Cirrus clouds' microphysical properties deduced from POLDER observations

During the EUCREX'94 (EUropean Cloud Radiation EXperiment 1994) campaign, observations of bidirectional reflected solar light above cirrus clouds off Brittany coast were collected with an airborne version of the POLDER (POLarization and Directionality of the Earth's Reflectances) instrument. This instrument was designed to measure the bidirectional total and polarized reflectances of the solar light scattered by a cloud. The polarization measurements provide a very good signature of the water phase (liquid or ice) in the observed cloud. A more precise analysis of the behaviour of the polarized reflectances shows that it is highly sensitive to the shape of the cirrus cloud ice crystals for scattering angles ranging between 60 degrees and 110 degrees. One sequence of the POLDER images collected during EUCREX'94 pointed out the presence of hexagonal plates with aspect ratio equal to 0.1, whereas the analysis of an another sequence of images collected during the same campaign clearly shows a specular reflection of the solar light corresponding to the presence of horizontally oriented prismatic ice crystals in the cloud. (C) 1998 Elsevier Science Ltd. All rights reserved.