CIRRUS CLOUDS MICROPHYSICAL PROPERTIES DEDUCED FROM POLDER OBSERVATIONS

During the EUCREX'94 (EUropean Cloud Radiation EXperiment 1994) campai gn, observations of bidirectional reflected solar light above cirrus c louds off Brittany coast were collected with an airborne version of th e 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 cl oud. The polarization measurements provide a very good signature of th e water phase (liquid or ice) in the observed cloud. A more precise an alysis of the behaviour of the polarized reflectances shows that it is highly sensitive to the shape of the cirrus cloud ice crystals for sc attering angles ranging between 60 degrees and 110 degrees. One sequen ce of the POLDER images collected during EUCREX'94 pointed out the pre sence of hexagonal plates with aspect ratio equal to 0.1, whereas the analysis of an another sequence of images collected during the same ca mpaign clearly shows a specular reflection of the solar light correspo nding to the presence of horizontally oriented prismatic ice crystals in the cloud. (C) 1998 Elsevier Science Ltd. All rights reserved.