COMPARISON OF IN-SITU AND LANDSAT THEMATIC MAPPER DERIVED SNOW GRAIN CHARACTERISTICS IN THE ALPS

Two experiments were performed in April and December 1992 in the Frenc h Alps, using simultaneous remote sensing and ground truth data at 11 different locations. The snow grain size and soot content were measure d from samples collected in the field. The snow reflectance tons compu ted from Landsat Thermatic Mapper (TM) data with and without atmospher ic correction. Then the data were compared with theoretical results ob tained from a bidirectional reflectance model. Most visible-channel si gnals are affected by the pollutants in the snow. The near-infrared ch annel signals are dependent on the grain size. For the near infrared c hannel TM4, the optical grain size is close to the measured one, but t he Landsat-derived snow reflectance is not accurate enough to allow an inversion of the grain size from the reflectance. For the middle infr ared channels TM5 and TM7, the computed optical grain size is very dif ferent from the measured one, but a linear relation was found between those different sizes for April and December data in spite of the very different solar irradiance conditions. Using ratios between channels instead of a single channel gives only a qualitative estimate of the g rain size. (C)Elsevier Science Inc., 1997.