Analysis of the POLDER polarization measurements performed overcloud covers

The POLDER instrument is designed to provide wide field of view bidimensional images in polarized light. During campaigns of the airborne version of the instrument, images of homogeneous cloud fields were acquired in polarized bands centered at 450 and 850 nm. The polarization of these images is analyzed. The bidirectional polarization distribution function measured in the 850 nm band is shown to make evident the liquid phase of the cloud droplets, by the large characteristic polarization of the cloudbows detected in backward scattering directions. The sensitivity of this feature to cloud parameters is discussed. On the contrary, for observation directions at about 90°-100° from the Sun, the cloud polarization is negligible. In these directions, the polarized light observed in the 450 nm band is characteristic of the molecular scattering about the cloud, which allows the cloud top altitude to be derived. The feasibility of the method is analyzed and is tested on cloud pictures acquired at different altitudes above cloud fields