Observations of horizontally oriented ice crystals in cirrus clouds with POLDER-1/ADEOS-1

Optical and radiative properties of cirrus clouds need to be accurately des cribed at global scale in order to correctly estimate the radiative impact of ice clouds. The orientation of ice crystals in cirrus is capable of havi ng a strong impact on their radiative budget: a cirrus cloud composed of ho rizontally oriented ice crystals has a larger plane albedo than a cirrus cl oud composed of randomly oriented particles. Until recently, space-borne se nsors were not adapted to observe ice crystal orientations. The POLDER inst rument onboard the ADEOS platform (October 1996-June 1997) enabled us to ob serve bidirectional polarized radiances. These data are useful to determine the frequency of occurrence of ice crystals horizontally oriented in space within cirrus clouds. This paper describes how the POLDER bidirectional po larized radiances have been analyzed to determine the fraction of ice cryst als preferably horizontally oriented. This preferred orientation is identif ied by observing specular reflection phenomenon above thick ice clouds. Thr ee different periods (January, March and June 1997) of ten days of POLDER/A DEOS data have been processed for this study, and show that at least 40% of the ice pixels exhibit specular reflection peaks that indicate preferred o rientation of ice crystals. The intensity and the distribution of specular reflection peaks are presented and discussed as a function of different par ameters (solar zenith angle, latitude, cloud reflectance). (C) 1999 Elsevie r Science Ltd. All rights reserved.