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| Titel |
A bulk parametrization of melting snowflakes with explicit liquid water fraction for the COSMO model |
| VerfasserIn |
C. Frick, A. Seifert, H. Wernli |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1991-959X
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| Digitales Dokument |
URL |
| Erschienen |
In: Geoscientific Model Development ; 6, no. 6 ; Nr. 6, no. 6 (2013-11-06), S.1925-1939 |
| Datensatznummer |
250085013
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| Publikation (Nr.) |
 copernicus.org/gmd-6-1925-2013.pdf |
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| Zusammenfassung |
| A new snow melting parametrization is presented for the
non-hydrostatic limited-area COSMO ("consortium for small-scale modelling")
model. In contrast to the standard cloud microphysics of the COSMO model,
which instantaneously transfers the meltwater from the snow to the rain
category, the new scheme explicitly considers the liquid water fraction of
the melting snowflakes. These semi-melted hydrometeors have characteristics
(e.g., shape and fall speed) that differ from those of dry snow and rain
droplets. Where possible, theoretical considerations and results from
vertical wind tunnel laboratory experiments of melting snowflakes are used as
the basis for characterising the melting snow as a function of its liquid
water fraction. These characteristics include the capacitance, the
ventilation coefficient, and the terminal fall speed. For the bulk
parametrization, a critical diameter is introduced. It is assumed that
particles smaller than this diameter, which increases during the melting
process, have completely melted, i.e., they are converted to the rain
category. The values of the bulk integrals are calculated with a finite
difference method and approximately represented by polynomial functions,
which allows an efficient implementation of the parametrization. Two case
studies of (wet) snowfall in Germany are presented to illustrate the
potential of the new snow melting parametrization. It is shown that the new
scheme (i) produces wet snow instead of rain in some regions with surface
temperatures slightly above the freezing point, (ii) simulates realistic
atmospheric melting layers with a gradual transition from dry snow to melting
snow to rain, and (iii) leads to a slower snow melting process. In the
future, it will be important to thoroughly validate the scheme, also with
radar data and to further explore its potential for improved surface
precipitation forecasts for various meteorological conditions. |
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