OBSERVATIONS OF HORIZONTALLY ORIENTED ICE CRYSTALS IN CIRRUS CLOUDS WITH POLDER-1/ADEOS-1

Abstract Optical and radiative properties of cirrus clouds need to be accurately described at global scale in order to correctly estimate the radiative impact of ice clouds. The orientation of ice crystals in cirrus is capable of having a strong impact on their radiative budget: a cirrus cloud composed of horizontally oriented ice crystals has a larger plane albedo than a cirrus cloud composed of randomly oriented particles. Until recently, space-borne sensors were not adapted to observe ice crystal orientations. The POLDER instrument onboard the ADEOS platform (October 1996–June 1997) enabled us to observe 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 polarized radiances have been analyzed to determine the fraction of ice crystals preferably horizontally oriented. This preferred orientation is identified by observing specular reflection phenomenon above thick ice clouds. Three different periods (January, March and June 1997) of ten days of POLDER/ADEOS data have been processed for this study, and show that at least 40% of the ice pixels exhibit specular reflection peaks that indicate preferred orientation of ice crystals. The intensity and the distribution of specular reflection peaks are presented and discussed as a function of different parameters (solar zenith angle, latitude, cloud reflectance).