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| Titel |
The first aerosol indirect effect quantified through airborne remote sensing during VOCALS-REx |
| VerfasserIn |
D. Painemal, P. Zuidema |
| 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. 2 ; Nr. 13, no. 2 (2013-01-22), S.917-931 |
| Datensatznummer |
250017608
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| Publikation (Nr.) |
 copernicus.org/acp-13-917-2013.pdf |
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| Zusammenfassung |
The first aerosol indirect effect (1AIE) is investigated using a combination of
in situ and remotely-sensed aircraft (NCAR C-130) observations acquired
during VOCALS-REx over the southeast Pacific stratocumulus cloud regime.
Satellite analyses have previously identified a high albedo susceptibitility
to changes in cloud microphysics and aerosols over this region. The 1AIE was
broken down into the product of two independently-estimated terms: the cloud
aerosol interaction metric ACIτ =dlnτ/dlnNa|LWP , and the relative albedo (A) susceptibility SR-τ
=dA/3dlnτ|LWP, with τ and Na
denoting retrieved cloud optical thickness and in situ aerosol concentration
respectively and calculated for fixed intervals of liquid water path (LWP).
ACIτ was estimated by combining in situ Na sampled below the cloud, with
τ and LWP derived from, respectively, simultaneous upward-looking
broadband irradiance and narrow field-of-view millimeter-wave radiometer
measurements, collected at 1 Hz during four eight-hour daytime flights by
the C-130 aircraft. ACIτ values were typically large, close to the
physical upper limit (0.33), with a modest increase with LWP. The high
ACIτ values slightly exceed values reported from many previous in situ
airborne studies in pristine marine stratocumulus and reflect the imposition
of a LWP constraint and simultaneity of aerosol and cloud measurements.
SR-τ increased with LWP and τ, reached a maximum SR-τ (0.086) for LWP (τ) of 58 g m−2 (~14), and
decreased slightly thereafter. The 1AIE thus increased with LWP and is
comparable to a radiative forcing of −3.2– −3.8 W m−2 for a 10%
increase in Na, exceeding previously-reported global-range values. The
aircraft-derived values are consistent with satellite estimates derived from
instantaneous, collocated Clouds and the Earth's Radiant Energy System
(CERES) albedo and MOderate resolution Imaging Spectroradiometer
(MODIS)-retrieved droplet number concentrations at 50 km resolution. The
consistency of the airborne and satellite estimates, despite their
independent approaches, differences in observational scales, and retrieval
assumptions, is hypothesized to reflect the ideal remote sensing conditions
for these homogeneous clouds. We recommend the southeast Pacific for
regional model assessments of the first aerosol indirect effect on this
basis. This airborne remotely-sensed approach towards quantifying 1AIE
should in theory be more robust than in situ calculations because of
increased sampling. However, although the technique does not explicitly
depend on a remotely-derived cloud droplet number concentration (Nd),
the at-times unrealistically-high Nd values suggest more emphasis on
accurate airborne radiometric measurements is needed to refine this
approach. |
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