The Antarctic ice sheet is the largest on the planet by a factor 10. It holds enough ice to raise global sea level by ~65 m. Small changes in the balance between losses and gains (the mass balance) can have, therefore, profound implications for sea level, ocean circulation and our understanding of the stability of the ice mass. Local variations in mass balance may be driven by short or long term changes in ice dynamics that may or may not be related to recent climatic change. They may also be due to trends in snowfall. There is now a general consensus that the ice sheet is losing mass but the range of estimates and uncertainties are still, in most cases, larger than the signal. To solve the open question of what the time evolving mass change is, we propose combining satellite observations, climate modelling and physical constraints to solve for the independent and uncorrelated errors that have hampered previous approaches. Sea level rise (SLR) since 1992 has averaged around 3.2 mm/yr, ~ twice the mean for the 20th Century. The cause is uncertain, but it is clear that a significant component is due to increased losses from both Greenland and Antarctica. Recent advances in regional climate modelling and analysis of gravity anomalies from the GRACE satellites have greatly improved our knowledge of both the magnitude and origin of mass losses from Greenland. Unfortunately, this is not the case for Antarctica for a range of reasons. The aim of this project is to address this shortcoming using a similar, but more comprehensive, approach to the one we used to improve our understanding of changes in Greenland. To do this, we must employ additional data and methods because i) the uncertainty in post glacial rebound for the West Antarctic Ice Sheet , in particular, is of a similar magnitude to the signal (unlike Greenland), ii) errors in observed and modelled variables are generally larger because of the paucity of in-situ data sets in, and around, Antarctica, and iii) observations in time and space are poorer for most of the ice sheet and, in particular, the areas showing the greatest change.

The central Sahara has one of the most extreme climates on Earth. During the northern summer months, a large low pressure system caused by intense sunshine develops over a huge, largely uninhabited expanse of northern Mali, southern Algeria and eastern Mauritania. Temperatures in the high 40s are normal and uplift of dry air through more than 6000m of the atmosphere is routine in what is thought to be the deepest such layer on the planet. This large zone is also where the thickest layer of dust anywhere in the Earth's atmosphere is to be found. Although the central Sahara is extremely remote, it turns out to be vitally important to the world's weather and climate. The large low pressure system drives the West African Monsoon and the dry, dusty air layers are closely related to the tropical cyclones which form over the Atlantic Ocean. Likewise, the dusty air has a strong influence on the way the atmosphere is heated, a process which is poorly understood. It is not surprising that the models we use to predict weather and climate and which are a crucial tool for understanding how the atmosphere works, all have problems in dealing with the central Sahara. Insights into how the climate system works, improving the models and therefore the predictions have all been held back in the case of the Sahara by a lack of measurements of the atmosphere and the processes that make dust and extreme weather. This will always be the case until a team goes to the central Sahara and makes these measurements. A key part of this proposal aims to do just that. We want to set up an array of special instruments, at the surface in two carefully chosen places in the central Sahara, which will monitor the winds, temperatures, dust and so on for an entire year. We will add to this collection for a shorter period of even more intense measurements during the core summer month of June. We plan also to fly a instruments attached to an aeroplane overhead the surface array and across the desert so that we can get an idea of the structure of the atmosphere and how it changes through the day. To find out how dust storms work, we will leave 10 weather stations at places where we think dust storms happen frequently. Satellites play an essential role in measuring weather and climate and are especially useful in remote places. The best available information from satellites will help to quantify how weather and climate works in the Sahara. We also expect to improve the way the satellites are able to make their measurements too. Because models are so important to understanding and predicting weather, we will make heavy use of them in this work. We want to know how well the models work over the Sahara and what can be done to improve them. We are especially interested in seeing whether the models work better if we allow them to deal with small parts of the climate system or whether we can still represent extreme places in the Sahara by ignoring these details in the models.

Small particles (known as aerosol) in the atmosphere play several critical roles. They affect the transmission of sunlight to the underlying surface; they affect the formation of clouds, and they host and enhance important chemical reactions. When they are deposited on ice they leave a record of past conditions that can be accessed by drilling ice cores. The most significant aerosol component over marine areas is sea salt aerosol. Over most of the world's oceans this is created by bubble bursting in sea spray. However there is strong evidence that another source of sea salt aerosol is important in the polar regions, and that this ultimately derives from the surface of sea ice. The existence of this source forms the basis for a proposed method using ice core data for determining changes in sea ice extent over long time periods. Additionally sea salt aerosol, along with salty sea ice surfaces, is the host for the production of halogen compounds which seem to play a key role in the oxidation chemistry of the polar regions. It is therefore important to understand the sources of polar sea salt aerosol and therefore to be able to predict how they may vary with, and feedback to, climate. It was recently proposed that the main source of this polar sea salt aerosol was the sublimation of salty blowing snow. The idea is that snow on sea ice has a significant salinity. When this salty snow is mobilised into blowing snow, sublimation from the (top of) the blowing snow layer will allow the formation of sea salt aerosol above the blowing snow layer, that can remain airborne after the blowing snow has ceased. First calculations suggested that this would provide a strong source of aerosol (greater than that from open ocean processes over an equivalent area). It was proposed that this would have a strong influence on polar halogen chemistry and a noticeable influence on halogens at lower latitudes. However, this was based on estimates of the relevant parameters as there were no data about aerosol production from this source, and almost no data about blowing snow over sea ice in general. Here we propose to take advantage of a very rare opportunity to penetrate the Antarctic sea ice zone during winter, as we have been allocated spaces on an unusual winter cruise into the sea ice zone on the German icebreaker Polarstern. During this cruise, we will be able to confirm that the blowing snow sea ice source exists, and make measurements that will provide a soundly-based parameterisation of the source. This will be done by making measurements of the snow on sea ice, of the blowing snow itself, and of aerosol above the blowing snow, as well as before and after such episodes. Measurements will include salinity, chemistry (looking at the amount of bromine present in each medium), and for blowing snow and aerosol, the amounts and size distributions. By combining our data with meteorological data, and by comparing them to satellite observations that have recently attempted to identify blowing snow episodes, we will be able to make estimates of the spatial and temporal distribution of sea salt aerosol from this source over the entire Antarctic sea ice zone. This will allow us to assess the importance of this source of sea salt (and of halogens) compared to others that have been proposed. We will then use existing models to assess how important such a source is to sea salt deposition in Antarctica, allowing us to determine how sea salt in ice cores is related to sea ice extent. This opens the possibility of turning a qualitative sea ice proxy into a quantitative one. Models will also be used to re-assess the importance of this source for halogen chemistry in the polar regions and globally. In summary this proposal will provide the first targeted measurements of the parameters needed to assess the importance of blowing snow sublimation as a source of sea salt, and to quantify its most relevant impacts.

Recent satellite observations of the Antarctic ice sheet show dramatic changes over the last decade or so. Two main types of change are seen. The first happens near the coast of the Amundsen Sea and affects several ice streams in the area, such as Pine Island and Thwaites Glaciers. Ice streams are rivers of fast-flowing (up to 1 km/yr) ice that are approximately 40 km wide and several hundred kilometres long, they are separated from the neighbouring slow-flowing (typically 10 m/yr) ice by abrupt shear margins. In these ice streams, the ice appears to be thinning at the rate of several metres per year. The other type of change is found deeper inland on the Siple Coast where one ice stream is thickenning and others show signs of lateral migration. Other evidence (such as buried crevasses) suggest that the flow of the ice streams in this area is very erratic and prone to the occasional shutdown. Air temperatures are so cold in Antarctica that there is very little surface melt and so changes in ice thickness are most likely caused by changes in the horizontal flow of ice, which can lead to thicker ice if the flow slows, or to thinning ice if it accelerates. Researchers believe that the first of the two observations highlighted above may be caused by warming ocean waters around Antarctica. This leads to increased melt from the underside of floating ice shelves, which therefore thin and tend (through buoyancy) to float more. This, in turn, reduces the amount of friction these ice masses experience as they flow over peaks and troughs in the subglacial topography. The net effect is that the ice shelves and their upstream ice streams accelerate and therefore thin. This type of process has been taken as an indicator of contemporary climate change. Until we know the cause of the oceanic warming (if it indeed exists), we will not be able to attribute this thinning to natural or anthropogenic causes. The strange behaviour of the ice streams along the Siple Coast is not thought to happen because of changes in the oceans. This is because the coast in this area is protected by the huge Ross ice shelf and water temperatures in the area are extremely cold. The observations of change in this area could be a reflection of the internal variability of ice flow and the analogy to 'weather' is often drawn. Ice streams are thought to be inherently unstable and prone to surges and periods of stagnation, like their smaller counterparts the valley glaciers. This behaviour may be caused by changes in the flow of water under the ice streams, which affects ice-steam flow because it lubricates any sediments at the base of the ice. Changes in water flow can therefore cause an ice stream to experience more friction and to stagnate. Both of the types of change that have been observed are therefore associated with the dynamics of ice streams. In this project, we want to understand this behaviour by constructing a numerical model of the ice sheet which has sufiiciently fine resolution to capture the the shapes of individual ice streams and ice shelves. This means that we will need to develop a method of doing calculations on a coarse grid for the whole of the ice sheet and on nested, finer grids for individual ice streams and shelves. In order to capture the behaviour described above, we will also have to develop models of new processes such as the transmission of stresses through an ice mass, the flow of water at its base and the interaction between this water and the softness of the underlying sediments. We will also have to integrate satellite observations of the ice sheet to produce an accurate model of its present-day flow. Once complete, the model will be used to assess the longer-term effects of changing ocean temperatures on the ice sheet. It will ultimately provide a tool to help us predict what Antarctica's contribution to future global sea level will be.

The volcanic plume from the Eyjafjallajökull eruption has caused significant disruption to air transport across Europe. The regulatory response, ensuring aviation safety, depends on dispersion models. The accuracy of the dispersion predictions depend on the intensity of the eruption, on the model representation of the plume dynamics and the physical properties of the ash and gases in the plume. Better characterisation of these processes and properties will require improved understanding of the near-source plume region. This project will bring to bear observations and modelling in order to achieve more accurate and validated dispersion predictions. The investigation will seek to integrate the volcanological and atmospheric science methods in order to initiate a complete system model of the near-field atmospheric processes. This study will integrate new modelling and insights into the dynamics of the volcanic plume and its gravitational equilibration in the stratified atmosphere, effects of meteorological conditions, physical and chemical behaviour of ash particles and gases, physical and chemical in situ measurements, ground-based remote sensing and satellite remote sensing of the plume with very high resolution numerical computational modelling. When integrated with characterisations of the emissions themselves, the research will lead to enhanced predictive capability. The Eyjafjallajökull eruption has now paused. However, all three previous historical eruptions of Eyjafjallajökull were followed by eruptions of the much larger Katla volcano. At least two other volcanic systems in Iceland are 'primed' ready to erupt. This project will ensure that the science and organisational lessons learned from the April/May 2010 response to Eyjafjallajökull are translated fully into preparedness for a further eruption of any other volcano over the coming years. Overall, the project will (a) complete the analysis of atmospheric data from the April/May eruption, (b) prepare for future observations and forecasting and (c) make additional observations if there is another eruption during within the forthcoming few years.