A better understanding of cloud optical thickness derived from the passive sensors MODIS/AQUA and POLDER/PARASOL in the A-Train constellation

摘要

Cloud optical thickness (COT) is one of the most important parameter for thecharacterization of cloud in the Earth radiative budget. Its retrievalstrongly depends on instrument characteristics and on many cloud andenvironment factors. Using coincident observations from POLDER/PARASOL andMODIS/AQUA in the A-Train constellation, geographical distributions andseasonal changes of COT are presented, in good agreement with general cloudclimatology characteristics. Retrieval uncertainties mainly associated tosensor spatial resolution, cloud inhomogeneity and microphysical assumptionsare discussed.Comparisons of COT derived from POLDER and MODIS illustrate that as theprimary factor, the sensor spatial resolution impacts COT retrievals andstatistics through both cloud detection and sub-pixel cloud inhomogeneitysensitivity.The uncertainties associated to cloud microphysics assumptions, namely cloudphase, particle size and shape, also impact significantly COT retrievals.For clouds with unambiguous cloud phase, strong correlations exist betweenthe two COTs, with MODIS values comparable to POLDER ones for liquid cloudsand MODIS values larger than POLDER ones for ice clouds. The largedifferences observed in ice phase cases are due to the use of differentmicrophysical models in the two retrieval schemes. In cases when the twosensors disagree on cloud phase decision, COT retrieved assuming liquidphase is systematically larger.The angular biases related to specific observation geometries are alsoquantified and discussed in particular based on POLDER observations. Thoseexhibit a clear increase of COT with decreasing sun elevation and a decreaseof COT in forward scattering directions due to sub-pixel inhomogeneities andshadowing effects, this especially for lower sun. It also demonstratesunrealistic COT variations in the cloudbow and backward directions due toinappropriate cloud optical properties representation and an importantincrease of COT in the sun-glint directions in case of broken cloud.