes travelled much further downwind due to stronger stratospheric zonal winds during austral winter, and many of these travelled so far east that they exited the local-area model domain

Numerical modelling: local-area simulations over South Georgia
AIRS 3-D satellite observations
Radiosondes
Model wind validation using co-located radiosonde measurements
Gravity waves over South Georgia in the full-resolution model
Applying the AIRS observational filter to the model
Horizontal sampling
Vertical resolution
Retrieval noise
Measuring 3-D gravity wave properties
Extracting gravity waves temperature perturbations
Measuring gravity wave properties with a 3-D S-transform
Zonal and meridional momentum fluxes
Time series of wave amplitude and directional GWMF
Wave amplitude growth with height
Large-amplitude mountain waves at short horizontal scales
Model sampled as AIRS: sensitivity to horizontal sampling and retrieval noise
Simulation of NGWs in the local-area model
Findings
Large-amplitude mountain waves directly over the island
10 Summary and conclusions
Full Text
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