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 and 1000 μm, respectively. For an effective radius of 100 μm, the albedo reduction caused by 100 ng g−1 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.

Impacts of absorbing aerosol deposition on snowpack and hydrologic cycle in the Rocky Mountain region based on variable-resolution CESM (VR-CESM) simulations

“…Note that the SNICAR model we use assumes spherical snow grains and aerosolsnow external mixing for the calculation of snowpack optical properties (Flanner et al, 2007;Oleson et al, 2010). Recent studies have shown that non-spherical snow grains play a critical role in snow albedo calculations and decrease the snow albedo reductions included by LAAs compared with spherical snow grains (e.g., Liou et al, 2014;Dang et al, 2016;He et al, 2014He et al, , 2017. Nonetheless, the knowledge of snow grain shape evolution is limited and thus spherical snow grains are assumed.…”

Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model

“…The derived scaling factor depends mainly on the aspect ratio (AR) of the non-spherical grains, varying between approximately 1.2 and 2.4. The largest values occur for equidimensional snow grains (AR ≈ 1; Dang et al, 2016, their Fig. 9), because g is smallest for these (for example, at λ = 0.5 µm, g ≈ 0.74 compared with g ≈ 0.89 for spheres).…”

“…The use of the OHC assumption with r e multiplied by roughly 1.7 led to results that are almost indistinguishable from those obtained with the spherical shape assumption. Recently, Dang et al (2016) demonstrated that the albedo of a snowpack consisting of non-spherical snow grains can be mimicked by using smaller grains of spherical shape. They derived the scaling factor between the size of non-spherical and spherical snow grains with the same albedo by using, in the non-spherical case, the parameterization of Fu (2007) for the asymmetry factor, which assumes hexagonal ice crystals with rough surfaces.…”

“…Liou et al (2014) suggested the use of hexagonal plates/columns and Koch snowflakes for new snow and spheroids for old snow. Dang et al (2016) evaluated the effects of snow grain non-sphericity on snow albedo using a parameterization of ice crystal asymmetry parameter developed for hexagonal prisms with rough surfaces (Fu, 2007). Räisänen et al (2015) developed a parameterization for the SSPs of non-spherical snow grains (the singlescattering co-albedo β, the asymmetry parameter g and the phase function P 11 ) based on a combination of three habits, a so-called optimized habit combination (OHC): severely rough (SR) droxtals, aggregates of SR plates, and strongly distorted Koch fractals.…”

“…determined based on albedo rather than microphysical measurements). Recently, Dang et al (2016) demonstrated that the spectral albedo of a snowpack consisting of non-spherical snow grains can indeed be mimicked by using smaller grains with spherical shape. They found that the scaling factor between the effective radius of non-spherical and spherical grains producing the same albedo depends on the aspect ratio of the nonspherical grains, peaking at 2.4 for equidimensional snow grains.…”