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| Titel |
Cirrus and water vapour transport in the tropical tropopause layer – Part 2: Roles of ice nucleation and sedimentation, cloud dynamics, and moisture conditions |
| VerfasserIn |
T. Dinh, S. Fueglistaler, D. Durran, T. Ackerman |
| 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 ; 14, no. 22 ; Nr. 14, no. 22 (2014-11-20), S.12225-12236 |
| Datensatznummer |
250119176
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| Publikation (Nr.) |
 copernicus.org/acp-14-12225-2014.pdf |
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| Zusammenfassung |
A high-resolution, two-dimensional numerical model is used to study the
moisture redistribution following homogeneous ice nucleation induced by
Kelvin waves in the tropical tropopause layer (TTL). We compare results for
dry/moist initial conditions and three levels of complexity for the
representation of cloud processes: complete microphysics and cloud
radiative effects, likewise but without radiative effects, and
instantaneous removal of moisture in excess of saturation upon nucleation.
Cloud evolution and moisture redistribution are found to be sensitive to
initial conditions and cloud processes. Ice sedimentation leads to a downward
flux of water, whereas the cloud radiative heating induces upward advection
of the cloudy air. The latter results in an upward (downward) flux of
water vapour if the cloudy air is moister (drier) than the environment,
which is typically when the environment is subsaturated (supersaturated).
Only a fraction (~25% or less) of the cloud experiences nucleation.
Post-nucleation processes (ice depositional growth, sedimentation, and
sublimation) are important to cloud morphology, and both dehydrated
and hydrated layers may be indicators of TTL cirrus occurrence. The
calculation with instantaneous removal of moisture not only misses the
hydration but also underestimates dehydration due to (i) nucleation before
reaching the minimum saturation mixing ratio, and (ii) lack of moisture removal
from sedimenting ice particles below the nucleation level.
The sensitivity to initial conditions and cloud processes suggests that it is
difficult to reach generic, quantitative estimates of cloud-induced moisture
redistribution on the basis of case-by-case calculations. |
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