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This study presents a comparison between the retrieval of optical propertiesof aerosol above clouds (AAC) from different techniques developed for theA-Train sensors CALIOP/CALIPSO and POLDER/PARASOL. The main objective is toanalyse the consistency between the results derived from the active and thepassive measurements. We compare the aerosol optical thickness (AOT) aboveoptically thick clouds (cloud optical thickness (COT) larger than 3) andtheir Ångström exponent (AE). These parameters are retrieved with theCALIOP operational method, the POLDER operational polarization method and theCALIOP-based depolarization ratio method (DRM) – for which we also propose acalibrated version (denominated DRMSODA, where SODA is theSynergized Optical Depth of Aerosols). We analyse 6 months of data over threedistinctive regions characterized by different types of aerosols and clouds.Additionally, for these regions, we select three case studies: abiomass-burning event over the South Atlantic Ocean, a Saharan dust case overthe North Atlantic Ocean and a Siberian biomass-burning event over the NorthPacific Ocean. Four and a half years of data are studied over the entireglobe for distinct situations where aerosol and cloud layers are in contactor vertically separated. Overall, the regional analysis shows a goodcorrelation between the POLDER and the DRMSODA AOTs when themicrophysics of aerosols is dominated by fine-mode particles ofbiomass-burning aerosols from southern Africa (correlation coefficient(R2) of 0.83) or coarse-mode aerosols of Saharan dust (R2 of 0.82).A good correlation between these methods (R2 of 0.68) is also observedin the global treatment, when the aerosol and cloud layers are separatedwell. The analysis of detached layers also shows a mean difference in AOT of0.07 at 532 nm between POLDER and DRMSODA at a global scale. Thecorrelation between the retrievals decreases when a complex mixture ofaerosols is expected (R2 of 0.37) – as in the East Asia region – andwhen the aerosol–cloud layers are in contact (R2 of 0.36). Thecorrelation coefficient between the CALIOP operational method and POLDER isfound to be low, as the CALIOP method largely underestimates the aerosolloading above clouds by a factor that ranges from 2 to 4. Potential biases on the retrieved AOT as a function of cloud properties arealso investigated. For different types of scenes, the retrieval ofabove-cloud AOT from POLDER and from DRM are compared for differentunderlying cloud properties (droplet effective radius (reff) andCOT retrieved with MODIS). The results reveal that DRM AOT vary withreff. When accounting for reff in the DRM algorithm,the consistency between the methods increases. The sensitivity study showsthat an additional polarized signal coming from aerosols locatedwithin the cloud could affect the polarization method, which leadsto an overestimation of the AOT retrieved with POLDER algorithm. In addition,the aerosols attached to or within the cloud can potentiallyimpact the DRM retrievals through the modification of the cloud dropletchemical composition and its ability to backscatter light. The next step ofthis work is to combine POLDER and CALIOP to investigate the impacts ofaerosols on clouds and climate when these particles are transported above orwithin clouds

Consistency of aerosols above clouds characterization from A-Train active and passive measurements