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| Titel |
Development and impact of hooks of high droplet concentration on remote southeast Pacific stratocumulus |
| VerfasserIn |
R. C. George, R. Wood, C. S. Bretherton, 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 ; 13, no. 13 ; Nr. 13, no. 13 (2013-07-04), S.6305-6328 |
| Datensatznummer |
250018735
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| Publikation (Nr.) |
 copernicus.org/acp-13-6305-2013.pdf |
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| Zusammenfassung |
Over the southeastern Pacific (SEP), droplet concentration
(Nd) in the typically unpolluted marine stratocumulus
west of 80° W (> 1000 km offshore) is periodically
strongly enhanced in zonally elongated "hook"-shaped features that
increase albedo. Here, we examine three hook events using the
chemistry version of the Weather Research and Forecasting model
(WRF-Chem) with 14 km horizontal resolution, satellite data,
and aircraft data from the VAMOS Ocean-Cloud-Atmosphere-Land Study
Regional Experiment (VOCALS-REx). A particularly strong hook yields
insights into the development, decay, and radiative impact of these
features. Hook development occurs with Nd increasing to
polluted levels over the remote ocean primarily due to entrainment of
cloud condensation nuclei (CCN) from the lower free troposphere (FT). The
feature advects northwestward until the FT CCN source is depleted,
after which Nd decreases over a few days due to
precipitation and dilution. The model suggests that the FT CCN source
supplying the hook consists of high concentrations of small
accumulation-mode aerosols that contribute a relatively small amount
of aerosol mass to the MBL, in agreement with near-coast VOCALS measurements of polluted layers
in the FT. The aerosol particles in this hook originate mainly from a pulse of offshore flow
that transports Santiago-region (33–35° S) emissions to the remote marine FT.
To provide pollution CCN that can sustain hooks, the FT transport of pollution plumes to the remote ocean requires strong, deep offshore flow. Such
flow is favored by a trough approaching the South American coast and
a southeastward shift of the climatological subtropical high-pressure
system. The model simulations show precipitation suppression in the
hook and a corresponding increase in liquid water path (LWP) compared
with a simulation without anthropogenic sources. LWP also increases as the hook evolves over time due to increasing stability and decreasing
subsidence. WRF-Chem suggests that dimethyl sulfide (DMS) significantly influences the
aerosol number and size distributions in a hook, but that hooks do not
form without FT CCN. The Twomey effect contributes
~ 50–70% of the albedo increase due to the presence of the
hook, while secondary aerosol indirect effects and meteorological
influences also contribute significantly.
The source of hook aerosols is difficult to determine with the
available observations alone. The model provides further explanation of the factors influencing hook formation. Two other weaker hooks during VOCALS-REx are not as well simulated but are
also associated with FT offshore flow near Santiago. Hooks demonstrate
the importance of free-tropospheric transport of aerosols in
modulating the droplet concentration in the southeastern Pacific
stratocumulus deck, and present a formidable challenge to simulate
accurately in large-scale models. |
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