Abstract:
The spatial distribution of surface snow grain size over the Antarctic snow cover has been determined from analysis of near-thermal-infra-red data acquired with the ATSR-2 instrument on the European Space Agency's ERS-2 satellite. Scattering from a snow surface in the short wave infra-red part of the spectrum (0.9 to 3.5 micron) is strongly dependent on grain size, and to a lesser extent on
... shape. A relation between snow surface reflectance, illumination incidence-angle, and grain size, has been established using a model of BRDF [Bi-Directional Reflectance Distribution Function] for snow and laboratory measurements of the BRDF behaviour of snow. This relation is used to invert the derived values of reflectance at Top-Of-Atmosphere to grain size. TOA reflectance values are derived from satellite observations of radiance in two channels with wave-band centred at 0.87 and 1.6 micron. A ratio of the two channels is used to correct for the effect of local surface slope variation on the apparent reflectance. Grain size is determined at a regular spacing in a grid of cells, each 16 km x 16 km. The ATSR-2 instrument provides observations on a 1 km pixel, so that 256 observations are accumulated to improve signal/ratio response. Calculations are made on a per orbit basis. Cloud affected cells are detected by two methods: a variance test within the 16 km cell; and a minimum grain size criterion. Very small grain size (less than 25 micron) occurs almost exclusively in clouds. A maximum solar incidence angle of 75 degrees is imposed and relatively small viewing incidence angles are used in order to minimise possible errors that could be introduced by the BRDF effect of the snow surface and its roughness.
Approximately 5400 scenes of ATSR-2 data (512 km x 512 km) were analysed. These data span the time interval from 16 November 1999 to 26 January 2000. The results are presented as a spatial distribution of grain size values on the regular array of 16 km cells. They include mean values accumulated over all orbits for the 1999-2000 summer season, together with standard deviation, and density of observations contributing to the mean. Time series of grain size values for each cell can also be extracted from the data set for the individual orbits. For the 1999-2000 austral summer season, incidence of cloud cover was very high in West Antarctica, and to a lesser extent around the near-coastal margin. Cloud cover over much of East Antarctica was low.
The data are currently in an internal format. The simple average grain size map can be provided as a shapefile or other suitable format. However, there is a considerable complexity to the underlying data set so that a format needs to be selected to suit the application if/when a request for the data are made.
This metadata record has been derived from work performed under the auspices of ASAC project 2200 (ASAC_2200). 
