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| Titel |
A glaciation indirect aerosol effect in a statistical analysis of modeled mixed-phase orographic precipitation over the Alps |
| VerfasserIn |
Elias Zubler, Ulrike Lohmann, Daniel Lüthi, Andreas Muhlbauer, Christoph Schär |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038481
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| Zusammenfassung |
| Aerosols act as cloud condensation or ice nuclei, altering the microphysical properties of
clouds and hydrometeors. Increasing the aerosol number in warm-phase clouds is thought to
cause a decrease in the rate of rain formation, whereas the physical processes that
affect a mixed-phase cloud and precipitation particles such as ice-crystals, graupel
and snowflakes, are more uncertain. Recent studies have shown that increasing
number concentrations of soluble aerosols may also reduce the riming efficiency and
therefore decrease precipitation. On the other hand, the glaciation of a cloud due to
heterogeneous freezing of cloud droplets may enhance the formation of graupel and
snow.
Using a numerical weather prediction model coupled to a 2-moment cloud and
aerosol microphysics at a horizontal resolution of 2.2Â km, we show in a statistical
framework with 310 2D-simulations of mixed-phase orographic precipitation over
complex terrain that the presence of the ice-phase determines the magnitude and the
sign of the effect of an increasing aerosol number concentration on orographic
precipitation. We show that this indirect aerosol effect is much less pronounced in
cold simulations compared to a warmer subset, and that cloud glaciation tends to
compensate and even overcome the loss of rain in polluted situations. Comparing the
simulated pairs of clean and polluted cases, we find a reduction of rain by 23% on
average (range of relative differences: -60% to 0%) in the polluted cases due to the
decline of autoconversion and accretion rates. In the cold subset of our simulations, a
much broader range of differences (standard deviation: 64%) and a tendency toward
increasing precipitation (mean: +17%) in the polluted cases is found through enhanced
ice-microphysical processes such as riming and aggregation of graupel and snow,
respectively. Furthermore, our study shows that in comparison with the clean cases
approximately 3.5-5% more precipitation spills over to the leeward side of the mountains in
the polluted cases as a consequence of the deceleration of precipitation formation. |
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