Optimizing cirrus optical depth retrievals over the ocean from collocated CALIPSO and AMSR-E observations

Retrievals of particulate optical depths and extinction coefficients from the cloud-aerosol lidar with orthogonal polarization (CALIOP) instrument deployed on the CALIPSO satellite mainly rely on a single global mean extinction-to-backscatter ratio, also known as the lidar ratio. However, the lidar ratio depends on the microphysical properties of particulates. An alternative approach is adopted to infer single-layer semi-transparent cirrus optical depths (CODs) over the open ocean that does not rely on an assumed lidar ratio. Instead, the COD is inferred directly from backscatter measurements obtained from the CALIOP lidar in conjunction with collocated sea surface wind speed data obtained from AMSR-E. This method is based on a Gram-Charlier ocean surface reflectance model relating wind-driven wave slope variances to sea surface wind speeds. To properly apply this method, the impact of multiple scattering between the sea surface and ice clouds should be taken into account. We take advantage of the 532 nm cross-polarization feature of CALIOP and introduce an empirical method based on the depolarization change at the sea surface to correct for potential bias in sea surface backscatter caused by whitecaps, bubbles, foam, and multiple scattering. After the correction, the COD can be derived for individual CALIOP retrievals in a single cloud layer over the ocean with this method. The global mean COD was found to be roughly 14% higher than the current values determined by the Version 4 CALIOP extinction retrieval algorithm. This study is relevant to future improvements of CALIOP operational products and is expected to lead to more accurate COD retrievals.