Schlüsselwörter
(Englisch)
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Cirrus clouds; water vapour; radiosonde; modelling
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Forschungsprogramme
(Englisch)
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COST-Action ES0604 - Atmospheric Water Vapour in the Climate System
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Kurzbeschreibung
(Englisch)
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The water vapour budget and in turn the radiative balance of the upper troposphere are determined by the occurrence of cirrus clouds and by the dehydration through sedimenting ice particles. However, there is evidence from field observations and laboratory studies that mass transfer of water from the gas phase to the condensed phase, i.e. growth of the ice particles and equilibration of the H2O partial pressure toward the ice vapour pressure, may at times be impeded. There is some evidence that extensive regions of supersaturation with respect to ice occur not only outside of cirrus clouds but also inside cirrus, which raises the question why these supersaturations do not readily relax to satu-rated conditions. The proposed project will perform Lagrangian measurements using the “match” technique on cirrus clouds in combination with state-of-the-art trajectory-based microphysical model-ling and 2-D cloud-resolved modelling focussing on the issue of cirrus cloud development. The measurements and modelling work will be performed in direct collaboration with COST partners at the Swiss water vapour profiling site in Payerne, Switzerland. The “match” will be enforced by measuring cirrus particle backscatter and the relative humidity with a mobile balloon sounding unit 2-3 hours downstream of Payerne. The examination of the same air mass at cirrus level by the two subsequent soundings in combination with comprehensive modelling promises unprecedented insight into the natural evolution of growth/evaporation processes and supersaturation in cirrus clouds.
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Weitere Hinweise und Angaben
(Englisch)
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Full name of research-institution/enterprise: ETH Zürich Institute for Atmospheric and Climate Science
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Partner und Internationale Organisationen
(Englisch)
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BE, BG, CH, CZ, DE, DK, ES, FI, FR, GR, HU, IE, IT, NO, PL, PT, SE, UK
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Abstract
(Englisch)
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The occurrence of cirrus clouds and dehydration through sedimenting ice particles affect the atmospheric water vapor budget and the Earth radiation balance. However, there is evidence from field observations and laboratory studies that mass transfer of water from the gas phase to the condensed phase may at times be impeded, leading to high supersaturations even within cirrus clouds. In order to improve our understanding of the life cycle of cirrus clouds in the mid-latitudes we introduce an unprecedented combination of techniques: (1) the state-of-the-art backscatter and relative humidity measurements sondes COBALD and SnowWhite; (2) a Lagrangian 'match technique' to observe the same cloud twice within 2-3 hours; (3) the use of one of the best available mesoscale numerical weather prediction models (COSMO-2); and finally (4) our comprehensive microphysical models based on the mesoscale model fields. For the 'matches', the MeteoSwiss radio sounding site at Payerne, Switzerland, and locations within the greater area of Zürich were taken as launch sites of the balloon pairs. We used a tandem of high quality hygrometers and optical backscatter sondes. During a 2.5 year period we analyzed about 500 nights with respect to their suitability for matches by means of COSMO-2 fields and the trajectory calculations, and based on this performed a total of 9 match flights. Detailed analysis and modeling of the match flight performed on 8 June 2010 reveal cirrus clouds between 6 and 12.5 km altitude with surprisingly strong and super-saturation of 20-30 % persistent between Payerne and Zürich. Backscatter and relative humidity were well captured by the microphysical models, showing close to perfect agreement at Payerne, but both quantities too low at Zürich. Improvements were achieved by introducing smallest-scale temperature fluctuations beyond the 2-km resolution of COSMO-2, which lead to higher number densities of smaller ice crystals with slower sedimentation speeds. However, the fluctuations also result in enhanced cloud inhomogeneities constraining the match quality. In conclusion, unexpected persistency in supersaturations in cirrus clouds was confirmed by the measurements, but could be largely explained by microphysical models, after properly taking the presence of smallest-scale temperature fluctuations (< 2 km) into account that remained unresolved by the mesoscale numerical weather prediction model.
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Datenbankreferenzen
(Englisch)
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Swiss Database: COST-DB of the State Secretariat for Education and Research Hallwylstrasse 4 CH-3003 Berne, Switzerland Tel. +41 31 322 74 82 Swiss Project-Number: C08.0037
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