DRAGON-WEX: The Drake Passage and Southern Ocean Wave Experiment

Gravity waves are atmospheric waves that can be generated by winds blowing over mountains, storms, unstable jet streams and strong convection. As the waves ascend from their sources in the lower atmosphere, into the stratosphere and mesosphere, they transport momentum in a "momentum flux". When the waves become unstable they "break", rather like ocean surface waves breaking on a beach. This acts to transfer their momentum into the atmosphere, exerting a "drag force" that dramatically influences the global atmospheric circulation.

Computer General Circulation Models (GCMs) used for numerical weather prediction and climate research must represent these waves realistically if they are to predict the behaviour of the real atmosphere.

However, the GCMs display "biases" in which the behaviour they predict does not match that revealed by observations. The largest biases in nearly all GCMs occur in the winter and springtime Antarctic stratosphere. There, they produce a polar region, the "polar vortex", that when compared to observations, is too cold by 5-10 K, has winds that are too strong by about 10 m/s and that persists some 2-3 weeks too long into spring before it breaks up. These significant biases are known as the "cold pole" problem.

It is now realised that the biases arise because the GCMs are missing large amounts of gravity-wave flux that must occur in the real atmosphere at latitudes near 60 degrees S. These latitudes include the stormy Southern Ocean and the Drake Passage. However, the nature, sources, variability and fluxes of these "missing" waves are currently very uncertain.

In DRAGON-WEX (DRake pAssaGe sOuthern oceaN - Wave EXperiment) we will use satellites, radiosondes and radars to directly measure the waves over the Southern Ocean and Drake Passage near 60 S, determine their properties and investigate their role in coupling together the troposphere, stratosphere and mesosphere. Our results will thus help resolve the cold pole problem.

We will apply a very powerful novel 3D method we have developed for analysing satellite data. With our method, we can detect individual gravity waves in the stratosphere in 3D and measure their momentum fluxes. Importantly, because it is a fully 3D method we can do this without the needing the assumptions that critically limit earlier 1D and 2D methods. We will use our method to identify an estimated 100,000 individual gravity waves near 60 S.

We will combine the satellite observations with measurements of gravity waves made by radiosondes ("weather balloons") and radars to characterise the "missing" gravity waves, determining their short-term and seasonal variability and investigate their sources - in particular, the contributions made to the waves by the mountains of the Southern Andes and Antarctic Peninsula, storms over the Southern Ocean/Drake Passage, unstable jet streams and by waves propagating into the 60 S region from latitudes to the North or South.

We will use a unique combination of meteor radars, one in the Antarctic and a new radar on the remote mountainous island of South Georgia to measure the winds, waves and tides of the mesosphere. We will determine the degree to which fluctuations in the waves we measure in the stratosphere drive the variability of the mesosphere and, in particular, the role of waves in driving anomalous events recently observed at heights near 90 km in the polar mesosphere, when the Northward winds of the general circulation appeared to briefly cease and when the occurrence frequency of polar mesospheric clouds was greatly reduced.

We will use meteor radars on the island of South Georgia and at Rothera in the Antarctic to investigate recent suggestions that waves generated by mountains can propagate to heights of 90 km or more - effectively the edge of space.

Finally, in Pathways to Impact we will work closely with the Met Office to use our results to test and improve their Unified Model GCM.

The project will have impact on two main beneficiaries. These are:

1. The Met Office and other operational weather forecasters

We have identified the Met Office and other operational weather forecasters as key potential user of our research. Such organisations will directly benefit from exploiting the knowledge generated. We will focus primarily on the UK Met Office because of our strong existing collaborations and ease of meeting/exchange visits. Here, we and the Met Office jointly explain how they will benefit.

The importance of gravity-wave drag on atmospheric circulation is well known. Errors in its representation in models significantly degrade the skill of weather forecasts and affect the prediction of climate-change effects on the large-scale circulation, with implications for regional-scale predictions of future weather extremes in, for example, wind and precipitation.

However, the parametrization schemes that represent this drag in weather and climate models are poorly constrained. In common with other models, the Met Office Unified Model's parametrizations are tuned with little consideration for the realism of the processes they represent.

Measurements of real gravity-wave properties of the kind which can be used to assess and improve the representation of gravity-wave drag in models are very difficult to obtain. DRAGON-WEX will deliver exactly the kind of measurements which are needed.

A 3D observation-derived dataset of gravity waves will enable the Met Office to assess the model's representation of gravity waves in detail, and so improve the parametrization schemes. For example, the results will enable the Met Office to determine the relative importance of the orographic and non-orographic gravity-wave parametrizations, improve the tuning of the schemes in their operational models, develop the non-orographic scheme to include representation of the different sources, and to determine whether effects such as horizontal propagation should be represented.

Furthermore, as forecasters move towards increasingly high-resolution global models for weather and climate applications, it is clear that the model explicitly resolves a portion of the gravity-wave spectrum. However, it is possible that some of these resolved waves are spurious (e.g., generated by errors in the model numerics or physics) and so it will be extremely valuable to evaluate these resolved waves against the satellite measurements in the DRAGON-WEX project.

THIS WILL HAVE IMMEDIATE IMPLICATIONS FOR THE CURRENT UNIFIED MODEL, BUT PERHAPS MORE IMPORTANTLY THE RESULTS WILL FEED INTO THE DEVELOPMENT OF ITS FUTURE REPLACEMENT OVER THE NEXT 5-10 YEARS (the LFRic project). This future model has a radically different design and potentially very different representation of gravity waves.

The Met Office will thus benefit from the use of our experimental results to improve the representation of gravity waves in the Unified Model and in the development of future models.

We will actively engage with the Met Office throughout all three years of the project and the Researcher Co-I, Dr Wright, will spend one week in each year at the Met Office in Exeter working on delivering the maximum impact from our results. We have identified three particular activities where we will concentrate our effort. These are described in the Pathways to Impact document.


2. The General Public

The public will benefit from the improvements in numerical weather prediction and climate modelling as will organisations that make use of such models in their businesses e.g. insurance industry.

The public will be informed of the exciting nature of our project and kept up to date on its progress by a dedicated website and social media, using the support of the Press Offices and Public Engagement Units of the University of Bath and British Antarctic Survey. The dramatic location of our work will help make it be particularly attractive.