Seasonal study of directional reflectance properties of snow

We present an analysis of the hemispherical-directional reflectance factor (HDRF) of snow, using 16 seasonal datasets of the spectral range from 400 to 2,500 nm. The data was measured under clear sky conditions in Davos Dorf (Grisons, Switzerland, 1,560 m a. s. l.). Fieldwork was carried out on seven days between February 5 and March 30 2004 with the Swiss Field Goniometer System (FIGOS). In addition to the HDRF measurements, snow stratigraphy, temperature and density were measured, and chemical and photomicroscopical analyses of snow samples were performed. Concentration of organic and elemental carbon was determined by chemical analysis. The grain size analyses through image processing of micrographs revealed relatively small differences of 0.21 to 0.33 mm mean radius in the top layers of snow cover. Seven datasets present HDRF of wet snow surfaces with similar anisotropy at smaller sun zenith angles (qI = 3.3 to 64.5°) compared to the nine surfaces measured at larger sun zenith angles (qI = 6.5 to 75.3°). Spectral albedo is relatively constant throughout datasets of different sun zenith angles of the same day, but has large variability among measurements of different days. With increasing wavelength, it decreases significantly faster for wet surfaces than for dry surfaces. The forward scattering peak was found to strengthen with increasing sun zenith angle and with increasing wavelength for both wet and dry surfaces at wavelengths above 700 nm. Finally, a spectral wet snow determination method is performed and the cross-sensitivity to HDRF variation could be derived. The best differentiability was found for 1,380 nm. This basis work increases the understanding of snow signatures for potential imaging spectroscopy applications in alpine regions.