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 French Alps, using simultaneous remote sensing and ground truth data at 11 different locations. The snow grain size and soot content were measured from samples collected in the fiel1. The snow reflectance was computed from Landsat Thermatic Mapper (TM) data with and without atmospheric correction. Then the data were compared with theoretical results obtained from a bidirectional reflectance model. Most visible-channel signals are affected by the pollutants in the snow. The near-infrared channel signals are dependent on the grain size. For the near infrared channel TM4, the optical grain size is close to the measured one, but the Landsat-derived snow reflectance is not accurate enough to allow an inversion of the grain size from the reflectance. For the middle infrared channels TM5 and TM7, the computed optical grain size is very different 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 irradianee conditions. Using ratios between channels instead of a single channel gives only a qualitative estimate of the grain size.