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| Titel |
A Lagrangian view of convective sources for transport of air across the Tropical Tropopause Layer: distribution, times and the radiative influence of clouds |
| VerfasserIn |
A. Tzella, B. Legras |
| 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. 23 ; Nr. 11, no. 23 (2011-12-13), S.12517-12534 |
| Datensatznummer |
250010262
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| Publikation (Nr.) |
 copernicus.org/acp-11-12517-2011.pdf |
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| Zusammenfassung |
The tropical tropopause layer (TTL) is a key region controlling transport
between the troposphere and the stratosphere. The efficiency of transport
across the TTL depends on the continuous interaction between the large-scale
advection and the small-scale intermittent convection that reaches the Level
of Zero radiative Heating (LZH). The wide range of scales involved presents a
significant challenge to determine the sources of convection and quantify
transport across the TTL. Here, we use a simple Lagrangian model, termed
TTL detrainment model, that combines a large ensemble of 200-day back
trajectory calculations with high-resolution fields of brightness
temperatures (provided by the CLAUS dataset) in order to determine the
ensemble of trajectories that are detrained from convective sources. The
trajectories are calculated using the ECMWF ERA-Interim winds and radiative
heating rates, and in order to establish the radiative influence of clouds,
the latter rates are derived both under all-sky and clear-sky conditions.
We show that most trajectories are detrained near the mean LZH with the
horizontal distributions of convective sources being highly-localized, even
within the space defined by deep convection. As well as modifying the degree
of source localization, the radiative heating from clouds facilitates the
rapid upwelling of air across the TTL. However, large-scale motion near the
fluctuating LZH can lead a significant proportion of trajectories to
alternating clear-sky and cloudy regions, thus generating a large dispersion
in the vertical transport times. The distributions of vertical transport
times are wide and skewed and are largely insensitive to a bias of about
±1 km (∓5 K) in the altitude of cloud top heights (the main
sensitivity appearing in the times to escape the immediate neighbourhood of
the LZH) while some seasonal and regional transport characteristics are
apparent for times up to 60 days. The strong horizontal mixing that
characterizes the TTL ensures that most air of convective origin is
well-mixed within the tropical and eventually within the extra-tropical
lower-stratosphere. |
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