Cloud Detection from the Spaceborne POLDER Instrument and Validation against Surface Synoptic Observations

This paper describes the cloud screening algorithms that have been developed for the processing of Polarization and Directionality of the Earth Reflectances (POLDER) measurements over land surfaces. Four tests are applied to the measurements. The first one is a threshold on the 0.44-μm reflectance after atmospheric correction. The second one is similar but with a smaller threshold and is applied only over targets with significant spectral variation. The third one compares the surface pressure to an estimate derived from two POLDER channels centered on an oxygen absorption band. The fourth one makes use of POLDER polarization capabilities and seeks the presence of a rainbow generated by water clouds. The performance of the method is evaluated using a large dataset of nearly coincident POLDER measurements and surface observations of cloud cover. The validation dataset is fully independent of the spaceborne measurements and allows the sampling of a wide range of situations. The results demonstrate the capability of the algorithm to distinguish clear and cloudy pixels, although a large fraction of pixels with a small cloud cover are incorrectly declared clear. This may be due in part to the different field of view of the spaceborne and surface observations. For a given cloud cover, the detection is less efficient for high clouds than for low- or medium-altitude clouds, which may result from their lower-optical thickness.