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Titel |
On the use of satellite remote sensing to determine aerosol direct radiative effect over land: A case study over China |
VerfasserIn |
Anu-Maija Sundström, Antti Arola, Pekka Kolmonen, Gerrit de Leeuw |
Konferenz |
EGU General Assembly 2014
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 16 (2014) |
Datensatznummer |
250092621
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Publikation (Nr.) |
EGU/EGU2014-6977.pdf |
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Zusammenfassung |
The quantification of aerosol radiative effects is complex and large uncertainties still exist,
mainly due to the high spatial and temporal variation of the aerosol concentration and mass as
well as their relatively short lifetime in the atmosphere. In this work a multi-sensor
satellite based approach is studied for defining the direct short wave aerosol radiative
effect (ADRE) over China. ADRE at the top of the atmosphere (TOA) is defined as
the difference between the net solar flux with (F ) and without (F0) aerosols. The
negative values of ADRE correspond to increased outgoing radiation and planetary
cooling, whereas positive values correspond to decreased outgoing radiation at
TOA and increased atmospheric warming. To derive instantaneous ADRE from the
satellite observations, the challenge is to estimate the value for F0. In this work F0 is
derived using the colocated observations of CERES (Clouds and the Earth’s Radian
Energy System) short wave broad band TOA-flux for cloud free sky and MODIS
(Moderate Imaging Spectroradiometer) aerosol optical depth (AOD). Assuming
that aerosol type does not change systematically within a 0.5 deg. grid cell over a
month, a linear relationship is established between the clear-sky TOA-fluxes and
AODs. Using the linear regression an estimate for instantaneous monthly F0 can
be obtained by extrapolating the line to AOD=0, while F is the monthly mean
of cloud free CERES observations. However, there are several other parameters
affecting the observed TOA flux than the aerosol loading and aerosol type, such
as solar zenith angle, water vapour, land surface albedo and Earth-Sun distance.
Changes in these parameters within a grid cell over a month inflect the correlation
between AOD and TOA fluxes. To minimize the effect of zenith angle, water vapour,
and Earth-Sun distance the CERES fluxes are normalized before the linear fitting
using reference fluxes calculated with a radiative transfer code (Libradtran). The
normalization, especially to a fixed zenith angle increases the correlation between
TOA flux and AOD significantly. For a comparison the F0 is also modeled using
Libradtran. Comparison shows that the modeled aerosol-free fluxes are mainly 5-10
Wm-2 lower than the estimate obtained from the linear fitting, but on the other
hand over bright surfaces the satellite based estimate is lower than the modeled
F0. Nevertheless, the fitting method in most of the cases produces qualitatively
similar results for instantaneous ADRE than what is obtained with modeled F0.
In some cases, the satellite based method gives positive ADRE over areas where
it is expected to be negative. This is most probably a method failure, related to
either subvisible cirrus contamination, systematic change of aerosol type or both. |
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