Ice cloud microphysics and its temperature dependence retrieved from Satellite data

A near-global (50 degrees S to 50 degrees N) survey of ice crystal sizes for both continental and oceanic areas is presented, along with a description of the methodology for retrieving ice cloud particle size from ISCCP data. To retrieve ice particle sizes, a radiative transfer model is used which includes all major absorbing gases and cloud scattering absorption to compute synthetic radiances as a function of satellite viewing geometry. The surface reflectance effect is considered according to retrieved monthly average reflectance values at each location. Ice crystal shapes are assumed to be hexagonal columns. The model results have been validated against clear sky observations and are consistent with the observed radiance range under cloudy conditions. We investigated the relationship between cloud temperatures/optical thickness and cirrus ice crystal sizes based on the retrieved cirrus particle size data. We found that for thick clouds (tau greater than or equal to 10), most of the regions over the globe show positive relationships between cloud temperature and cirrus ice crystal sizes. However, if relatively thin clouds are included (tau greater than or equal to 3), this correlation becomes negative for tropical areas but remains mostly positive for midlatitudes.