POLDER 2 - CLOUD PROPERTIES: Cloud Pressures

• Cloud Rayleigh pressure

Cloud Rayleigh pressure is retrieved from the polarization measurements at 443 nm (Buriez et al., 1997). Indeed, the polarized reflectance at 443 nm is mainly related to the atmospheric molecular optical thickness above the observed surface if the radiance originating from this surface is negligibly polarized. Therefore, this method is restricted to the region of maximum molecular scattering polarization (80° < < 120°), outside the sunglint area and only for thick enough clouds. A small contamination of polarized reflectance at 443 nm caused by the cloud layer is removed using polarized reflectance at 865 nm where molecular scattering is negligible. A new correction is applied to Stokes parameters and liquid and ice clouds are now treated separately. The corrected polarized measurements are first converted into pressure according to single-scattering approximation. A better estimate of the cloud top pressure is then calculated taking into account multiple scattering. This estimation based on off-line simulations has also been improved in the new algorithm. It induces a drastic reduction of the number of abnormally high values of retrieved cloud top pressure observed in the original POLDER-1 cloud products.

• Cloud Oxygen pressure

Cloud Oxygen pressure Poxy is determined from differential absorption between the radiances measured in the channels centered at 763 and 765 nm respectively (Buriez et al., 1997). In a first step, the ground reflectivity effect is neglected. An apparent pressure Papp is inferred by assuming that the atmosphere behaves as a pure absorbing medium overlying a perfect cloud reflector located at pressure Papp (Vanbauce et al., 1998). Because of the effects of surface reflection and multiple scattering inside the cloud, the apparent pressure is almost always higher than the cloud top pressure. It is even higher than the cloud base pressure when a considerable amount of photons reaches the surface before being reflected back to space, that is in the case of thin cloud layer above a bright surface. Cloud Oxygen pressure Poxy is determined from apparent pressure by removing the surface contribution. This correction is made using the cloud optical thickness. From comparisons of POLDER-1 cloud oxygen pressure and ARM/MMCR cloud boundaries pressures, Poxy appears to indicate the cloud middle pressure rather than the cloud top pressure (Vanbauce et al., 2003).