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| Titel |
Quantifying the climatological cloud-free direct radiative forcing of aerosol over the Red Sea |
| VerfasserIn |
Helen Brindley, Serega Osipov, Richard Bantges, Alexander Smirnov, Jamie Banks, Robert Levy, P.-Jish Prakash, Georgiy Stenchikov |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250111606
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| Publikation (Nr.) |
 EGU/EGU2015-11742.pdf |
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| Zusammenfassung |
| A combination of ground-based and satellite observations are used, in conjunction with
column radiative transfer modelling, to assess the climatological aerosol loading and quantify
its corresponding cloud-free direct radiative forcing (DRF) over the Red Sea. While
there have been campaigns designed to probe aerosol-climate interactions over
much of the world, relatively little attention has been paid to this region. Because of
the remoteness of the area, satellite retrievals provide a crucial tool for assessing
aerosol loading over the Sea. However, agreement between aerosol properties inferred
from measurements from different instruments, and even in some cases from the
same measurements using different retrieval algorithms can be poor, particularly
in the case of mineral dust. Ground based measurements which can be used to
evaluate retrievals are thus highly desirable. Here we take advantage of ship-based
sun-photometer micro-tops observations gathered from a series of cruises which took place
across the Red Sea during 2011 and 2013. To our knowledge these data represent
the first set of detailed aerosol measurements from the Sea. They thus provide a
unique opportunity to assess the performance of satellite retrieval algorithms in this
region.
Initially two aerosol optical depth (AOD) retrieval algorithms developed for the
MODerate Resolution Imaging Spectroradiometer (MODIS) and Spinning Enhanced Visible
and InfraRed Imager (SEVIRI) instruments are evaluated via comparison with the
co-located cruise observations. These show excellent agreement, with correlations
typically better than 0.9 and very small root-mean-square and bias differences.
Calculations of radiative fluxes and DRF along one of the cruises using the observed
aerosol and meteorological conditions also show good agreement with co-located
estimates from the Geostationary Earth Radiation Budget (GERB) instrument if
the aerosol asymmetry parameter is adjusted to account for the presence of large
particles.
A monthly climatology of AOD over the Red Sea is then created from 5yrs of SEVIRI
retrievals and shows both enhanced aerosol loading and the development of a distinct north to
south gradient across the basin in the summer relative to the winter months. The climatology
is used as input to radiative transfer calculations to generate corresponding estimates of the
DRF at the top and bottom of the atmosphere and the atmospheric absorption due to
aerosol. These estimates indicate that although longwave effects can reach 10s
W m-2, shortwave cooling typically dominates the net radiative effect over the
basin and is particularly pronounced in the summer, exceeding 130 W m-2 at the
surface. The spatial gradient in summer-time AOD is reflected in both the aerosol
forcing at the surface and in associated differential heating by aerosol within the
atmosphere above the Sea. This asymmetric forcing would be expected to exert a
significant influence on the regional atmospheric and oceanic circulation and warrants
further study in the context of coupled aerosol-atmosphere-ocean regional models. |
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