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| Titel |
Absorbing aerosols above clouds: Getting hold of the direct aerosol effect |
| VerfasserIn |
Karsten Peters, J. Quaas |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250020758
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| Zusammenfassung |
| Aerosol effects, direct as well as indirect, constitute the biggest
uncertainties when it comes to quantifying human induced climate change.
Understanding these will thus increase the credibility of climate
predictions.
This study focuses on the direct aerosol effect when absorbing aerosols
reside above clouds. In cloud-free conditions, absorbing aerosols
usually exert a negative radiative forcing at TOA due to an overall
enhancement of the shortwave local planetary albedo (LPA). When located
above clouds, these aerosols can potentially reduce the shortwave LPA,
resulting in an often significant local positive radiative forcing.
Investigation of these situations on a global scale is particularly
interesting, since this has not been done to satisfaction using
measurements only.
A method for deriving the direct aerosol effect of absorbing aerosols
above clouds solely from measurements is presented in this study. Global
data of 2005 and 2006 from a number of sensors aboard satellites of the
"A-Train" constellation is used, since these guarantee a very good
correlation in time and space. Aerosol properties are taken from MODIS
and OMI, cloud properties from MODIS and AMSR-E and radiation
measurements from CERES (all on board EOS-Aqua, except for OMI on board
EOS-Aura). A multiple linear regression is used to identify the
dependence of the LPA in cloudy scenes on aerosol optical depth (AOD),
using the OMI UV-Aerosolindex (UV-AI) as an indicator.
The results show a decrease of the LPA with increasing amount of
absorbing aerosols in cloudy scenes, demonstrating that it is feasible
to derive the direct aerosol effect of absorbing aerosols above clouds
using a combination of remote sensing instruments. The direct radiative
forcing of anthropogenic absorbing aerosols in cloudy scenes is then
computed, showing pronounced positive radiative forcing values
especially off the coast of southern Africa, depending on the season. |
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