Analyses of Semi-transparent Cirrus Clouds Using Combined Retrievals of CALIPSO Radiometer and Lidar Measurements

Abstract:

We present a detailed evaluation of cloud optical depths retrieved from perfectly co-located observations of semi-transparent cirrus made by the Imaging Infrared Radiometer (IIR) and the Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP) flying on-board the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite. Analyses use Version 3 CALIOP Level 2 5-km cloud layer products and the corresponding Version 3 IIR Level 2 track products. Uncertainties are assessed by comparing IIR absorption optical depths retrieved at 12.05 microns to CALIOP visible optical depths for single-layered clouds over ocean when the latter can be derived using the so-called constrained retrieval technique. A bias in the optical depths used in CALIOP constrained retrievals is evidenced for optical depths smaller than about 0.6. This bias can be largely eliminated by relaxing the criteria applied in the current version 3 of the CALIOP algorithm to select the qualifying clouds. Ratios of CALIOP visible to IIR absorption optical depths are found to increase quasi-linearly by about 50% as the temperature at the layer centroid altitude decreases from 250 K to 200 K, whereas they are expected between 1.9 and 2 according to simulations and ice crystal effective diameters retrieved from the IIR. The analysis of simultaneously derived CALIOP lidar ratios of moderately deep clouds (1-4 km) strongly suggests that these observations can be explained by variations of the multiple scattering correction factor, which is taken to be constant and equal to 0.6 in the CALIOP algorithm. Multiple scattering factors derived by reconciling observed and expected ratios of visible to infrared absorption optical extend from 0.68 +/- 0.05 at 200 K to 0.46 +/- 0.03 at 250 K.