Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
1680-7316
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8
19
2008
10.5194/acp-8-5771-2008
http://www.atmos-chem-phys.net/8/5771/2008/
http://www.atmos-chem-phys.net/8/5771/2008/acp-8-5771-2008.html
http://www.atmos-chem-phys.net/8/5771/2008/acp-8-5771-2008.pdf
5771
5786
2008-10-02
Direct SW aerosol radiative forcing over Portugal
D. Santos
dinas@uevora.pt
M. J. Costa
A. M. Silva
Geophysics Centre of Évora, Univ. Évora, Évora, Portugal
Departament of Physics, Univ. Évora, Évora, Portugal
In this work, the evaluation of the aerosol radiative forcing at the top of
the atmosphere as well as at the surface over the south of Portugal is made,
particularly in the regions of Évora (38°34' N, 7°54' W) and of
Cabo da Roca (38°46' N, 9°38' W), during years 2004 and 2005.
<br><br>
The radiative transfer calculations, using the radiative transfer code
Second Simulation of the Satellite Signal in the Solar Spectrum (6S),
combine ground-based measurements, from Aerosol Robotic NETwork (AERONET),
and satellite measurements, from MODerate Imaging Spectroradiometer (MODIS),
to estimate the direct SW aerosol radiative forcing. The method developed to
retrieve the surface spectral reflectance is also presented, based on
ground-based measurements (AERONET) of the aerosol optical properties
combined with the satellite-measured radiances (MODIS).
<br><br>
The instantaneous direct SW aerosol radiative forcing values obtained at the
top of the atmosphere are, in the majority of the cases, negative,
indicating a tendency for cooling the Earth at the top of the atmosphere.
For Desert Dust aerosols, over the Évora land region, the average
forcing efficiency is estimated to be −25 Wm<sup>−2</sup>/AOT<sub>0.55</sub> whereas for
the Cabo da Roca area, the average forcing efficiency is −46 Wm<sup>−2</sup>/AOT<sub>0.55</sub>.
In the presence of Forest Fire aerosols, both from
short and long distances, the average value of forcing efficiency at the top
of the atmosphere over Cabo da Roca is found to be −28 Wm<sup>−2</sup>/AOT<sub>0.55</sub> and, over Évora,
−27 Wm<sup>−2</sup>/AOT<sub>0.55</sub>. For
specific situations, discussed in this work, the average surface direct SW
aerosol radiative forcing efficiency due to the Desert Dust aerosols, in
Évora region, is −66 Wm<sup>−2</sup>/AOT<sub>0.55</sub>, whereas in Cabo da Roca
region, the corresponding average value is −38 Wm<sup>−2</sup>/AOT<sub>0.55</sub>. Considering the Forest Fire aerosols,
over Évora region, the average surface direct
SW aerosol radiative forcing efficiency can vary between −36 and −113 Wm<sup>−2</sup>/AOT<sub>0.55</sub>, the
more negative value corresponding to forest fire aerosols coming only from shorter distances.
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