Study of Ice Crystal Orientation in Cirrus Clouds Based on Satellite Polarized Radiance Measurements

The goal of this paper is to retrieve information about ice particle orientation in cirrus clouds. This is achieved by comparing simulations of sunlight reflection on a cirrus cloud with measurements of polarized radiances from the spaceborne instrument Polarization and Directionality of the Earth's Reflectance (POLDER-1) on Advanced Earth Observing Satellite-1 (ADEOS-1). Results show that horizontal orientation of crystals can be spotted by the presence of a local maximum of polarized radiance in the direction of specular reflection. The angular width of the local maximum is shown to contain information on the particle maximum deviation angle, while the maximum intensity can provide information on particle shape and relative concentrations of ice crystals, horizontally and randomly oriented. The study of 31 ice cloud cases show that in 80% of them, the deviation angle is less than 3. Also, the relative concentration of horizontally oriented crystals is less than 21%, depending on the angular distribution used for crystal deviation.