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| Titel |
The potential influence of Asian and African mineral dust on ice, mixed-phase and liquid water clouds |
| VerfasserIn |
A. Wiacek, T. Peter, U. Lohmann |
| 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 ; 10, no. 18 ; Nr. 10, no. 18 (2010-09-16), S.8649-8667 |
| Datensatznummer |
250008774
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| Publikation (Nr.) |
 copernicus.org/acp-10-8649-2010.pdf |
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| Zusammenfassung |
| This modelling study explores the availability of mineral dust particles as
ice nuclei for interactions with ice, mixed-phase and liquid water clouds,
also tracking the particles' history of cloud-processing. We performed
61 320 one-week forward trajectory calculations originating near the surface
of major dust emitting regions in Africa and Asia using high-resolution
meteorological analysis fields for the year 2007. Dust-bearing trajectories
were assumed to be those coinciding with known dust emission seasons,
without explicitly modelling dust emission and deposition processes. We
found that dust emissions from Asian deserts lead to a higher potential for
interactions with high ice clouds, despite being the climatologically much
smaller dust emission source. This is due to Asian regions experiencing
significantly more ascent than African regions, with strongest ascent in the
Asian Taklimakan desert at ~25%, ~40% and 10% of
trajectories ascending to 300 hPa in spring, summer and fall, respectively.
The specific humidity at each trajectory's starting point was transported in
a Lagrangian manner and relative humidities with respect to water and ice
were calculated in 6-h steps downstream, allowing us to estimate the
formation of liquid, mixed-phase and ice clouds. Downstream of the
investigated dust sources, practically none of the simulated air parcels
reached conditions of homogeneous ice nucleation (T≲−40 °C)
along trajectories that have not experienced water saturation first. By
far the largest fraction of cloud forming trajectories entered conditions of
mixed-phase clouds, where mineral dust will potentially exert the biggest
influence. The majority of trajectories also passed through atmospheric
regions supersaturated with respect to ice but subsaturated with respect to
water, where so-called "warm ice clouds" (T≳−40 °C)
theoretically may form prior to supercooled water or mixed-phase clouds. The
importance of "warm ice clouds" and the general influence of dust in the
mixed-phase cloud region are highly uncertain due to both a considerable
scatter in recent laboratory data from ice nucleation experiments, which we
briefly review in this work, and due to uncertainties in sub-grid scale
vertical transport processes unresolved by the present trajectory analysis.
For "classical" cirrus-forming temperatures (T≲−40 °C),
our results show that only mineral dust ice nuclei that underwent
mixed-phase cloud-processing, most likely acquiring coatings of organic or
inorganic material, are likely to be relevant. While the potential paucity
of deposition ice nuclei shown in this work dimishes the possibility of
deposition nucleation, the absence of liquid water droplets at
T≲−40 °C makes the less explored contact freezing
mechanism (involving droplet collisions with bare ice nuclei) highly
inefficient. These factors together indicate the necessity of further
systematic studies of immersion mode ice nucleation on mineral dust
suspended in atmospherically relevant coatings. |
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