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| Titel |
The roles of dynamical variability and aerosols in cirrus cloud formation |
| VerfasserIn |
B. Kärcher, J. Ström |
| 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 ; 3, no. 3 ; Nr. 3, no. 3 (2003-06-23), S.823-838 |
| Datensatznummer |
250001050
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| Publikation (Nr.) |
 copernicus.org/acp-3-823-2003.pdf |
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| Zusammenfassung |
| The probability of occurrence of ice crystal number densities in young cirrus clouds is examined
based on airborne measurements. The observations have been carried out at midlatitudes in both
hemispheres at equivalent latitudes (52-55°N/S) during the same season (local autumn
in 2000). The in situ measurements considered in the present study include temperatures, vertical
velocities, and total ice crystal concentrations, the latter determined with high precision and
accuracy using a counterflow virtual impactor. Most young cirrus clouds typically contain high number densities
(1-10 cm-3) of small (diameter <20 mm) ice crystals. This mode dominates the probability distributions
and is shown to be caused by rapid cooling rates associated with updraft speeds
in the range 10-100 cm s-1. A second mode containing larger crystals extends from
~1 cm-3 to low concentrations close to the detection threshold
(~3 x 10-4 cm-3) and could be associated with lower updraft speeds.
Results of a statistical analysis provide compelling evidence that the dynamical variability of
vertical air motions on the mesoscale is the key factor determining the observed probability
distributions of pristine ice crystal concentrations in cirrus. Other factors considered are
changes of temperature as well as size, number, and ice nucleation thresholds of the freezing
aerosol particles. The variability in vertical velocities is caused by atmospheric gravity waves leading to small-scale
temperature fluctuations. Inasmuch as gravity waves are widespread, mesoscale variability in
vertical velocities can be viewed as a universal feature of young cirrus clouds. Large-scale
models that do not account for this subgrid-scale variability yield erroneous predictions of the
variability of basic cirrus cloud properties. Climate change may bring about changes in the global
distribution of updraft speeds, mean air temperatures, and aerosol properties. As shown in this work,
these changes could significantly modify the probability distribution of cirrus ice crystal
concentrations. This study emphasizes the key role of vertical velocities and mesoscale variability
in vertical velocities in controlling cirrus properties. The results suggest that, in any effort to
ascribe cause to trends of cirrus cloud properties, a careful evaluation of dynamical changes in
cloud formation should be done before conclusions regarding the role of other anthropogenic factors,
such as changes in aerosol composition, are made. |
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