Effect of snow grain shape on snow albedo

Abstract

Radiative transfer models of snow albedo have usually assumed a spherical shape for the snow grains, using Mie theory to compute single-scattering properties. The scattering by more 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 particles in the snowpack will be exposed to less sunlight. These effects are examined here for nonspherical snow grains with aspect ratios from 0.1 to 10. The albedo of an opaque snowpack with equidimensional (i.e. aspect ratio 1) nonspherical snow grains is higher than that with spherical snow grains by 0.032 and 0.050, for effective grain radii of 100 μm and 1000 μm, respectively. For an effective radius of 100 μm, the albedo reduction caused by 100 ng/g of black carbon is 0.019 for spherical snow grains, but only 0.012 for equidimensional nonspherical snow grains. The albedo of a snowpack consisting of nonspherical snow grains can be mimicked by using a smaller grain of spherical shape; this is why radiative-transfer models using spherical grains were able to match measurements of spectral albedo. The scaling factor for snow grain radius is different for nonspherical grains with different aspect ratios, and is about 2.4 for equidimensional snow grains.