Solar radiation processes on the East Antarctic Plateau: interaction of clouds, snow, and atmospheric gases
The bidirectional reflectance distribution function (BRDF) of snow was measured from a 32-meter tower at Dome C, at latitude 75°S on the East Antarctic Plateau. These measurements were made at 96 solar zenith angles between 51° and 87°, and cover wavelengths 350--2400 nm, over the full range of viewing geometry.Parameterizations are presented for the anisotropic reflectance factor using a small number of empirical orthogonal functions. The parameterizations cover nearly all viewing angles and are applicable to the high parts of the Antarctic Plateau that have small surface roughness.It has been a long-standing puzzle why clouds, which should interact with solar radiation similarly to a thin layer of snow, have such a dramatic effect on the reflectance observed by satellites over snow-covered regions. The presence of a cloud over the snow strongly enhances the anisotropy of the scene; by contrast, when a plane-parallel cloud is placed above a plane-parallel snow surface in a model, it slightly decreases the anisotropy of the system due to the cloud's smaller particles.Using the surface-reflectance parameterizations, I show that this effect of clouds over snow is due to the non-plane-parallel nature of the snow surface, not to unexpected features of the clouds. The snow-surface roughness reduces the anisotropy of the reflected sunlight compared to that from a plane-parallel snow surface. Clouds hide this roughness with a surface that is very smooth in units of optical depth. I use the surface parameterization to accurately model reflectance observations made from above cloud-covered snow.I also use these parameterizations in a model to calculate the directional reflectance above Dome C, integrated over the solar spectrum, for comparison with observations from Clouds and the Earth's Radiant Energy System (CERES). These comparisons suggest that the CERES radiances may be biased low, by about 5%, but that the anisotropic reflectance factors used by CERES to convert radiance to flux are appropriate for use over the region. A study of the effect of atmospheric variations on anisotropy suggests this is not likely to introduce significant uncertainty into CERES results.
- Atmospheric sciences