The objective of this proposal is to improve imaging techniques which exploit the interaction of waves with matter to reconstruct the physical properties of an object. To date these techniques have been limited by the tradeoff between resolution and imaging depth. While long wavelengths can penetrate deep into a medium, the classical diffraction limit precludes the possibility of observing subwavelength structures. Over the past twenty years, near-field microscopy has demonstrated that the diffraction limit can be broken by bringing a probing sensor within one wavelength distance from the surface of the object to be imaged. Now, the scope of near field microscopy has been extended to the reconstruction of subwavelength structures from measurements performed in the far-field, this approach having a much wider range of applications since often the structure to be imaged is not directly accessible. The key to subwavelength resolution lies in the physical models employed to describe wave scattering. Conventional imaging methods use the Born approximation which does not take into account the distortion of the probing wave as it travels through the medium to be imaged, so neglecting what is known as multiple scattering. On the other hand, multiple scattering is the key mechanism which can encode subwavelength information in the far-field, thus leading to a potentially unlimited resolution. New experimental evidence has shown that a resolution better than a quarter of the wavelength can be achieved for an object more than 70 wavelengths away from the probing sensors. This preliminary work has established the fundamental principle that subwavelength resolution from far-field measurements is possible as long as the Born approximation is abandoned and more accurate models for the wave-matter interaction are adopted. The aim of this proposal is to pursue this new and exciting idea and to focus on more specific applications such as medical diagnostics and geophysical imaging.

In response to global warming, the ice covers of the Arctic and Antarctic are changing, with a significant reduction in the summer extent of Arctic sea ice. The reduction of Arctic sea ice is more rapid and extreme than climate models predict, suggesting that these models do not adequately represent the processes controlling this reduction. The reduced summer Arctic sea ice cover, and changes to the winter sea ice cover, affect the mechanical and thermodynamic coupling between the air and ocean. In fact, observations show that the sea ice cover has become more mobile in the last 15 years and that there has been an increase in the mean ocean circulation beneath the sea ice. Since, over the same period, there has not been an observed increase in wind strength, this suggests that changes to the sea ice cover itself are responsible for an enhanced ice motion and transfer of wind stress to the ocean beneath sea ice. Our project hypothesis is: Changes in the Arctic sea ice cover have resulted in a more efficient transfer of momentum between the air and ocean, resulting in spin up of sea ice and the Arctic Ocean. We will test this hypothesis with a combination of new data, theory and numerical modelling. We will investigate how changes in the roughness of the ice cover, e.g. through a more dilute ice cover having more floe edges exposed, change the drag forces exerted by the air on the ice and the ice on the ocean. We will investigate how a reduction in the ice cover may reduce the resistance of the ice cover to the wind, allowing it to move more easily. In particular we address the question: to what extent is acceleration of the Arctic sea ice gyre the result of decreased ice forces versus increased drag? We will use climate models containing new physics calibrated with, and derived from, new observations, to examine the prediction that: Changes in the sea ice cover will continue to lead to enhanced momentum transfer between the air and ocean, resulting in a more mobile and responsive ice cover and enhanced flow and mixing in the Arctic Ocean. Although we focus our analysis on the Arctic Ocean, where sea ice changes have been more dramatic, we will also examine air-ice-ocean momentum exchanges in the Southern Ocean. This proposal brings together leading researchers in sea ice dynamics, remote sensing, ocean and climate modelling, and builds upon existing expertise in satellite observation, theory, and modelling of sea ice in the Centre for Polar Observation and Modelling. In addition to the scientific outcomes, the proposed work will result in new sea ice drag physics being incorporated into a sea ice climate model and delivered to climate modelling groups. This will directly help scientists investigating and predicting future changes to the sea ice cover in the Arctic and Southern Oceans and also help scientists trying to understand and predict changes in the global climate system.

The global ocean is populated by a vigorous and dynamic eddy field. This eddy field has a significant impact on the wider structure of the ocean circulation, but it is computationally extremely expensive to resolve directly. While recent simulations have allowed for direct eddy resolving calculations to be performed, equilibrated eddy resolving calculations are unlikely to become routine at any point in the near future. There therefore remains an urgent need for eddy parameterisation schemes, which aim to capture the aggregated effects of the eddy field in coarse resolution numerical models. The development of ocean eddy closures remains a formidable challenge. However, one can apply fundamental principles in order to rule out eddy closures which lead to unphysical behaviour. For example, the eddies require energy in order to mix ocean properties, and must observe the principle of conservation of momentum. A somewhat more subtle property is that ocean eddies must, on average, mix out gradients in the fluid potential vorticity. Eddy parameterisations which fail to observe these constraints can be ruled out as a possible solution for the underlying eddy dynamics. A particularly important challenge for ocean eddy parameterisations is the accurate reproduction of ocean sensitivities to forcing changes. These responses have implications for the long term ocean response to climate change. Recent studies indicate that the Southern Ocean may exhibit reduced responses to wind forcing changes when the influence of the eddies is well sufficiently resolved. Coarse resolution models, which parameterise the eddies and do not resolve them directly, often give wildly incorrect predictions for the ocean responses. This study proposes the implementation of a suite of new and existing ocean eddy parameterisations. The parameterisations are to be implemented in a simplified context (in a "quasi-geostrophic" model), and then in a full ocean circulation model. This research has three key novel aspects. First, optimisation techniques will be employed in order to assess the performance of the closures in the simplified quasi-geostrophic case. These techniques can be used to identify the best possible configuration of a given eddy parameterisation scheme, enabling the best-case performance of the scheme to be rigorously identified. This will provide a robust comparison of a broad range of possible approaches for eddy parameterisation. Secondly, the research aims to impose the three physical principles, imposed by energetic constraints, momentum conservation, and the need to mix potential vorticity, simultaneously. Thirdly, the research will investigate the performance of a broad range of parameterisations in determining the ocean sensitivity to wind forcing changes. Particularly important questions to be addressed are: How important are the fundamental physical constraints in controlling the ocean eddy dynamics? Can the restoration of these constraints in ocean eddy parameterisation schemes lead to improved coarse resolution simulations?

Summary of research for a general audience One of the most exciting frontiers of science is the study of phenomena that take place on the timescale of attoseconds (as, 10^-18 s). To imagine such an incredibly short period of time, consider that light travels from here to the moon in one second, but only travels 0.0003mm in one femtosecond (fs, 10^-15 s). To put it in context, that is about 1/300th the width of a human hair in 10^-15 s (or 1000 as). Attoseconds are the timescales on which atomic processes/transitions occur - for example, an electron circles the hydrogen atom in ~24 as (the so called 'atomic unit of time'). To investigate, and in future control, the dynamics of such ultrafast processes measurement tools of unprecedented quality and precision are required - pulses of light with attosecond duration. This is much shorter than a single cycle of any visible light wave (violet~1.3fs, red~2.5fs), requiring instead extreme-ultraviolet (XUV)/ X-ray radiation to be controlled with clinical accuracy to achieve ultrashort durations. However, the pay-off for this effort is substantial; researchers can investigate the microcosm with a degree of spatial clarity and on shorter time scales than previously possible, thus allowing them to see events that are ordinarily 'blurred' using conventional XUV/X-ray sources such as synchrotrons. Attosecond pulses must be synthesized using wavelengths shorter than those in the visible region of the spectrum and therein lies a significant problem - wavelengths shorter than the visible spectrum i.e. ultraviolet and X-rays, are strongly absorbed in most materials. It is therefore impossible to build an attosecond laser using conventional laser building techniques. Instead next generation methods are required. Two principle media are currently being studied at laser laboratories around the world - intense laser-gas interactions and relativistic laser plasmas formed using solid density targets - for the production of attosecond pulses. In the proposed research we focus on the second medium - relativistic laser plasma. The underlying mechanism under investigation for the generation of intense attosecond pulses is the production of relativistic electron nanobunches during high power optical laser interactions with ultrathin carbon foils. This novel concept is based on our recent work showing that dense bunches of electrons with sub 10nm scale (nm = nanometer = 10^-9m) can be formed and rapidly accelerated on the front surface by the relativistically intense driving laser field and subsequently emerge from the rear surface of ultrathin carbon foils. Two resulting mechanisms will be studied in detail in this research - Coherent Synchrotron Emission (CSE) and Relativistic Electron Mirrors (REM). Only recently demonstrated, CSE and REM offer a novel window onto the relativistic laser plasma interaction and our work will not only reveal the microscopic dynamics of these mechanisms but also show a direct path to the generation of bright attosecond pulses.