Gravity waves are an important type of atmospheric wave. They play a key role in many atmospheric processes, ranging from convection to the mixing of chemical species to influencing the global-scale circulation of the stratosphere and mesosphere. Because of this, it is essential to represent their effects in numerical weather prediction and climate models.
Gravity waves are generated by sources including winds blowing over mountains, jet-stream instabilities and strong convection. The waves can transport energy and momentum away from these sources and deposit them at greater heights, thus exerting a significant "drag" on the circulation and so coupling together different layers of the atmosphere.
Recent studies have shown that isolated mountainous islands in regions of strong winds can be intense sources of gravity waves that can have climatologically-significant effects on atmospheric circulation. However, most climate and numerical weather prediction models cannot accurately model waves from such small, intense island sources because the islands are too small compared to the resolution of the models - this is the "small island problem".
The South Geogia -Wave EXperiment (SG-WEX), was a NERC funded project (grant awards NE/K015117/1, NE/K012614/1 and NE/K012584/1) proposed a major coordinated observational and modelling experiment to determine the nature and impacts of gravity waves generated by the most important of all these islands, South Georgia in the Southern Atlantic.
The SG-WEx project sought to answer the following questions:
1. What is the nature of gravity waves generated by South Georgia and what is their variability?
2. What is the contribution of these gravity waves to the total field of gravity waves over the South Atlantic?
3. What is the influence of gravity waves from South Georgia on the mesosphere?
4. How can these observations be used to improve gravity-wave parametrizations in models?
5. How important is South Georgia in comparison to other gravity-wave sources and how does it impact local winds and the development of synoptic systems?
To answer these questions the project made measurements of gravity waves over and around South Georgia in two radiosondes campaigns in which meteorological balloons were launched from South Georgia. The observations were then placed in context with measurements made by satellite across the whole South Atlantic. Significantly, the project also deploy the first atmospheric radar on South Georgia - a meteor radar that making the first ever measurements of gravity waves (and winds, tides and large-scale planetary waves) in the mesosphere over South Georgia at heights of 80 - 100 km.
These experimental results were then complemented by a programme of modelling work that explored the propagation of gravity waves away from their sources. The observations will be used to help guide the development of new, improved, mathematical representations of gravity waves (so-called "parametrizations") allowing such islands to be better represented in the Met Office's Unified Model used for numerical weather prediction and climate studies. Finally, modelling studies integrated these studies and determine the relative importance of South Georgia compared to other waves sources and investigate the impact of Gravity waves from South Georgia on local winds and the development of synoptic (weather) systems.
The primary academic beneficiaries will be the community of atmospheric scientists who have interests in the structure and dynamics of the middle atmosphere and in understanding how it is coupled to the underlying troposphere.
Numerical Weather Prediction (NWP) and climate models all rely on parametrizations of gravity waves to produce realistic middle atmospheres. The project's results will help constrain such parametrizations and thus will thus also be of interest to these communities of scientists - including those meteorologists working with NWP models. This work will thus contribute to the broader activities of weather prediction and climate-change prediction which benefits society at large.
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