Sensitivity of retrieved POLDER directional cloud optical thickness to various ice particle models

The "directional" values of cloud optical thickness (or cloud spherical alb edo) retrieved from ADEOS-POLDER data constitute a strong constraint on the microphysical model used in the retrieval algorithm. In this paper, we foc us on ice clouds. We quantify the departure of the directional values of sp herical albedo from:their averaged value. By so doing, we can assess the su itability of different ice particle scattering models. (i.e., spheres, frac tal polycrystal, inhomogeneous hexagonal monocrystal, or a synthesized phas e function). The liquid water droplet and the ice fractal polycrystal model s appear to be inappropriate since they induce a notable dependence of the ice cloud spherical albedo on scattering angle. On the contrary, much bette r:agreement is achieved by using a synthesized phase function or an inhomeg eneous hexagonal monocrystal model. Using these models, the retrieved ice c loud optical thickness is then found to be 40% smaller than using the dropl et model.