Effects of snow physical parameters on spectral albedo and bidirectional reflectance of snow surface

Observations of spectral albedo and bidirectional reflectance in the wavele ngth region of lambda = 0.35-2.5 mu m were made together with snow pit work on a flat snowfield in eastern Hokkaido, Japan. The effects of snow impuri ties, density, layer structure, and grain size attained by in situ and labo ratory measurements were taken into account in snow models for which spectr al albedos were calculated using a multiple-scattering model for the atmosp here-snow system. Comparisons of these theoretical albedos with measured on es suggest that the snow impurities were concentrated at the snow surface b y dry fallout of atmospheric aerosols. The optically equivalent snow grain size was found to be of the order of a branch width of dendrites or of a di mension of narrower portion of broken crystals. This size was smaller than both the mean grain size and the effective grain size obtained from microgr aphs by image processing. The observational results for the. bidirectional reflection distribution function (BRDF) normalized by the radiance at the n adir showed that the anisotropic reflection was very significant in the nea r-infrared region, especially for lambda > 1.4 mu m, while the visible norm alized BRDF (NBRDF) patterns were relatively flat. Comparison of this resul t with two kinds of theoretical NBRDFs, where one having been calculated us ing single-scattering parameters by Mie theory and the other using the same parameters except for Henyey-Greenstein (HG) phase function obtained from the same asymmetry factor as in the Mie theory, showed that the observed NB RDF agreed with the theoretical one using the HG phase function rather than with that using the Mie phase function, while the albedos calculated with both phase functions agreed well with each other.