An explanation for the effect of clouds over snow on the top‐of‐atmosphere bidirectional reflectance

Abstract

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, so that a cloud‐over‐snow scene appears darker than clear sky over snow when observed near nadir, but much brighter when observed at large zenith angles in the forward reflected direction. 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 because of the cloud's smaller particles. Using a parameterization for the directional reflectance from East Antarctic snow, developed from extensive near‐surface observations from a tower, we show that the unexpected effect of clouds over snow in this region 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, by hiding this roughness with a surface that is very smooth in units of optical depth, increase the anisotropy by bringing the system closer to the plane‐parallel case. We use the surface parameterization to accurately model reflectance observations made from the tower over a ground fog and from the top of the atmosphere over cloud‐covered snow by the MISR satellite instrument.