On the validity of the plane-parallel approximation for cloud reflectances as measured from POLDER during ASTEX

The POLDER instrument provides multidirectional radiance measurements in the visible and near-infrared range and thus allows to measure the anisotropy of the reflected solar radiation. As a result POLDER allows to investigate the validity of the usual plane-parallel approximation for cloud reflectances. The airborne instrument was flown over stratocumulus clouds in the Azores region during June 1992. The bidirectional reflectances of overcast scenes have been analyzed. These scenes are not similar to homogeneous plane-parallel layers but their bidirectional reflectances compare favorably with those of heterogeneous plane-parallel layers within 2-3%