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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
Sprache Englisch
ISSN 1680-7316
Digitales Dokument URL
Erschienen In: Atmospheric Chemistry and Physics ; 13, no. 11 ; Nr. 13, no. 11 (2013-06-06), S.5567-5585
Datensatznummer 250018689
Publikation (Nr.) Volltext-Dokument vorhandencopernicus.org/acp-13-5567-2013.pdf
 
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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