Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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8
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2008
10.5194/acpd-8-8585-2008
http://www.atmos-chem-phys-discuss.net/8/8585/2008/
http://www.atmos-chem-phys-discuss.net/8/8585/2008/acpd-8-8585-2008.html
http://www.atmos-chem-phys-discuss.net/8/8585/2008/acpd-8-8585-2008.pdf
8585
8628
2008-05-09
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
The estimation of radiative forcing due to desert dust and forest fires
aerosols is a very important issue since these particles are very efficient
at scattering and absorbing both short and longwave radiation. In this work,
the evaluation of the aerosol radiative forcing at the top of the atmosphere
over the south of Portugal is made, particularly in the regions of Évora
and of Cabo da Roca.
<br></br>
The radiative transfer calculations combine ground-based and satellite
measurements, to estimate the top of the atmosphere direct SW aerosol
radiative forcing. The method developed to retrieve the surface spectral
reflectance is also presented, based on ground-based measurements of the
aerosol optical properties combined with the satellite-measured radiances.
<br></br>
The aerosol direct radiative effect is shown to be very sensitive to the
underlying surface, since different surface spectral reflectance values may
originate different forcing values. The results obtained also illustrate the
importance of considering the actual aerosol properties, in this case
measured by ground-based instrumentation, particularly the aerosol single
scattering albedo, because different aerosol single scattering albedo values
can flip the sign of the direct SW aerosol radiative forcing.
<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. For Desert Dust aerosols, over Évora land region, the average forcing efficiency is
estimated to be −25 W/m<sup>2</sup>/AOT<sub>0.55</sub> whereas for Cabo da Roca area, the average forcing
efficiency is −46 W/m<sup>2</sup>/AOT<sub>0.55</sub>. In the presence of Forest Fire aerosols, over Cabo da Roca
region, the average value of forcing efficiency is −28 W/m<sup>2</sup>/AOT<sub>0.55</sub> and over Évora region an
average value of −33 W/m<sup>2</sup>/AOT<sub>0.55</sub> is found.
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