Effect of Snow Grain Shape on Snow Albedo

Abstract:

Radiative transfer models of snow albedo have usually assumed spherical snow grains, using Mie theory to compute single-scattering quantities. The scattering by realistic nonspherical snow grains is less in the forward direction and more to the sides, resulting in a smaller asymmetry factor g (the mean cosine of the scattering angle). Compared to a snowpack of spherical grains with the same area-to-mass ratio, a snowpack of nonspherical grains will have a higher albedo, thin snowpacks of nonspherical grains will more effectively hide the underlying surface, and light-absorbing impurities in the snowpack will be exposed to less sunlight. These effects are examined here for a variety of snow grain shapes, with aspect ratio from 0.1 to 10. The albedo of opaque snowpack with equidimensional non-spherical snow grains (aspect ratio 1) is higher than that with spherical snow grains by 0.036 and 0.05, for grain radius 100 mm and 1000 mm; respectively. For snow grain radius of 100 mm, the albedo reduction caused by 100 ppb black carbon is 0.02 for spherical snow grains, but only 0.01 for equidimensional non-spherical snow grains.