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 strato cumulus clouds in the Azores region during June 1992. The bidirectiona l reflectances of overcast scenes have been analyzed. These scenes are not similar to homogeneous plane-parallel layers but their bidirectio nal reflectances compare favorably with those of heterogeneous plane-p arallel layers within 2 - 3%.