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VEGETATION - 2000

Lake Maggiore - Italy, 3-6 April 2000


Antarctic snow characteristics from POLDER and VEGETATION data

Michel FILY, Olivier MANSE, Jean-Pierre BENOIST
Laboratoire de Glaciologie et Géophysique de l'Environnement, CNRS-UJF,
Grenoble, France
fily@glaciog.ujf-grenoble.fr

Paper (pdf file, 346 k)

The energy balance at the surface of the ice sheets is mainly dependent on the radiative fluxes. During daylight the albedo of snow is one of the main parameters which control these fluxes. As the albedo is high (about 0.8) a small difference can change drastically the amount of absorbed energy and then the surface temperature. Therefore a good parameterization of the snow albedo is necessary when modelling the polar climate over the ice sheets. The snow albedo depends on its pollution in the visible part of the spectrum and on the grain characteristics (size, shape) in the short wave infrared (SWIR). The sun incidence angles and the surface roughness also modify the snow albedo. For radiative effects, the snow can be considered as clean on the ice sheets. Good parameterization of the grain size and of of the sun incidence angle effects are found in the litterature but, so far, there is very poor knowledge about the surface grain size and about the roughness of the ice sheets at a global scale.

The objective of our investigation is to derive the snow grain size from the Vegetation SWIR (1.6 µm) channel which is very sensitive to this parameter. For this purpose Vegetation data were acquired above Antarctica during the 98-99 and 99-00 austral summers. The cloud selection is based on the facts that the grain size is usually smaller in clouds than in snow and that the cloud surface is often rougher than the snow one. From POLDER data it was found that the reflectance obtained at large angle are affected by the surface roughness and only view angles less than 35° are used. The results of a radiative transfer model is used to derive the grain size from the reflectance and finally the albedo is computed.