Abstract: Megadune fields occupy large areas in the interior of the East Antarctic ice sheet and are the result of unusual snow accumulation and redistribution processes. They therefore are important to surface mass balance and ice core interpretation. Field observations (GPS, GPR, and surface measurements) have provided a detailed description of megadune sedimentation and morphology over a 70 km2 area, located 200 km east of Dome C. A combination of remote sensing analysis (using Landsat and satellite radar altimetry) and field measurements indicate that slope in the prevailing wind direction (SPWD) and climatic conditions play a crucial role in megadune genesis. The megadune areas tend to be characterized by slightly steeper regional slope and the presence of highly persistent katabatic winds. The megadunes represent 2 to 4 m amplitude waves of 2 to 5 km wavelength formed by variable net accumulation, ranging between 25% (leeward faces) to 120% (windward faces) of the accumulation in adjacent nonmegadune areas. Leeward faces are characterized by glazed, sastrugi-free surfaces and extensive depth hoar formation. Windward faces are covered by large rough sastrugi up to 1.5 m in height.

Abstract: The surface area of snow that is accessible to gases is an essential parameter for quantifying the exchange of trace gases between the snowpack and the atmosphere and is called the specific surface area (SSA). Snow SSA decreases during metamorphism, but this is not described in current snow models owing to the complexity of the physics and geometry of snow. In this paper, we test whether it is possible to model snow SSA changes during isothermal metamorphism without accounting for all the complexity of the three-dimensional (3-D) structure of real snow. We have developed a mean field model of snow metamorphism under isothermal conditions, grounded in the theoretical framework of transient Ostwald ripening and representing snow as a distribution of spherical particles. Analytical expressions of the growth rates of these spheres are obtained, and the evolution of two measurable parameters that characterize snow geometry, the SSA and the distribution of radii of curvature (DRC), are simulated and compared to experimental data obtained by X-ray tomography. The qualitative effects of temperature, snow density, and the condensation coefficient on the rate of SSA decrease are examined. The model predicts very well the rate of evolution of the particle size distribution, which validates our physical description of isothermal metamorphism. In particular, we find that vapor phase diffusion is rate limiting. However, the calculation of the SSA from the DRC appears delicate and evidences too crude approximations in our description of the 3-D geometry of snow. Finally, it is stressed that the initial DRC can greatly influence the rate of SSA decrease, while experimental measurements of the rate of SSA decrease suggest that all snow types evolve in a similar way. It is thus proposed that most natural fresh snows have similar DRCs.

Abstract: A new scheme is proposed for the inversion of surface waves using a continuous formulation of the inverse problem and the least squares criterion. Like some earlier schemes a Gaussian a priori covariance function controls the horizontal degree of smoothing in the inverted model, which minimizes some artifacts observed with spherical harmonic parameterizations. Unlike earlier schemes the new approach incorporates some sophisticated geometrical algorithms which dramatically increase computational efficiency and render possible the inversion of several tens of thousands of seismograms in few hours on a typical workstation. The new algorithm is also highly suited to parallelization which makes practical the inversion of data sets with more than 50,000 ray paths. The constraint on structural and anisotropic parameters is assessed using a new geometric approach based on Voronoi diagrams, polygonal cells covering the Earth's surface. The size of the Voronoi cells is used to give an indication of the length scale of the structures that can be resolved, while their shape provides information on the variation of azimuthal resolution. The efficiency of the scheme is illustrated with realistic uneven ray path configurations. A preliminary global tomographic model has been built for SV wave heterogeneities and azimuthal variations through the inversion of 24,124 fundamental and higher-mode Rayleigh waveforms. Our results suggest that the use of relatively short paths (<10,000 km) in a global inversion should minimize multipathing, or focusing/defocusing effects and provide lateral resolution of a few hundred kilometers across the globe.