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| Titel |
Influence of heterogeneous freezing on the microphysical and radiative properties of orographic cirrus clouds |
| VerfasserIn |
H. Joos, P. Spichtinger, P. Reutter, F. Fusina |
| 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. 13 ; Nr. 14, no. 13 (2014-07-04), S.6835-6852 |
| Datensatznummer |
250118866
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| Publikation (Nr.) |
 copernicus.org/acp-14-6835-2014.pdf |
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| Zusammenfassung |
| The influence of heterogeneous freezing on the microphysical and optical
properties of orographic cirrus clouds has been simulated with the large eddy
simulation model EULAG. Idealised simulations with different concentrations
of ice nuclei (IN) in a dynamically dominated regime with high vertical
velocities have been performed. Furthermore the temperature at cloud
formation as well as the critical supersaturation for initiation of
heterogenous freezing have been varied. The shortwave, longwave and net
cloud forcing has been calculated under the assumption that the clouds form
between 06:00 and 12:00 local time (LT) or between 12:00 and 18:00 LT. In
general it can be seen that the onset of homogeneous freezing is shifted in
time depending on the IN concentration, as part of the available water vapour
is depleted before the critical threshold for homogeneous freezing is
reached. Although the high vertical velocities in an orographic gravity wave
lead to a strong adiabatic cooling followed by high ice supersaturations, even a
small number concentration of IN of the order of 5 L−1 is
able to strongly decrease the simulated ice crystal number burden (ICNB), ice
water path (IWP) and optical depth of the cloud. In general,
the ICNB, IWP and optical depth strongly decrease when the IN concentrations
are increased from 0 to 50 L−1. The absolute values of the
shortwave, longwave and net cloud forcing are also reduced with increasing IN
concentrations. A cloud will produce a net warming or cooling depending on
the IN concentration, the temperature and the time of day when the cloud
forms. The clouds that form between 06:00 and 12:00 LT are mainly cooling,
whereas the clouds with the same microphysical properties can lead to a
warming when they form between 12:00 and 18:00 LT. In order to predict the
radiative forcing of cirrus clouds it is therefore necessary to take the
correct dynamical and thermodynamical processes as well as the possible
existence and freezing threshold of heterogeneous IN into account, not only
for low vertical velocities but also for dynamically dominated regimes like
orographic cirrus. |
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