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| Titel |
Residual circulation trajectories and transit times into the extratropical lowermost stratosphere |
| VerfasserIn |
T. Birner, H. Bönisch |
| 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 ; 11, no. 2 ; Nr. 11, no. 2 (2011-01-28), S.817-827 |
| Datensatznummer |
250009200
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| Publikation (Nr.) |
 copernicus.org/acp-11-817-2011.pdf |
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| Zusammenfassung |
| Transport into the extratropical lowermost stratosphere (LMS) can be divided
into a slow part (time-scale of several months to years) associated with the
global-scale stratospheric residual circulation and a fast part (time-scale
of days to a few months) associated with (mostly quasi-horizontal) mixing
(i.e. two-way irreversible transport, including extratropical
stratosphere-troposphere exchange). The stratospheric residual circulation
may be considered to consist of two branches: a deep branch more
strongly associated with planetary waves breaking in the middle to upper
stratosphere, and a shallow branch associated with synoptic and
planetary scale waves breaking in the subtropical lower stratosphere. In
this study the contribution due to the stratospheric residual circulation
alone to transport into the LMS is quantified using residual circulation
trajectories, i.e. trajectories driven by the (time-dependent) residual mean
meridional and vertical velocities. This contribution represents the
advective part of the overall transport into the LMS and can be viewed as
providing a background onto which the effect of mixing has to be added.
Residual mean velocities are obtained from a comprehensive chemistry-climate
model as well as from reanalysis data. Transit times of air traveling from
the tropical tropopause to the LMS along the residual circulation
streamfunction are evaluated and compared to recent mean age of air
estimates. A time-scale separation with much smaller transit times into the
mid-latitudinal LMS than into polar LMS is found that is indicative of a
separation of the shallow from the deep branch of the residual circulation.
This separation between the shallow and the deep circulation branch is
further manifested in a distinction in the aspect ratio of the vertical to
meridional extent of the trajectories, the integrated mass flux along the
residual circulation trajectories, as well as the stratospheric entry
latitude of the trajectories. The residual transit time distribution
reproduces qualitatively the observed seasonal cycle of youngest air in the
extratropical LMS in fall and oldest air in spring. |
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