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| Titel |
On the Variability of Giant Sea-Salt Size Distributions as Observed from Aircraft during the VOCALS Campaign: Relationship to Marine Stratocumulus, Cold Pools and Implications for Drizzle Formation |
| VerfasserIn |
J. Jensen |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070664
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| Zusammenfassung |
| This study presents a quantum leap in the observations of marine sea-salt size distributions.
The observations are obtained with a new instrument, the Giant Nucleus Impactor (GNI),
which consists of polycarbonate slides exposed in the free airstream outside an aircraft,
followed by storage in desiccated test tubes. The slides are subsequently analyzed in the
laboratory, using a humidified automatic optical digital microscope system. The instrument
has been extensively characterized in terms of uncertainty.
450 slides, each with typical sample volumes of 300 liters and ~50,000 particles per
slide, were exposed from the NSF/NCAR C-130 research aircraft during the 2008 VOCALS
campaign over the Pacific Ocean off the coast of Chile. The observed particle size range is
0.7 μm – 12 μm dry radius range for relatively low wind speeds of 1-14 m/s in the marine
boundary layer.
Figure 1 shows the aerosol mass loading as a function of wind speed for 286 slides that
were exposed within a narrow altitude range 120 – 180 m above the sea surface. All slides
obtained within a single flight are shown with the same symbol. The figure shows a general
trend of higher mass loading with increasing wind speed, but at the same time a clustering of
the measurements obtained within a given day. This suggest that wind speed is not the sole
determinant of the giant sea salt mass loading, but the action of other factors, possibly cold
pools or wave fields that are not in balance with the wind at the time of the slide
exposures.
The implications for drizzle precipitation formation are considerable. The concentration
of the largest of the giant sea-salt particles are related to the observations of drizzle drops in
the VOCALS stratocumulus in the vicinity of the exposed slides. In-cloud observations
of cloud droplet size spectra as well as drizzle drop spectra shows two important
conclusions:
In lightly drizzling clouds (low drizzle drop concentrations of a few per litre), the drizzle
drop concentrations can be explained if drizzle drops form on all particles larger than about 4
μm dry radius.
In more intense drizzling clouds (drizzle drop concentrations of about 100 per litre),
almost all giant sea-salt particles with dry radius > 1 μm dry radius are needed to explain the
observed drizzle drop concentrations. Even so, these intensely precipitating clouds are also
characterized by large cloud droplets.
The implementation of giant sea-salt particles as nuclei for warm rain in large-scale
models is implemented using a “toss-box” model, which includes both condensational and
coalescence growth. |
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