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| Titel |
Aircraft observations of aerosol, cloud, precipitation, and boundary layer properties in pockets of open cells over the southeast Pacific |
| VerfasserIn |
C. R. Terai, C. S. Bretherton, R. Wood, G. Painter |
| 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. 15 ; Nr. 14, no. 15 (2014-08-13), S.8071-8088 |
| Datensatznummer |
250118941
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| Publikation (Nr.) |
 copernicus.org/acp-14-8071-2014.pdf |
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| Zusammenfassung |
Five pockets of open cells (POCs) are studied using aircraft flights from the
VOCALS Regional Experiment (VOCALS-REx), conducted in October and November 2008 over the
southeast Pacific Ocean. Satellite imagery from the geostationary satellite
GOES-10 is used to distinguish POC areas, and measurements from the aircraft
flights are used to compare aerosol, cloud, precipitation, and boundary layer
conditions inside and outside of POCs. Conditions observed across individual
POC cases are also compared.
POCs are observed in boundary layers with a wide range of inversion heights
(1250 to 1600 m) and surface wind speeds (5 to 11 m s−1) and
show no remarkable difference from the observed surface and free-tropospheric
conditions during the two months of the field campaign. In all cases,
compared to the surrounding overcast region the POC boundary layer is more
decoupled, supporting both thin stratiform and deeper cumulus clouds.
Although cloud-base precipitation rates are higher in the POC than the
overcast region in each case, a threshold precipitation rate that
differentiates POC precipitation from overcast precipitation does not exist.
Mean cloud-base precipitation rates in POCs can range from 1.7 to
5.8 mm d−1 across different POC cases. The occurrence of heavy
drizzle (> 0 dBZ) lower in the boundary layer better differentiates POC
precipitation from overcast precipitation, likely leading to the more active
cold pool formation in POCs. Cloud droplet number concentration is at least a
factor of 8 smaller in the POC clouds, and the ratio of drizzle water to
cloud water in POC clouds is over an order of magnitude larger than that in
overcast clouds, indicating an enhancement of collision–coalescence
processes in POC clouds.
Despite large variations in the accumulation-mode aerosol concentrations
observed in the surrounding overcast region (65 to 324 cm−3), the
accumulation-mode aerosol concentrations observed in the subcloud layer of
all five POCs exhibit a much narrower range (24 to 40 cm−3), and
cloud droplet concentrations within the cumulus updrafts originating in this
layer reflect this limited variability. Above the POC subcloud layer exists
an ultraclean layer with accumulation-mode aerosol concentrations
< 5 cm−3, demonstrating that in-cloud collision–coalescence
processes efficiently remove aerosols. The existence of the ultraclean layer
also suggests that the major source of accumulation-mode aerosols, and hence
of cloud condensation nuclei in POCs, is the ocean surface, while entrainment
of free-tropospheric aerosols is weak. The measurements also suggest that at
approximately 30 cm−3 a balance of surface source and coalescence
scavenging sinks of accumulation-mode aerosols maintain the narrow range of
observed subcloud aerosol concentrations. |
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