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| Titel |
Evaluation of CALIOP 532 nm aerosol optical depth over opaque water clouds |
| VerfasserIn |
Z. Liu, D. Winker, A. Omar, M. Vaughan, J. Kar, C. Trepte, Y. Hu, G. Schuster |
| 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 ; 15, no. 3 ; Nr. 15, no. 3 (2015-02-05), S.1265-1288 |
| Datensatznummer |
250119390
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| Publikation (Nr.) |
 copernicus.org/acp-15-1265-2015.pdf |
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| Zusammenfassung |
| With its height-resolved measurements and near global coverage, the CALIOP
lidar onboard the CALIPSO satellite offers a new capability for aerosol
retrievals in cloudy skies. Validation of these retrievals is difficult,
however, as independent, collocated and co-temporal data sets are generally
not available. In this paper, we evaluate CALIOP aerosol products above
opaque water clouds by applying multiple retrieval techniques to CALIOP
Level 1 profile data and comparing the results. This approach allows us to
both characterize the accuracy of the CALIOP above-cloud aerosol optical
depth (AOD) and develop an error budget that quantifies the relative
contributions of different error sources. We focus on two spatial domains:
the African dust transport pathway over the tropical North Atlantic and the
African smoke transport pathway over the southeastern Atlantic. Six years of
CALIOP observations (2007–2012) from the northern hemisphere summer and
early fall are analyzed. The analysis is limited to cases where aerosol
layers are located above opaque water clouds so that a constrained retrieval
technique can be used to directly retrieve 532 nm aerosol optical depth and
lidar ratio. For the moderately dense Sahara dust layers detected in the
CALIOP data used in this study, the mean/median values of the lidar ratios
derived from a constrained opaque water cloud (OWC) technique are 45.1/44.4 ± 8.8 sr,
which are somewhat larger than the value of 40 ± 20 sr
used in the CALIOP Level 2 (L2) data products. Comparisons of CALIOP L2 AOD
with the OWC-retrieved AOD reveal that for nighttime conditions the L2 AOD
in the dust region is underestimated on average by ~26%
(0.183 vs. 0.247). Examination of the error sources indicates that errors in
the L2 dust AOD are primarily due to using a lidar ratio that is somewhat
too small. The mean/median lidar ratio retrieved for smoke is 70.8/70.4 ± 16.2 sr, which is consistent with the modeled value of
70 ± 28 sr used in the CALIOP L2 retrieval. Smoke AOD is found to be underestimated,
on average, by ~39% (0.191 vs. 0.311). The primary cause
of AOD differences in the smoke transport region is the tendency of the
CALIOP layer detection scheme to prematurely assign layer base altitudes and
thus underestimate the geometric thickness of smoke layers. |
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