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| Titel |
Improved information about the vertical location and extent of monolayer clouds from POLDER3 measurements in the oxygen A-band |
| VerfasserIn |
M. Desmons, N. Ferlay, F. Parol, L. Mcharek, 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. 8 ; Nr. 6, no. 8 (2013-08-30), S.2221-2238 |
| Datensatznummer |
250085051
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| Publikation (Nr.) |
 copernicus.org/amt-6-2221-2013.pdf |
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| Zusammenfassung |
| This paper describes new advances in the exploitation of oxygen A-band
measurements from POLDER3 sensor onboard PARASOL, satellite platform within
the A-Train. These developments result from not only an account of the dependence of
POLDER oxygen parameters to cloud optical thickness τ and to the scene's
geometrical conditions but also, and more importantly, from the finer
understanding of the sensitivity of these parameters to cloud vertical
extent. This sensitivity is made possible thanks to the multidirectional
character of POLDER measurements. In the case of monolayer clouds that
represent most of cloudy conditions, new oxygen parameters are obtained and
calibrated from POLDER3 data colocalized with the measurements of the two
active sensors of the A-Train: CALIOP/CALIPSO and CPR/CloudSat. From a
parameterization that is (μs, τ) dependent, with
μs the cosine of the solar zenith angle, a cloud top oxygen
pressure (CTOP) and a cloud middle oxygen pressure (CMOP) are obtained, which
are estimates of actual cloud top and middle pressures (CTP and CMP).
Performances of CTOP and CMOP are presented by class of clouds following the
ISCCP classification. In 2008, the coefficient of the correlation between
CMOP and CMP is 0.81 for cirrostratus, 0.79 for stratocumulus, 0.75 for deep
convective clouds. The coefficient of the correlation between CTOP and CTP is
0.75, 0.73, and 0.79 for the same cloud types. The score obtained by CTOP,
defined as the confidence in the retrieval for a particular range of inferred
value and for a given error, is higher than the one of MODIS CTP estimate.
Scores of CTOP are the highest for bin value of CTP superior in numbers. For
liquid (ice) clouds and an error of 30 hPa (50 hPa), the score
of CTOP reaches 50% (70%). From the difference between CTOP and
CMOP, a first estimate of the cloud vertical extent h is possible. A second
estimate of h comes from the correlation between the angular standard
deviation of POLDER oxygen pressure σPO2 and the cloud
vertical extent. This correlation is studied in detail in the case of liquid
clouds. It is shown to be spatially and temporally robust, except for clouds
above land during winter months. The analysis of the correlation's dependence
on the scene's characteristics leads to a parameterization providing h from
σPO2. For liquid water clouds above ocean in 2008, the
mean difference between the actual cloud vertical extent and the one
retrieved from σPO2 (from the pressure difference)
is 5 m (−12 m). The standard deviation of the mean difference is
close to 1000 m for the two methods. POLDER estimates of the cloud
geometrical thickness obtain a global score of 50% confidence for a
relative error of 20% (40%) of the estimate for ice (liquid) clouds over ocean. These results need to be validated outside of the
CALIPSO/CloudSat track. |
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