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| Titel |
On the origin of subvisible cirrus clouds in the tropical upper troposphere |
| VerfasserIn |
M. Reverdy, V. Noel, H. Chepfer, B. Legras |
| 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 ; 12, no. 24 ; Nr. 12, no. 24 (2012-12-20), S.12081-12101 |
| Datensatznummer |
250011680
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| Publikation (Nr.) |
 copernicus.org/acp-12-12081-2012.pdf |
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| Zusammenfassung |
| Spaceborne lidar observations have recently revealed a previously undetected
significant population of Subvisible Cirrus (SVC). We show them to be colder
than −74 °, with an optical depth below 0.0015 on average. The
formation and persistence over time of this new cloud population could be
related to several atmospheric phenomena. In this paper, we investigate if
these clouds follow the same formation mechanisms as the general tropical
cirrus population (including convection and in-situ ice nucleation), or if
specific nucleation sites and trace species play a role in their formation.
The importance of three scenarios in the formation of the global SVC
population is investigated through different approaches that include
comparisons with data imaging from several spaceborne instruments and
back-trajectories that document the history and behavior of air masses
leading to the point in time and space where subvisible cirrus were
detected. In order to simplify the study of their formation, we singled out
SVC with coherent temperature histories (mean variance lower than 4 K)
according to back-trajectories along 5, 10 or 15 days (respectively 58, 25
and 11% of SVC). Our results suggest that external processes, including
local increases in liquid and hygroscopic aerosol concentration (either
through biomass burning or volcanic injection forming sulfate-based aerosols
in the troposphere or the stratosphere) have very limited short-term or
mid-term impact on the SVC population. On the other hand, we find that
~20% of air masses leading to SVC formation interacted
with convective activity 5 days before they led to cloud formation and
detection, a number that climbs to 60% over 15 days. SVC formation
appears especially linked to convection over Africa and Central America,
more so during JJA than DJF. These results support the view that the SVC
population observed by CALIOP is an extension of the general upper
tropospheric ice clouds population with its extreme thinness as its only
differentiating factor. |
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