Polarized light scattering by inhomogeneous hexagonal monocrystals: Validation with ADEOS-POLDER measurements

Various in situ measurements of the light-scattering diagram in ice clouds were performed with a new nephelometer during several airborne campaigns. T hese measurements were favorably compared with a theoretical scattering mod el called Inhomogeneous Hexagonal Monocrystal (IHM) model. This model consi sts in computing the scattering of light by an ensemble of randomly oriente d hexagonal ice crystals containing spherical impurities of soot and air bu bbles. It is achieved by using a combination of ray tracing, Mie theory, an d Monte Carlo techniques and enables to retrieve the six independent elemen ts of the scattering matrix. This good agreement between nephelometer measu rements and IHM model provides an opportunity to use this model in order to analyze ADEOS-POLDER total and polarized reflectance measurements over ice clouds. POLDER uses an original concept to observe ice cloud properties, e nabling to measure reflectances and polarized reflectances, for a given sce ne, under several (up to 14) viewing directions. A first analysis of ice cl oud spherical albedoes over the terrestrial globe for November 10, 1996 and April 23, 1997, shows a rather good agreement between measurements and mod eling. Moreover, polarized reflectances are also calculated and show a sati sfactory agreement with measurements.