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| Titel |
Ice supersaturations exceeding 100% at the cold tropical tropopause: implications for cirrus formation and dehydration |
| VerfasserIn |
E. J. Jensen, J. B. Smith, L. Pfister, J. V. Pittman, E. M. Weinstock, D. S. Sayres, R. L. Herman, R. F. Troy, K. Rosenlof, T. L. Thompson, A. M. Fridlind, P. K. Hudson, D. J. Cziczo, A. J. Heymsfield, C. Schmitt, J. C. Wilson |
| 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 ; 5, no. 3 ; Nr. 5, no. 3 (2005-03-14), S.851-862 |
| Datensatznummer |
250002510
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| Publikation (Nr.) |
 copernicus.org/acp-5-851-2005.pdf |
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| Zusammenfassung |
| Recent in situ measurements at tropical tropopause temperatures as low as
187 K indicate supersaturations with respect to ice exceeding 100% with
little or no ice present. In contrast, models used to simulate cloud
formation near the tropopause assume a supersaturation threshold for ice
nucleation of about 65% based on laboratory measurements of aqueous aerosol
freezing. The high supersaturations reported here, along with cloud
simulations assuming a plausible range of temperature histories in the
sampled air mass, indicate that the vast majority of aerosols in the air
sampled on this flight must have had supersaturation thresholds for ice
nucleation exceeding 100% (i.e. near liquid water saturation at these
temperatures). Possible explanations for this high threshold are that (1) the
expressions used for calculating vapor pressure over supercooled water at low
temperatures give values are at least 20% too low, (2) organic films on the
aerosol surfaces reduce their accommodation coefficient for uptake of water,
resulting in aerosols with more concentrated solutions when moderate-rapid
cooling occurs and correspondingly inhibited homogeneous freezing, and (3) if
surface freezing dominates, organic coatings may increase the surface energy
of the ice embryo/vapor interface resulting in suppressed ice nucleation.
Simulations of in situ cloud formation in the tropical tropopause layer (TTL)
throughout the tropics indicate that if decreased accommodation coefficients
and resulting high thresholds for ice nucleation prevailed throughout the
tropics, then the calculated occurrence frequency and areal coverage of TTL
cirrus would be significantly suppressed. However, the simulations also show
that even if in situ TTL cirrus form only over a very small fraction of the
tropics in the western Pacific, enough air passes through them due to rapid
horizontal transport such that they can still effectively freeze-dry air
entering the stratosphere. The TTL cirrus simulations show that even if very
large supersaturations are required for ice nucleation, these large
supersaturations should occur very rarely. |
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