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Home > News > Ph.D. Thesis > Ph.D. Thesis 2014

Mercredi 26 novembre 2014, soutenance de thèse de Hélène BARRAL - 14h00, Salle Louis Lliboutry, LGGE, Campus

Couches limites atmosphériques en Antarctique

Thesis supervisor

- M Christophe Brun (LEGI),
Associate Professor, UJF, Co-supervisor
- M Christophe Genthon (LGGE),
Directeur de recherche, CNRS, Co-supervisor

Abstract

Except during a few summer afternoon hours, the snow-covered surface of Antarctica is constantly cooling because of radiative processes. This results in a stable, persisting stratification of the atmospheric boundary layer that feeds katabatic winds along the slopes descending from the Plateau to the Ocean. Temperature inversions and wind speeds both peak during the winter, with inversions
regularly reaching 25 degrees (C) over the Plateau and winds exceeding 200 km/h along the coast. In the summer, significant inversions remain at night but solar heating leads to the formation of convective layers near the surface in the afternoon. With berms and large, empty slopes constantly
covered with snow, Antarctica is a unique and perfect laboratory for the study of transitions between turbulent regimes and of the turbulence within stable and katabatic boundary layers. The investigation of these processes is usually made dificult by their sensitivity to heterogeneities at the surface.

This thesis work documents three typical "text-book" summer cases: the diurnal cycle on the Antarctic Plateau, the generation of a local katabatic wind and the katabatic forcing of the boundary layer. The investigation of these three cases uses in-situ data. For two of these cases, the observational data has fed and been completed with some Meso-NH model simulation outputs.

The first case focusses on the diurnal cycle at Dome C. On the Antarctic Plateau, Dome C is a flat, homogeneous area far from oceanic perturbations. Since a few years, a 45 meters tower samples the boundary layer there. In the summer, the diurnal cycle there is characterized by clean signals in both temperature and winds, with a nocturnal low-level jet within the boundary layer. A two-days data set representative of the rest of the summer has been selected for analysis and is used in the GABLS4 comparison study prepared in collaboration with Meteo France. Single-column simulations have been run for this comparison work launched in June.

The second case examines a local katabatic flow generated at sunset over a 600 by 300 meters slope in Terre Adelie. Characteristics of the turbulence of this flow, in particular, its anisotropy, are investigated using small-scale model simulations. A measuring station has been deployed in order to prepare and evaluate these simulations.

The third case is concerned with boundary layers typical of coastal areas with strong winds of katabatic origins, which have flown over 1000 km-long slopes towards the sea. By moving around the snow at the surface, these winds interact with turbulent mixing processes. For this final case, the work is interested in the impact of blowing snow on atmospheric moisture and with the calculation of turbulent fluxes based on temperature, wind and humidity profiles.

Keywords

Antarctica, stable atmospheric boundary layer, katabatic wind, turbulent mixing, numerical simulation, observation