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| Titel |
A distribution law for relative humidity in the upper troposphere and lower stratosphere derived from three years of MOZAIC measurements |
| VerfasserIn |
K. Gierens, U. Schumann, M. Helten, H. Smit, A. Marenco |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 17, no. 9 ; Nr. 17, no. 9, S.1218-1226 |
| Datensatznummer |
250013814
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| Publikation (Nr.) |
 copernicus.org/angeo-17-1218-1999.pdf |
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| Zusammenfassung |
Data from three years of MOZAIC measurements
made it possible to determine a distribution law for the relative humidity in
the upper troposphere and lower stratosphere. Data amounting to 13.5% of the
total were obtained in regions with ice supersaturation. Troposphere and
stratosphere are distinguished by an ozone concentration of 130 ppbv as
threshold. The probability of measuring a certain amount of ice supersaturation
in the troposphere decreases exponentially with the degree of ice
supersaturation. The probability of measuring a certain relative humidity in the
stratosphere (both with respect to water and ice) decreases exponentially with
the relative humidity. A stochastic model that naturally leads to the
exponential distribution is provided. Mean supersaturation in the troposphere is
about 15%, whereas ice nucleation requires 30% supersaturation on the average.
This explains the frequency of regions in which aircraft induce persistent
contrails but which are otherwise free of clouds. Ice supersaturated regions are
3-4 K colder and contain more than 50% more vapour than other regions in the
upper troposphere. The stratospheric air masses sampled are dry, as expected,
having mean relative humidity over water of 12% and over ice of 23%,
respectively. However, 2% of the stratospheric data indicate ice
supersaturation. As the MOZAIC measurements have been obtained on commercial
flights mainly between Europe and North America, the data do not provide a
complete global picture, but the exponential character of the distribution laws
found is probably valid globally. Since water vapour is the most important
greenhouse gas and since it might enhance the anthropogenic greenhouse effects
via positive feedback mechanisms, it is important to represent its distribution
correctly in climate models. The discovery of the distribution law of the
relative humidity makes possible simple tests to show whether the hydrological
cycle in climate models is represented in an adequate way or not.
Key words. Atmospheric composition and structure
(troposphere · composition and chemistry) |
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