Cloud thermodynamical phase classification from the POLDER spaceborne instrument

Cloud phase recognition is important for cloud studies. Ice crystals corres pond to physical process and properties that differ from those of liquid wa ter drops. The angular polarization signature is a good mean to discriminat e between spherical and nonspherical particles (liquid and ice phase, respe ctively). POLDER (Polarization and Directionality of Earth Reflectances) ha s been launched on the Japanese ADEOS platform in August 1996. Because of i ts multidirectional, multispectral, and multipolarization capabilities this new radiometer gives useful information on clouds and their influence on r adiation in the shortwave range. The POLDER bidirectional observation capab ility provides the polarization signatures within a large range of scatteri ng angles in three spectral bands centered on 0.443, 0.670, and 0.865 mu m with a spatial resolution of 6.2 km x 6.2 km. These original features allow to obtain some information both on cloud thermodynamic phase and on cloud microphysics (size/shape). According to POLDER airborne observations, liqui d cloud droplets exhibit very specific polarization features of a rainbow f or scattering angles near 140 degrees. Conversely, theoretical studies of s cattering, by various crystalline particles and also airborne measurements show that the rainbow characteristics disappear as soon as the particles de part from the spherical shape. In the paper the POLDER algorithm for cloud phase classification is presented, as well as the physical principle of thi s algorithm. Results derived from the POLDER spaceborne version are also pr esented and compared with lidar ground-based observations and satellite clo ud classification. This cloud phase classification method is shown to be re liable. The major limitation appears when thin cirrus clouds overlap the li quid cloud layer, In this case, if the cirrus optical thickness is smaller than 2, the liquid phase may be retrieved. Otherwise, the ice phase is corr ectly detected as long as cloud detection works.