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| Titel |
Impacts of cloud heterogeneities on cirrus optical properties retrieved from space-based thermal infrared radiometry |
| VerfasserIn |
T. Fauchez, P. Dubuisson, C. Cornet, F. Szczap, A. Garnier, J. Pelon, K. Meyer |
| 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 ; 8, no. 2 ; Nr. 8, no. 2 (2015-02-09), S.633-647 |
| Datensatznummer |
250116125
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| Publikation (Nr.) |
 copernicus.org/amt-8-633-2015.pdf |
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| Zusammenfassung |
| This paper presents a study, based on simulations, of the impact of cirrus
cloud heterogeneities on the retrieval of cloud parameters (optical thickness
and effective diameter) for the Imaging Infrared Radiometer (IIR) on board
CALIPSO. Cirrus clouds are generated by the stochastic model 3DCLOUD for two
different cloud fields and for several averaged cloud parameters. One cloud
field is obtained from a cirrus observed on 25 May 2007 during the
airborne campaign CIRCLE-2 and the other is a cirrus uncinus. The radiative
transfer is simulated with the 3DMCPOL code. To assess the errors due to
cloud heterogeneities, two related retrieval algorithms are used: (i) the
split-window technique to retrieve the ice crystal effective diameter and
(ii) an algorithm similar to the IIR operational algorithm to retrieve the
effective emissivity and the effective optical thickness. Differences between
input parameters and retrieved parameters are compared as a function of
different cloud properties such as the mean optical thickness, the
heterogeneity parameter and the effective diameter. The optical thickness
heterogeneity for each 1 km × 1 km observation pixel is
represented by the optical thickness standard deviation computed using
100 m × 100 m subpixels. We show that optical thickness
heterogeneity may have a strong impact on the retrieved parameters, mainly
due to the plane-parallel approximation (PPA assumption). In particular, for
cirrus clouds with ice crystal diameter of approximately 10 μm, the
averaged error on the retrieved effective diameter and optical thickness is
about 2.5 μm (~ 25%) and −0.20 (~ 12%),
respectively. Then, these biases decrease with increasing effective size due
to a decrease of the cloud absorption and, thus, the PPA bias. Cloud
horizontal heterogeneity effects are greater than other possible sources of
retrieval errors such as those due to cloud vertical heterogeneity impact,
surface temperature or atmospheric temperature profile uncertainty and IIR
retrieval uncertainty. Cloud horizontal heterogeneity effects are larger than
the IIR retrieval uncertainty if the standard deviation of the optical
thickness, inside the observation pixel, is greater than 1. |
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