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| Titel |
Tropical tropopause ice clouds: a dynamic approach to the mystery of low crystal numbers |
| VerfasserIn |
P. Spichtinger, M. Krämer |
| 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 ; 13, no. 19 ; Nr. 13, no. 19 (2013-10-07), S.9801-9818 |
| Datensatznummer |
250085732
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| Publikation (Nr.) |
 copernicus.org/acp-13-9801-2013.pdf |
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| Zusammenfassung |
| The occurrence of high, persistent ice supersaturation inside and
outside cold cirrus in the tropical tropopause layer (TTL) remains
an enigma that is intensely debated as the "ice supersaturation
puzzle". However, it was recently confirmed that observed
supersaturations are consistent with very low ice crystal
concentrations, which is incompatible with the idea that homogeneous
freezing is the major method of ice formation in the TTL. Thus, the
tropical tropopause "ice supersaturation puzzle" has become an "ice
nucleation puzzle". To explain the low ice crystal concentrations,
a number of mainly heterogeneous freezing methods have been
proposed. Here, we reproduce in situ measurements of frequencies of
occurrence of ice crystal concentrations by extensive model
simulations, driven by the special dynamic conditions in the TTL,
namely the superposition of slow large-scale updraughts with
high-frequency short waves. From the simulations, it follows that
the full range of observed ice crystal concentrations can be
explained when the model results are composed from scenarios with
consecutive heterogeneous and homogeneous ice formation and scenarios
with pure homogeneous ice formation occurring in very slow (< 1 cm s−1)
and faster (> 1 cm s−1) large-scale updraughts,
respectively. This statistical analysis shows that about 80% of
TTL cirrus can be explained by "classical" homogeneous ice
nucleation, while the remaining 20% stem from heterogeneous and
homogeneous freezing occurring within the same environment. The
mechanism limiting ice crystal production via homogeneous freezing
in an environment full of gravity waves is the shortness of the
gravity waves, which stalls freezing events before a higher ice
crystal concentration can be formed. |
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