The South Georgia Wave Experiment (SG-WEX)

Lead Research Organisation: University of Bath
Department Name: Electronic and Electrical Engineering

Abstract

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".

Here, we propose 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.

Our experiment will 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 we will make measurements of gravity waves over and around South Georgia in two radiosondes campaigns in which meteorological balloons will be launched from South Georgia. We will place these observations in context with measurements made by satellite across the whole South Atlantic. Significantly, we will also deploy the first atmospheric radar on South Georgia. This is a meteor radar that will make 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 will be complemented by a programme of modelling work that will explore 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 will integrate 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.

Planned Impact

There are three main groups of beneficiaries:

a) The academic community who work in tropospheric, stratospheric and mesospheric science. We will present our results and make a particular effort to bridge the gaps between experimental scientists studying different regions of the atmosphere and those modellers working in numerical weather prediction.

b) The general public, who will be informed of the nature of our project and kept up to date on its development by a dedicated web site and the Press Office and Public Engagement Unit of The University of Bath. The public will also ultimately benefit from the improvements in numerical weather prediction and climate modelling.

c) The Met Office, who will benefit by being able to use the knowledge generated by the project to assess and improve the representation of gravity waves in future Model development for numerical weather prediction and climate research.

Publications

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Hindley N (2016) A two-dimensional Stockwell Transform for gravity wave analysis of AIRS measurements in Atmospheric Measurement Techniques Discussions

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Hindley N (2016) A two-dimensional Stockwell transform for gravity wave analysis of AIRS measurements in Atmospheric Measurement Techniques

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Moss A (2016) Does the Madden-Julian Oscillation modulate stratospheric gravity waves? in Geophysical Research Letters

 
Description In this research project we investigated atmospheric gravity waves in the stratosphere and mesosphere over the South Atlantic and Southern Ocean. In particular, we used radars and satellites to study waves generated by wins blowing over the isolated mountainous island of South Georgia and compared them with the waves generated by the mountains of the Southern Andes/Antarctic peninsula and over the Drake Passage.

This region is particularly interesting because it is recognised that the Global Circulation Models (GCMs) used for numerical weather prediction and climate research are deficient in waves at latitudes near 60S, i.e., the real atmosphere has more waves in it than the GCMs can account for. There is thus a need to actually measure the waves of this region and use them to guide the development of future GCMs.

We used Global Positioning System radio-occultation (GPS-RO) data from the COSMIC satellite constellation to determine the properties of gravity waves in this region of the Earth. We found that there is considerable southward propagation to latitudes near 60S of waves apparently generated over the southern Andes. We propose that this propagation may account for a significant part of the waves missing from the models. Furthermore, there is a long leeward region of increased gravity-wave energy that sweeps eastwards from the mountains, out over the Southern Ocean.

Despite its striking nature, the nature and source of this region of high gravity-wave energy has proved difficult to determine. Our observations suggest that this region includes both waves generated locally and orographic waves advected downwind from the southern Andes mountains and Drake Passage.

We developed a new wavelet-based analysis technique for the quantitative identification of individual waves in COSMIC temperature profiles. This analysis reveals different geographical regimes of wave amplitude and short-timescale variability in the wave field over the Southern Ocean. These results were published as Hindley et al. (2015) and Wright et al. (2015).

However, our results highlighted the limitations of existing 1D or 2D satellite methods for estimating the important momentum fluxes of gravity waves. We therefore developed a second new analysis technique that combines data from two different satellite instruments (NASA's AIRS and MLS) to measure the waves in 3D and so determine their momentum fluxes. We published this technique and illustrated its use in Wright et al. (2016).

Finally, as part of the research we deployed an advanced meteor radar on South Georgia. The radar detects meteors drifting in the upper mesosphere and so can measure the winds at heights of ~ 80 - 100 km, heights where measurements of winds are otherwise exceptionally difficult to make. These radar observations have revealed a complex motion field that includes semi-diurnal atmospheric tides of large amplitude, in addition to variable fluxes of gravity waves and planetary waves.
Exploitation Route We have met with staff from the Met Office who work with the representation of gravity waves in their Unified Model GCM and discussed our results in depth. The improved understanding of gravity waves arising from our results will help constrain future developments of the Unified Model and similar GCMs.
Sectors Environment