Low-cloud geometrical thickness over the ocean from CALIPSO

The potential of CALIPSO's Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument to detect cloud geometrical thickness is evaluated in marine boundary layers for single layer clouds with top heights below 3 km. Three resolutions CALIPSO level-2 cloud top and base height products – 1/3 km, 1 km and 5 km resolution – are analyzed. Overall, 1 km and 5 km retrievals reproduce expected global features, such as the occurrence of deep clouds in the Intertropical Convergence Zone, and the westward cloud thickening in marine boundary layer cloud regimes. In contrast, 1/3 km cloud thickness yields an unrealistically homogeneous global pattern. In addition, the cloud thickness tends to decrease with the product resolution, that is, 1/3 km retrievals are thinner than their 1km counterpart, and the latter are thinner than those from the 5km product. Adiabatic liquid water path calculated from CALIOP 1 km cloud depth correlates well (r~0.55) with collocated liquid water path retrievals from The Advanced Microwave Scanning Radiometer – EOS (AMSR-E) over cloudy scenes, with both 1km and 5km resolution data correctly reproducing the sub-adiabatic nature of boundary layer clouds. The causes for the differences between the three resolution products will be analyzed, and the accuracy of CALIOP will be further evaluated with the use of in-situ observations.