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Retrieval of Aerosol Microphysical and Optical Properties Above Liquid Clouds from Polder/Parasol Polarization Measurements : Volume 5, Issue 4 (27/08/2012)

By Waquet, F.

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Book Id: WPLBN0003976795
Format Type: PDF Article :
File Size: Pages 63
Reproduction Date: 2015

Title: Retrieval of Aerosol Microphysical and Optical Properties Above Liquid Clouds from Polder/Parasol Polarization Measurements : Volume 5, Issue 4 (27/08/2012)  
Author: Waquet, F.
Volume: Vol. 5, Issue 4
Language: English
Subject: Science, Atmospheric, Measurement
Collections: Periodicals: Journal and Magazine Collection (Contemporary), Copernicus GmbH
Publication Date:
Publisher: Copernicus Gmbh, Göttingen, Germany
Member Page: Copernicus Publications


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Herman, M., Labonnote, L., Vanbauce, C., Waquet, F., Dubovik, O., Cornet, C.,...Thieuleux, F. (2012). Retrieval of Aerosol Microphysical and Optical Properties Above Liquid Clouds from Polder/Parasol Polarization Measurements : Volume 5, Issue 4 (27/08/2012). Retrieved from

Description: Laboratoire d'Optique Atmosphérique, CNRS-INSU – UMR8518, Université Lille 1, Villeneuve d'Ascq, France. Most of the current aerosol retrievals from passive sensors are restricted to cloud-free scenes, which strongly reduces our ability to monitor the aerosol properties at a global scale. The presence of Aerosols Above Clouds (AAC) affects the polarized light reflected by the cloud layer, as shown by the spaceborne measurements provided by the POlarization and Directionality of Earth Reflectances (POLDER) instrument. We present new developments that allow retrieving the properties of mineral dust particles when they are present above clouds. These particles do not much polarize light but strongly attenuate the polarized cloud bow generated by the beneath liquid cloud layer. The spectral attenuation can be used to qualitatively identify the nature of the particles (i.e. mineral dust particles or biomass burning aerosols) whereas the magnitude of the attenuation is related to the optical thickness of the aerosol layer. We provide accurate polarized radiance calculations for AAC scenes and evaluate the contribution of the POLDER polarization measurements for the simultaneous retrieval of the aerosol and clouds properties. We investigate various scenes with mineral dust particles and biomass burning aerosols above clouds. We found that the magnitude of the primary cloud bow cannot be accurately estimated with a plane parallel transfer radiative code. The errors for the modelling of the polarized cloud bow are between 5 and 8% for homogenous cloudy scenes, as shown by a 3-D radiative transfer code. For clouds, our results confirm that the droplets size distribution is narrow in high latitude ocean regions and that the droplets effective radii retrieved from polarization measurements and from total radiance measurements are generally close for AAC scenes (departures smaller than 2 μm). For the aerosols, the POLDER polarization measurements are primarily sensitive to the particles load, size distribution, shape and real refractive index. An algorithm was developed to retrieve the Aerosol Optical Thickness (AOT) and the Angström exponent above clouds in an operational way. This method was applied to various regions of the world and time period. Large mean AOTs above clouds at 0.865 μm (>0.3) are retrieved for oceanic regions near the coasts of South Africa and California (>0.1) that correspond to biomass burning aerosols whereas even larger mean AOTs above clouds for mineral dust particles (>0.6) are also retrieved near the coasts of Senegal (for June–August 2008). For these regions and time period, the direct AAC radiative forcing is likely to be significant. The final aim of this work is the global monitoring of the aerosol above clouds properties and the estimation of the direct aerosol radiative forcing in cloudy scenes.

Retrieval of aerosol microphysical and optical properties above liquid clouds from POLDER/PARASOL polarization measurements

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