A simple method for the determination of the cloud phase from near-infrared spectra is presented. It is based on the shape of the reflectivity spectrum around 1.67 Ám, which is different for ice and water clouds. The strength of the method is found in the fact that it directly exploits the differing absorption properties of water and ice that show up in cloud reflectivity spectra. The method is shown to be succesful by means of near-infrared reflectivity spectra made by the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) over a well-defined cloud system of stratocumulus and cirrus over the Pacific Ocean. The method will be applied to global measurements to be made by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) which is scheduled for launch on ESA's ENVISAT on 1 March 2002.
The second aspect of cloud property determination concerns the question which ice crystal shape should be used for the retrieval of ice cloud optical thickness and crystal size. For this purpose global multi-angle (polarized) radiance measurements made by the POLDER satellite instrument are compared with model calculations of the angular-dependent radiation field over ice clouds. For this comparison, model clouds consisting of different ice particles are used: spheres, hexagonal crystals with smooth/rough surfaces and with/without air bubble inclusions, and polycrystals. It is found that the POLDER measurements are best simulated using smooth hexagons with air bubble inclusions or rough hexagons without inclusions as a model for real ice crystals.