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| Titel |
Particle backscatter and relative humidity measured across cirrus clouds and comparison with microphysical cirrus modelling |
| VerfasserIn |
M. Brabec, F. G. Wienhold, B. P. Luo, H. Vömel, F. Immler, P. Steiner, E. Hausammann, U. Weers, 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 ; 12, no. 19 ; Nr. 12, no. 19 (2012-10-05), S.9135-9148 |
| Datensatznummer |
250011498
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| Publikation (Nr.) |
 copernicus.org/acp-12-9135-2012.pdf |
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| Zusammenfassung |
| Advanced measurement and modelling techniques are employed to estimate the
partitioning of atmospheric water between the gas phase and the condensed
phase in and around cirrus clouds, and thus to identify in-cloud and
out-of-cloud supersaturations with respect to ice. In November 2008 the newly
developed balloon-borne backscatter sonde COBALD (Compact Optical Backscatter
and AerosoL Detector) was flown 14 times together with a CFH (Cryogenic Frost
point Hygrometer) from Lindenberg, Germany (52° N, 14° E).
The case discussed here in detail shows two cirrus layers
with in-cloud relative humidities with respect to ice between 50% and
130%. Global operational analysis data of ECMWF (roughly
1° × 1° horizontal and 1 km vertical resolution, 6-hourly stored fields)
fail to represent ice water contents and relative
humidities. Conversely, regional COSMO-7 forecasts
(6.6 km × 6.6 km, 5-min stored fields) capture the measured
humidities and cloud positions remarkably well. The main difference between
ECMWF and COSMO data is the resolution of small-scale vertical features
responsible for cirrus formation. Nevertheless, ice water contents in COSMO-7
are still off by factors 2–10, likely reflecting limitations in COSMO's ice
phase bulk scheme. Significant improvements can be achieved by comprehensive
size-resolved microphysical and optical modelling along backward trajectories
based on COSMO-7 wind and temperature fields, which allow accurate
computation of humidities, homogeneous ice nucleation, resulting ice particle
size distributions and backscatter ratios at the COBALD wavelengths. However,
only by superimposing small-scale temperature fluctuations, which remain
unresolved by the numerical weather prediction models, can we obtain a
satisfying agreement with the observations and reconcile the measured
in-cloud non-equilibrium humidities with conventional ice cloud microphysics.
Conversely, the model-data comparison provides no evidence that additional
changes to ice-cloud microphysics – such as heterogeneous nucleation or
changing the water vapour accommodation coefficient on ice – are required. |
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