Cirrus cloud properties derived from POLDER-1/ADEOS polarized radiances: First validation using a ground-based lidar network

Bidirectional polarized reflectances measured with the POLDER-I instrument on board Advanced Earth Observing Satellite-1 have been used to infer cloud altitude and thermodynamical phase (ice/liquid) at a global scale. This pa per presents a validation of these properties for cirrus clouds. The valida tion presented here is based on comparisons between POLDER-I retrievals and measurements collected with a ground-based lidar network. The scale differ ences between POLDER measurements and lidar data are treated by selecting h omogeneous and stable cloud layers. These comparisons show that the cloud altitude retrieval with POLDER is val id for optically thick cloud, and nonvalid for semitransparent and thin cir rus clouds. The limitations of the cloud altitude retrieval method are anal yzed by using both comparisons between POLDER and lidar and simulations of the bidirectional polarized reflectances performed with a radiative transfe r code to assess a threshold of validity of the POLDER retrieval method. Th e comparisons of lidar and POLDER data show that the cloud thermodynamical phase (ice/liquid) retrieval is satisfactory, and examples of cloud thermod ynamical phase retrieval are presented as a function of cloud temperatures.