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Titel |
Estimating the direct aerosol radiative effect over China using multi-sensor satellite remote sensing measurements |
VerfasserIn |
Anu-Maija Sundström, Antti Arola, Pekka Kolmonen, Gerrit de Leeuw |
Konferenz |
EGU General Assembly 2013
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 15 (2013) |
Datensatznummer |
250078942
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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 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 TOA-flux and AOD when AOD < 2.0. Using the
linear fit an estimate for F0 can be obtained and F is the monthly mean of 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. 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 theF0 is also
modeled using Libradtran. Comparison shows that the modeled aerosol-free fluxes are
mainly 5 - 10Wm-2 lower than the estimate 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 over the region
of interest. 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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