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| Titel |
Observations of cirrus clouds in the lowermost stratosphere: common feature, rare incident, or observational artefact? |
| VerfasserIn |
Reinhold Spang, Gebhard Günther, Rolf Müller, Lars Hoffmann, Sabine Griessbach, Christian Rolf, Martin Riese |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250092932
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| Publikation (Nr.) |
 EGU/EGU2014-7297.pdf |
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| Zusammenfassung |
| Ground based observations by lidar instruments show evidential occurrence of optically and
vertically thin cirrus clouds in the lowermost stratosphere (LMS). The knowledge about the
potential formation processes of these clouds, their occurrence and distribution, and
their radiative impact is very limited. Global observations of LMS cirrus clouds by
satellites would be very helpful to better characterise these clouds. However, this is a
difficult task because the optical thickness of LMS cirrus is usually at the edge of the
detection limit (for space borne limb-sounders) or even below (for infra red nadir
sounders).In addition, instrument characteristics can make it difficult to judge if a cloud
observation is inside the LMS of just at or below the tropopause. Consequently it is
not really proven if LMS cirrus clouds are a rare occasion or a globally common
feature.
We will give a brief overview of the history of LMS cirrus observations from
ground and space borne sensors and are highlighting the sometimes controversial
discussion on the observation of clouds in the LMS. Then we will focus on results from
measurements of the Cryogenic Infrared Spectrometers and Telescopes for the
Atmosphere (CRISTA) satellite instrument. CRISTA made a number of snapshot
measurements of the UT/LS during its two Space Shuttle missions in 1994 and 1997.
The measurements demonstrate the potential of the IR limb viewing technique to
provide information on several trace constituents and optically thin cirrus clouds with
comparably high spatial resolution. The CRISTA data are still unique for IR limb
sounders in the sense of vertical (1.5 km) and horizontal (300-500 km) resolution as
well as daily global coverage by using three telescopes for three different viewing
directions simultaneously. The detection sensitivity for optically thin cirrus clouds is
extremely high. Depending on the vertical and horizontal extent of a cirrus cloud, the
detection of an ice water content > 10-5g-m3 is achievable. This detection limit is
even better than for the CALIPSO lidar, the most sensitive instrument currently in
space.
An overview of CRISTA-2 water vapour and cirrus observations in August 1997 will be
given, and compared with corresponding ECMWF reanalyses and simulations by
the Chemical Lagrangian Model of the Stratosphere (CLaMS). The observations
indicate a considerable flux of moisture from the upper tropical troposphere into the
extra-tropical lowermost stratosphere, accompanied by high cirrus cloud occurrence
frequencies even at rather high northern latitudes (65N) and at altitudes above the
tropopause (around 350 K potential temperature). The observations are compared
with CLaMS simulations, which include a simple parameterisation for cirrus cloud
formation and sedimentation of ice particles. The results suggest a connection between
isentropic, quasi-horizontal transport of water vapour from the sub-tropics and the
occurrence of optically thin cirrus clouds in the lowermost stratosphere well above the
tropopause. |
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