Use of polarization for clouds study

POLDER (POLarization and Directionality of the Earth Reflectance) has been launched on the Japonese ADEOS (ADvanced Earth Observing System) platform in August 1996. Due to its multi-directional, multispectral and multi-polarization capabilities, this new radiometer gives usefull informations on the clouds and on their influence on the radiation in the shortwave range. The POLDER bidirectional observation capability provides the polarization signatures within a large range of scattering angles in three spectral bands centered on 443, 670 and 865 nm with a Spatial resolution of 7km x 7km. These original features allow to obtain some informations both on cloud thermodynamic phase and even on the cloud microphysic (size/shape) of particles. Cloud phase recognition is important for cloud studies. Ice crytals correspond to physical process and properties that differ from those of liquid water drops. The angular polarization signatures is a good mean to discriminate between liquid and ice phases. According to POLDER airborne observations, liquid cloud droplets exhibit the very specific polarisation features of the rainbow for scattering angles near 140 degrees. Conversely, according to theoretical studies and also to airborne measurements, scattering by various crystaline particles all show that the rainbow characteristics disappear as soon as the particles depart from the spherical shape. In the paper, preliminary results about cloud observations using polarized light measured by the spaceborne version of POLDER are presented, with emphasis on cloud phase detection and microphysic retrieval.