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| Titel |
Remote sensing of cloud top pressure/height from SEVIRI: analysis of ten current retrieval algorithms |
| VerfasserIn |
U. Hamann, A. Walther, B. Baum, R. Bennartz, L. Bugliaro, M. Derrien, P. N. Francis, A. Heidinger, S. Joro, A. Kniffka, H. Le Gléau, M. Lockhoff, H.-J. Lutz, J. F. Meirink, P. Minnis, R. Palikonda, R. Roebeling, A. Thoss, S. Platnick, P. Watts, G. Wind |
| 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 ; 7, no. 9 ; Nr. 7, no. 9 (2014-09-09), S.2839-2867 |
| Datensatznummer |
250115891
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| Publikation (Nr.) |
 copernicus.org/amt-7-2839-2014.pdf |
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| Zusammenfassung |
The role of clouds remains the largest uncertainty in climate
projections. They influence solar and thermal radiative transfer and
the earth's water cycle. Therefore, there is an urgent need for
accurate cloud observations to validate climate models and to
monitor climate change. Passive satellite imagers measuring
radiation at visible to thermal infrared (IR) wavelengths provide
a wealth of information on cloud properties. Among others, the cloud
top height (CTH) – a crucial parameter to estimate the thermal cloud
radiative forcing – can be retrieved. In this paper we investigate
the skill of ten current retrieval algorithms to estimate the CTH
using observations from the Spinning Enhanced Visible and InfraRed
Imager (SEVIRI) onboard Meteosat Second Generation (MSG). In the
first part we compare ten SEVIRI cloud top pressure (CTP)
data sets with each other. The SEVIRI algorithms catch the
latitudinal variation of the CTP in a similar way. The agreement is
better in the extratropics than in the tropics. In the tropics
multi-layer clouds and thin cirrus layers complicate the CTP
retrieval, whereas a good agreement among the algorithms is found for trade wind cumulus,
marine stratocumulus and the optically thick cores of the deep
convective system.
In the second part of the paper the SEVIRI retrievals are compared
to CTH observations from the Cloud–Aerosol LIdar with Orthogonal
Polarization (CALIOP) and Cloud Profiling Radar (CPR)
instruments. It is important to note that the different measurement
techniques cause differences in the retrieved CTH data. SEVIRI
measures a radiatively effective CTH, while the CTH of the active
instruments is derived from the return time of the emitted radar or lidar
signal. Therefore, some systematic differences are expected. On
average the CTHs detected by the SEVIRI algorithms are 1.0 to
2.5 km lower than CALIOP observations, and the correlation
coefficients between the SEVIRI and the CALIOP data sets range
between 0.77 and 0.90. The average CTHs derived by the SEVIRI algorithms are closer
to the CPR measurements than to CALIOP measurements. The biases between SEVIRI and
CPR retrievals range from −0.8 km to 0.6 km.
The correlation coefficients of CPR and
SEVIRI observations vary between 0.82 and 0.89. To discuss the
origin of the CTH deviation, we investigate three
cloud categories: optically thin and thick single layer as well as
multi-layer clouds. For optically thick clouds the correlation
coefficients between the SEVIRI and the reference data sets are
usually above 0.95. For optically thin single layer clouds the
correlation coefficients are still above 0.92. For this cloud
category the SEVIRI algorithms yield CTHs that are lower than CALIOP
and similar to CPR observations. Most challenging are the
multi-layer clouds, where the correlation coefficients are for most
algorithms between 0.6 and 0.8. Finally, we evaluate the
performance of the SEVIRI retrievals for boundary layer clouds.
While the CTH retrieval for this cloud type is relatively accurate,
there are still considerable differences between the
algorithms. These are related to the uncertainties and limited
vertical resolution of the assumed temperature profiles in
combination with the presence of temperature inversions, which lead
to ambiguities in the CTH retrieval. Alternative approaches for the
CTH retrieval of low clouds are discussed. |
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