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| Titel |
Global retrieval of ATSR cloud parameters and evaluation (GRAPE): dataset assessment |
| VerfasserIn |
A. M. Sayer, C. A. Poulsen, C. Arnold, E. Campmany, S. Dean, G. B. L. Ewen, R. G. Grainger, B. N. Lawrence, R. Siddans, G. E. Thomas, P. D. Watts |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 11, no. 8 ; Nr. 11, no. 8 (2011-04-28), S.3913-3936 |
| Datensatznummer |
250009658
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| Publikation (Nr.) |
 copernicus.org/acp-11-3913-2011.pdf |
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| Zusammenfassung |
| The Along-Track Scanning Radiometers (ATSRs) provide a long
time-series of measurements suitable for the retrieval of cloud
properties. This work evaluates the freely-available Global Retrieval
of ATSR Cloud Parameters and Evaluation (GRAPE) dataset (version 3)
created from the ATSR-2 (1995–2003) and Advanced ATSR (AATSR; 2002
onwards) records. Users are recommended to consider only retrievals
flagged as high-quality, where there is a good consistency between the
measurements and the retrieved state (corresponding to about 60% of
converged retrievals over sea, and more than 80% over land). Cloud
properties are found to be generally free of any significant spurious
trends relating to satellite zenith angle. Estimates of the random
error on retrieved cloud properties are suggested to be generally
appropriate for optically-thick clouds, and up to a factor of two too
small for optically-thin cases. The correspondence between ATSR-2 and
AATSR cloud properties is high, but a relative calibration difference
between the sensors of order 5–10% at 660 nm and 870 nm limits
the potential of the current version of the dataset for trend
analysis. As ATSR-2 is thought to have the better absolute
calibration, the discussion focusses on this portion of the
record. Cloud-top heights from GRAPE compare well to ground-based data
at four sites, particularly for shallow clouds. Clouds forming in
boundary-layer inversions are typically around 1 km too high in GRAPE
due to poorly-resolved inversions in the modelled temperature profiles
used. Global cloud fields are compared to satellite products derived
from the Moderate Resolution Imaging Spectroradiometer (MODIS),
Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)
measurements, and a climatology of liquid water content derived from
satellite microwave radiometers. In all cases the main reasons for
differences are linked to differing sensitivity to, and treatment of,
multi-layer cloud systems. The correlation coefficient between GRAPE
and the two MODIS products considered is generally high (greater than
0.7 for most cloud properties), except for liquid and ice cloud
effective radius, which also show biases between the datasets. For
liquid clouds, part of the difference is linked to choice of
wavelengths used in the retrieval. Total cloud cover is slightly lower
in GRAPE (0.64) than the CALIOP dataset (0.66). GRAPE underestimates
liquid cloud water path relative to microwave radiometers by up to
100 g m−2 near the Equator and overestimates by around
50 g m−2 in the storm tracks. Finally, potential future
improvements to the algorithm are outlined. |
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