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| Titel |
Balloon-borne match measurements of midlatitude cirrus clouds |
| VerfasserIn |
A. Cirisan, B. P. Luo, I. Engel, F. G. Wienhold, M. Sprenger, U. K. Krieger, U. Weers, G. Romanens, G. Levrat, P. Jeannet, D. Ruffieux, R. Philipona, B. Calpini, P. Spichtinger, T. Peter |
| 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. 14 ; Nr. 14, no. 14 (2014-07-18), S.7341-7365 |
| Datensatznummer |
250118895
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| Publikation (Nr.) |
 copernicus.org/acp-14-7341-2014.pdf |
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| Zusammenfassung |
| Observations of high supersaturations with respect to ice inside
cirrus clouds with high ice water content (> 0.01 g kg−1)
and high crystal number densities (> 1 cm−3) are challenging
our understanding of cloud microphysics and of climate feedback processes
in the upper troposphere. However, single measurements of a
cloudy air mass provide only a snapshot from which the persistence of ice
supersaturation cannot be judged. We introduce here the "cirrus match
technique" to obtain information about the evolution of clouds and their
saturation ratio. The aim of these coordinated balloon soundings is to
analyze the same air mass twice. To this end the standard radiosonde
equipment is complemented by a frost point hygrometer, "SnowWhite", and a
particle backscatter detector, "COBALD" (Compact Optical Backscatter
AerosoL Detector). Extensive trajectory calculations based on regional weather model
COSMO (Consortium for Small-Scale Modeling) forecasts are performed for flight planning, and COSMO analyses are used
as a basis for comprehensive microphysical box modeling (with grid scale of 2
and 7 km, respectively). Here we present the results of matching a cirrus
cloud to within 2–15 km, realized on 8 June 2010 over Payerne, Switzerland,
and a location 120 km downstream close to Zurich. A thick cirrus cloud was detected
over both measurement sites. We show that in order to quantitatively
reproduce the measured particle backscatter ratios, the small-scale
temperature fluctuations not resolved by COSMO must be superimposed on the
trajectories. The stochastic nature of the fluctuations is captured by
ensemble calculations. Possibilities for further improvements in the
agreement with the measured backscatter data are investigated by assuming a
very slow mass accommodation of water on ice, the presence of heterogeneous
ice nuclei, or a wide span of (spheroidal) particle shapes. However, the
resulting improvements from these microphysical refinements are moderate and
comparable in magnitude with changes caused by assuming different regimes of
temperature fluctuations for clear-sky or cloudy-sky conditions, highlighting
the importance of proper treatment of subscale fluctuations. The model
yields good agreement with the measured backscatter over both sites and
reproduces the measured saturation ratios with respect to ice over Payerne.
Conversely, the 30% in-cloud supersaturation measured in a massive 4 km
thick cloud layer over Zurich cannot be reproduced, irrespective of the
choice of meteorological or microphysical model parameters. The measured
supersaturation can only be explained by either resorting to an unknown
physical process, which prevents the ice particles from consuming the excess
humidity, or – much more likely – by a measurement error, such as a
contamination of the sensor housing of the SnowWhite hygrometer by a
precipitation drop from a mixed-phase cloud just below the cirrus layer or
from some very slight rain in the boundary layer. This uncertainty calls for
in-flight checks or calibrations of hygrometers under the special humidity
conditions in the upper troposphere. |
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