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

The ”directional” values of cloud optical thickness (or cloud spherical albedo) retrieved from ADEOS-POLDER data constitute a strong constraint on the microphysical model used in the retrieval algorithm. In this paper, we focus on ice clouds. We quantify the departure of the directional values of spherical albedo from their averaged value. By so doing, we can assess the suitability of different ice particle scattering models (i.e., spheres, fractal polycrystal, inhomogeneous hexagonal monocrystal, or a synthesized phase function). The liquid water droplet and the ice fractal polycrystal models appear to be inappropriate since they induce a notable dependence of the ice cloud spherical albedo on scattering angle. On the contrary, much better agreement is achieved by using a synthesized phase function or an inhomegeneous hexagonal monocrystal model. Using these models, the retrieved ice cloud optical thickness is then found to be 40% smaller than using the droplet model.