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| Titel |
Rare temperature histories and cirrus ice number density in a parcel and a one-dimensional model |
| VerfasserIn |
D. M. Murphy |
| 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 ; 14, no. 23 ; Nr. 14, no. 23 (2014-12-09), S.13013-13022 |
| Datensatznummer |
250119222
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| Publikation (Nr.) |
 copernicus.org/acp-14-13013-2014.pdf |
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| Zusammenfassung |
| A parcel and a one-dimensional model are used to investigate the temperature
dependence of ice crystal number density. The number of ice crystals
initially formed in a cold cirrus cloud is very sensitive to the nucleation
mechanism and the detailed history of cooling rates during nucleation. A
possible small spread in the homogeneous freezing threshold due to varying
particle composition is identified as a sensitive nucleation parameter. In a
parcel model, the slow growth rate of ice crystals at low temperatures
inherently leads to a strong increase in ice number density at low
temperatures. This temperature dependence is not observed. The model
temperature dependence occurs for a wide range of assumptions and for either
homogeneous or, less strongly, heterogeneous freezing. However, the parcel
model also shows that random temperature fluctuations result in an extremely
wide range of ice number densities. A one-dimensional model is used to show
that the rare temperature trajectories resulting in the lowest number
densities are disproportionately important. Low number density ice crystals
sediment and influence a large volume of air. When such fall streaks are
included, the ice number becomes less sensitive to the details of nucleation
than it is in a parcel model. The one-dimensional simulations have a more
realistic temperature dependence than the parcel mode. The one-dimensional
model also produces layers with vertical dimensions of meters even if the
temperature forcing has a much broader vertical wavelength. Unlike warm
clouds, cirrus clouds are frequently surrounded by supersaturated air.
Sedimentation through supersaturated air increases the importance of any
process that produces small numbers of ice crystals. This paper emphasizes
the relatively rare temperature trajectories that produce the fewest crystals.
Other processes are heterogeneous nucleation, sedimentation from
the very bottom of clouds, annealing of disordered to hexagonal ice, and
entrainment. |
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