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| Titel |
Retrieval of aerosol microphysical and optical properties above liquid clouds from POLDER/PARASOL polarization measurements |
| VerfasserIn |
F. Waquet, C. Cornet, J.-L. Deuzé, O. Dubovik, F. Ducos, P. Goloub, M. Herman, T. Lapyonok, L. C. Labonnote, J. Riedi, D. Tanré, F. Thieuleux, C. Vanbauce |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 6, no. 4 ; Nr. 6, no. 4 (2013-04-15), S.991-1016 |
| Datensatznummer |
250017874
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| Publikation (Nr.) |
 copernicus.org/amt-6-991-2013.pdf |
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| Zusammenfassung |
| 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 and to estimate their radiative forcing. The
presence of aerosol 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 on
the PARASOL satellite. In a previous work, a first retrieval method was developed
for AAC scenes and evaluated for biomass-burning aerosols transported over
stratocumulus clouds. The method was restricted to the use of observations
acquired at forward scattering angles (90–120°) where polarized
measurements are highly sensitive to fine-mode particle scattering.
Non-spherical particles in the coarse mode, such as mineral dust particles,
do not much polarize light and cannot be handled with this method. In this
paper, we present new developments that allow retrieving also the properties
of mineral dust particles above clouds. These particles do not much polarize
light but strongly reduce the polarized cloud bow generated by the
liquid cloud layer beneath and observed for scattering angles around 140°. The
spectral attenuation can be used to qualitatively identify the nature of the
particles (i.e. accumulation mode versus coarse mode, i.e. mineral dust
particles versus biomass-burning aerosols), whereas the magnitude of the
attenuation is related to the optical thickness of the aerosol layer. We also
use the polarized measurements acquired in the cloud bow to improve the
retrieval of both the biomass-burning aerosol properties and the cloud
microphysical properties. 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 cloud
properties. We investigate various scenes with mineral dust particles and
biomass-burning aerosols above clouds. For clouds, our results confirm that
the droplet size distribution is narrow in high-latitude ocean regions and
that the droplet effective radii retrieved from both polarization
measurements and from total radiance measurements are generally close for AAC
scenes (departures smaller than 2 μm). 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 modeling of the polarized cloud
bow are between 4 and 8% for homogenous cloudy scenes, as shown by a 3-D
radiative transfer code. These effects only weakly impact the retrieval of
the Aerosol Optical Thickness (AOT) performed with a mineral dust particle model for which the
microphysical properties are entirely known (relative error smaller than
6%). We show that the POLDER polarization measurements allow retrieving
the AOT, the fine-mode particle size, the
Ångström exponent and the fraction of spherical particles. However, the
complex refractive index and the coarse-mode particle size cannot be
accurately retrieved with the present polarization measurements. Our complete
and accurate algorithm cannot be applied to process large amounts of data, so
a simpler algorithm was developed to retrieve the
AOT and the Ångström exponent above clouds in an operational way.
Illustrations are provided for July–August 2008 near the African coast. Large
mean AOTs above clouds at 0.865 μm (>0.3) are retrieved for
oceanic regions near the coasts of South Africa 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 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 AAC
properties and the estimation of the direct aerosol radiative forcing in
cloudy scenes. |
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