Effects Due to Cloud Geometry on Biases in the Albedo Derived from Radiance Measurements

Abstract

The usual procedure in determining the earth's radiation budget is that observed radiances are first used to derive cloud properties. The properties are then used in a one-dimensional model to deduct cloud albedos. This conventional procedure yields more accurate estimates than a simple scheme that uses a bidirectional function. The procedure is, however, known to give rise to biases in the inferred albedos. In order to assess the effects due to cloud geometry on the biases, the reflected radiances are calculated for broken cloud fields, making use of a Monte Carlo model. The results of the calculations indicate that the anisotropy of the upwelling radiance from broken clouds differs markedly from that of plane-parallel clouds, which gives rise to the biases in the inferred albedos. Potential errors in the inferred albedos for broken clouds are, however, significantly reduced by the choice of appropriate viewing directions at which there are less effects due to the cloud geometry. In general, the biases are negligible for viewing zenith angles near the zenith, even if the cloud-contaminated field of view is incorrectly identified as overcast.