Universality in azimuthal asymmetry of anisotropic sea-ice reflectance

We study the azimuthal asymmetry of the spectral bidirectional reflectance distribution function (BRDF) of an optically thick sea-ice layer. From radiative transfer modeling, we find that the azimuth-dependent part of the BRDF ceases to change rather rapidly with an increase in the optical thickness of the layer and tends to a nearly universal angular dependence, insensitive to the specific law of single scattering in the ice. The effect of the scattering phase function manifests itself only in the value of a pre-factor in the azimuth-dependent part of the BRDF. The universality stems from the refractive index mismatch at the air/ice interface and should occur for any natural ice containing large (compared to the wavelength) inclusions. The specified part of the BRDF is governed by sub-diffusive radiation transport and, at large illumination zenith angles, can make a noticeable contribution to the spectral albedo of the layer in the visible range.