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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
13 Mar 2017
Review status
This discussion paper is under review for the journal Atmospheric Measurement Techniques (AMT).
Consistency of aerosols above clouds characterisation from A-Train active and passive measurements
Lucia T. Deaconu1, Fabien Waquet1, Damien Josset2, Nicolas Ferlay1, Fanny Peers3, François Thieuleux1, Fabrice Ducos1, Nicolas Pascal4, Didier Tanré1, and Philippe Goloub1 1University of Lille, CNRS, UMR 8518 - LOA - Laboratoire d’Optique Atmosphérique, F - 59000 Lille, France
2US Naval Research Laboratory, NASA Stennis Space Center, 39529, Mississippi
3College of Engineering, Mathematics and Physical Scien ces, University of Exeter, UK
4ICARE Data and Service center, University of Lille, Villeneuve d’Ascq, 59655, France
Abstract. This study presents a comparison between the retrieval of aerosol above clouds (AAC) optical properties from different techniques developed for the A-Train sensors CALIOP and POLDER/PARASOL. The main objective is to analyse the consistency between the results of the active and the passive measurements. We assess the Aerosol Optical Thickness (AOT) of above optically thick clouds (Cloud Optical Thickness (COT) larger than 3) and their Ångström Exponent (AE). These parameters are retrieved with the CALIOP operational method, the POLDER operational polarization method and the CALIOP-based depolarization ratio method (DRM) – for which we also propose a calibrated version (denominated DRMSODA, SODA as Synergized Optical Depth of Aerosols). We analyse six months of data over three distinctive regions characterized by different types of aerosols and clouds. Additionally, for these regions, we select three case studies: a biomass-burning event over the South Atlantic Ocean, a Saharan dust case over the North Atlantic Ocean and a Siberian biomass-burning event over the North Pacific Ocean. 4.5 years of data are studied over the entire globe for distinct situations where aerosol and cloud layers are in contact or vertically separated. Overall, the regional analysis shows a good correlation between the POLDER and the DRMSODA AOTs when the microphysics of aerosols is dominated by fine-mode particles of biomass-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 observed in the global treatment, when the aerosol and cloud layers are well separated. The analysis of detached layers also shows a mean difference in AOT of 0.07 at 532 nm between POLDER and DRMSODA, at a global scale. The correlation between the retrievals decreases when a complex mixture of aerosols is expected (R2 of 0.37) – as in the East Asia region, and when the aerosol-cloud layers are in contact (R2 of 0.36). The correlation coefficient between the CALIOP operational method and POLDER is low, as the CALIOP method largely underestimates the aerosol loading above clouds by a factor that ranges from two to four.

Potential biases on the retrieved AOT as a function of cloud properties are also investigated. For different types of scenes, the retrieval of above-cloud AOT from POLDER and from DRM are compared for different underlying cloud properties (droplet effective radius (reff) and COT retrieved with MODIS). The results reveal that DRM AOT vary with reff. When accounting for reff in the DRM algorithm, the consistency between the methods increases. The sensitivity study shows that an additional polarized signal coming from aerosols located within the cloud could affect the polarization method, which leads to an overestimation of the AOT retrieved with POLDER algorithm. In addition, the aerosols attached or within the cloud can potentially impact the DRM retrievals through the modification of the cloud droplet chemical composition and its ability to backscatter light. The next step of this work is to combine POLDER and CALIOP to investigate the impacts of aerosols on clouds and climate when these particles are transported above or within clouds.

Citation: Deaconu, L. T., Waquet, F., Josset, D., Ferlay, N., Peers, F., Thieuleux, F., Ducos, F., Pascal, N., Tanré, D., and Goloub, P.: Consistency of aerosols above clouds characterisation from A-Train active and passive measurements, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2017-42, in review, 2017.
Lucia T. Deaconu et al.
Lucia T. Deaconu et al.
Lucia T. Deaconu et al.


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Short summary
This study presents a comparison between active (CALIOP) and passive (POLDER) remote sensing methods, developed for the retrieval of aerosol above-cloud optical and microphysical properties. Main results show a good agreement between the retrievals when the microphysics of aerosols is dominated by fine-mode particles or coarse-mode dust, or when the aerosol layer is well separated from the cloud below. The study is also focused in understanding the origin of the biases between the measurements.
This study presents a comparison between active (CALIOP) and passive (POLDER) remote sensing...