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| Titel |
Arctic low-level boundary layer clouds: in situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the top layer of liquid phase clouds |
| VerfasserIn |
M. Klingebiel, A. de Lozar, S. Molleker, R. Weigel, A. Roth, L. Schmidt, J. Meyer, A. Ehrlich, R. Neuber, M. Wendisch, S. Borrmann |
| 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 ; 15, no. 2 ; Nr. 15, no. 2 (2015-01-16), S.617-631 |
| Datensatznummer |
250119329
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| Publikation (Nr.) |
 copernicus.org/acp-15-617-2015.pdf |
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| Zusammenfassung |
| Aircraft borne optical in situ size distribution measurements were performed
within Arctic boundary layer clouds with a special emphasis on the cloud top
layer during the VERtical Distribution of Ice in Arctic clouds (VERDI)
campaign in April and May 2012. An instrumented Basler BT-67 research
aircraft operated out of Inuvik over the Mackenzie River delta and the
Beaufort Sea in the Northwest Territories of Canada. Besides the cloud
particle and hydrometeor size spectrometers the aircraft was equipped with
instrumentation for aerosol, radiation and other parameters. Inside the
cloud, droplet size distributions with monomodal shapes were observed for
predominantly liquid-phase Arctic stratocumulus. With increasing altitude
inside the cloud the droplet mean diameters grew from 10 to 20 μm.
In the upper transition zone (i.e., adjacent to the cloud-free air aloft)
changes from monomodal to bimodal droplet size distributions (Mode 1 with
20 μm and Mode 2 with 10 μm diameter) were observed. It
is shown that droplets of both modes co-exist in the same (small) air volume
and the bimodal shape of the measured size distributions cannot be explained
as an observational artifact caused by accumulating data point populations
from different air volumes. The formation of the second size mode can be
explained by (a) entrainment and activation/condensation of fresh aerosol
particles, or (b) by differential evaporation processes occurring with cloud
droplets engulfed in different eddies. Activation of entrained particles
seemed a viable possibility as a layer of dry Arctic enhanced background
aerosol (which was detected directly above the stratus cloud) might form a
second mode of small cloud droplets. However, theoretical considerations and
model calculations (adopting direct numerical simulation, DNS) revealed that,
instead, turbulent mixing and evaporation of larger droplets are the most
likely reasons for the formation of the second droplet size mode in the
uppermost region of the clouds. |
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