Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys. Discuss., 13, 23295-23324, 2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
05 Sep 2013
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). A final paper in ACP is not foreseen.
Satellite-based estimate of aerosol direct radiative effect over the South-East Atlantic
L. Costantino and F.-M. Bréon
Laboratoire des Sciences du Climat et de l'Environnement Unité Mixte de Recherche CEA-CNRS-UVSQ, UMR8212, 91191 Gif sur Yvette, France

Abstract. The net effect of aerosol Direct Radiative Forcing (DRF) is the balance between the scattering effect that reflects solar radiation back to space (cooling), and the absorption that decreases the reflected sunlight (warming). The amplitude of these two effects and their balance depends on the aerosol load, its absorptivity, the cloud fraction and the respective position of aerosol and cloud layers.

In this study, we use the information provided by CALIOP (CALIPSO satellite) and MODIS (AQUA satellite) instruments as input data to a Rapid Radiative Transfer Model (RRTM) and quantify the shortwave (SW) aerosol direct atmospheric forcing, over the South-East Atlantic. The combination of the passive and active measurements allows estimates of the horizontal and vertical distributions of the aerosol and cloud parameters. We use a parametrization of the Single Scattering Albedo (SSA) based on the satellite-derived Angstrom coefficient.

The South East Atlantic is a particular region, where bright stratocumulus clouds are often topped by absorbing smoke particles. Results from radiative transfer simulations confirm the similar amplitude of the cooling effect, due to light scattering by the aerosols, and the warming effect, due to the absorption by the same particles. Over six years of satellite retrievals, from 2005 to 2010, the South-East Atlantic all-sky SW DRF is −0.03 W m−2, with a spatial standard deviation of 8.03 W m−2. In good agreement with previous estimates, statistics show that a cloud fraction larger than 0.5 is generally associated with positive all-sky DRF. In case of cloudy-sky and aerosol located only above the cloud top, a SSA larger than 0.91 and cloud optical thickness larger than 4 can be considered as threshold values, beyond which the resulting radiative forcing becomes positive.

Citation: Costantino, L. and Bréon, F.-M.: Satellite-based estimate of aerosol direct radiative effect over the South-East Atlantic, Atmos. Chem. Phys. Discuss., 13, 23295-23324, doi:10.5194/acpd-13-23295-2013, 2013.
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