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| Titel |
Properties of subvisible cirrus clouds formed by homogeneous freezing |
| VerfasserIn |
B. Kärcher |
| 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 ; 2, no. 2 ; Nr. 2, no. 2 (2002-06-24), S.161-170 |
| Datensatznummer |
250000485
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| Publikation (Nr.) |
 copernicus.org/acp-2-161-2002.pdf |
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| Zusammenfassung |
| Number concentrations and mean sizes of ice crystals and derived microphysical and optical
properties of subvisible cirrus clouds (SVCs) formed by homogeneous freezing of supercooled
aerosols are investigated as a function of temperature and updraft speed of adiabatically ascending
air parcels. The properties of such clouds are insensitive to variations of the aerosol number and
size distribution. Based on criteria constraining the optical extinction, sedimentation time, and
existence time of SVCs, longer-lived (>10min) clouds, capable of exerting a measurable
radiative or chemical impact, are generated within a narrow range of updraft speeds below
1-2cm s-1 at temperatures below about 215K, with concentrations of ice crystals
not exceeding 0.1cm-3. The clouds do not reach an equilibrium state because the ice
crystals sediment out of the formation layer typically before the supersaturation is removed. Two
important conclusions emerge from this work. First, the above characteristics of SVCs may provide
an explanation for why SVCs are more common in the cold tropical than in the warmer midlatitude
tropopause region. Second, it seems likely that a limited number (<0.1cm-3) of
effective heterogeneous freezing nuclei that nucleate ice below the homogeneous freezing threshold
can control the formation and properties of SVCs, although homogeneous freezing nuclei are far more
abundant. |
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