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| Titel |
On the "well-mixed" assumption and numerical 2-D tracing of atmospheric moisture |
| VerfasserIn |
H. F. Goessling, C. H. Reick |
| 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. 11 ; Nr. 13, no. 11 (2013-06-06), S.5567-5585 |
| Datensatznummer |
250018689
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| Publikation (Nr.) |
 copernicus.org/acp-13-5567-2013.pdf |
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| Zusammenfassung |
Atmospheric water vapour tracers (WVTs) are an elegant tool to
determine source–sink relations of moisture "online" in atmospheric
general circulation models (AGCMs). However, it is sometimes
desirable to establish such relations "offline" based on already
existing atmospheric data (e.g. reanalysis data). One simple and
frequently applied offline method is 2-D moisture tracing. It makes
use of the "well-mixed" assumption, which allows for treating the
vertical dimension integratively. Here we scrutinise the "well-mixed"
assumption and 2-D moisture
tracing by means of analytical considerations in combination with
AGCM-WVT simulations.
We find that vertically well-mixed conditions
are seldom met. Due to the presence of vertical inhomogeneities, 2-D
moisture tracing (i) neglects a significant degree of fast-recycling,
and (ii) results in erroneous advection where the direction of the
horizontal winds varies vertically. The latter is not so much the case
in the extratropics, but in the tropics this can lead to large
errors. For example, computed by 2-D moisture tracing, the fraction of
precipitation in the western Sahel that originates from beyond the
Sahara is ~40%, whereas the fraction that originates from the
tropical and Southern Atlantic is only ~4%. According to full
(i.e. 3-D) moisture tracing, however, both regions contribute roughly
equally, showing that the errors introduced by the 2-D
approximation can be substantial. |
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