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| Titel |
MERIS albedo climatology for FRESCO+ O2 A-band cloud retrieval |
| VerfasserIn |
C. Popp, P. Wang, D. Brunner, P. Stammes, Y. Zhou, M. Grzegorski |
| 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 ; 4, no. 3 ; Nr. 4, no. 3 (2011-03-08), S.463-483 |
| Datensatznummer |
250001796
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| Publikation (Nr.) |
 copernicus.org/amt-4-463-2011.pdf |
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| Zusammenfassung |
A new global albedo climatology for Oxygen A-band cloud retrievals is
presented. The climatology is based on MEdium Resolution Imaging Spectrometer
(MERIS) Albedomap data and its favourable impact on the derivation of cloud
fraction is demonstrated for the FRESCO+ (Fast Retrieval Scheme for Clouds
from the Oxygen A-band) algorithm. To date, a relatively coarse resolution
(1° × 1°) surface reflectance dataset from GOME
(Global Ozone Monitoring Experiment) Lambert-equivalent reflectivity (LER) is
used in FRESCO+. The GOME LER climatology does not account for the usually
higher spatial resolution of UV/VIS instruments designed for trace gas remote
sensing which introduces several artefacts, e.g. in regions with sharp
spectral contrasts like coastlines or over bright surface targets. Therefore,
MERIS black-sky albedo (BSA) data from the period October 2002 to October
2006 were aggregated to a grid of 0.25° × 0.25° for
each month of the year and for different spectral channels. In contrary to
other available surface reflectivity datasets, MERIS includes channels at
754 nm and 775 nm which are located close to the spectral windows required
for O2 A-band cloud retrievals.
The MERIS BSA in the near-infrared compares well to Moderate Resolution
Imaging Spectroradiometer (MODIS) derived BSA with an average difference
lower than 1% and a correlation coefficient of 0.98. However, when relating
MERIS BSA to GOME LER a distinctly lower correlation (0.80) and enhanced
scatter is found. Effective cloud fractions from two exemplary months
(January and July 2006) of Scanning Imaging Absorption Spectrometer for
Atmospheric Chartography (SCIAMACHY) data were subsequently derived with
FRESCO+ and compared to those from the Heidelberg Iterative Cloud Retrieval
Utilities (HICRU) algorithm. The MERIS climatology generally improves FRESCO+
effective cloud fractions. In particular small cloud fractions are in better
agreement with HICRU. This is of importance for atmospheric trace gas
retrieval which relies on accurate cloud information at small cloud
fractions. In addition, overestimates along coastlines and underestimates in
the Intertropical Convergence Zone introduced by the GOME LER were
eliminated. While effective cloud fractions over the Saharan desert and the
Arabian peninsula are successfully reduced in January, they are still too
high in July relative to HICRU due to FRESCO+'s large sensitivity to albedo
inaccuracies of highly reflecting targets and inappropriate aerosol
information which hampers an accurate albedo retrieval. Finally, NO2 tropospheric vertical column
densities and O3 total columns were derived with the FRESCO+ cloud parameters from the new dataset
and it is found that the MERIS BSA climatology has a pronounced and beneficial effect on regional scale. Apart from FRESCO+,
the new MERIS albedo dataset is applicable to any cloud retrieval
algorithms using the O2 A-band or the O2-O2 absorption band
around 477 nm. Moreover, the by-product of BSA at 442 nm can be used in
NO2 remote sensing and the BSA at 620 nm, 665 nm, and 681 nm could be
integrated in current H2O retrievals. |
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