PhD Defense

The aim of this work is to provide a comprehensive analysis of cloud and aerosol interaction over South-East Atlantic, to quantify the overall aerosol impact on the regional radiation budget. We used data from MODIS, PARASOL and CALIPSO satellites, that fly in close proximity on the same sun-synchronous orbit and allow for complementary observations of the same portion of the atmosphere, within a few minutes.

The main idea is to use CALIPSO vertical information to define whether or not aerosol and cloud layers observed by MODIS and PARASOL are mixed and interacting. We found evidences that, in case of interaction, cloud properties are strongly influenced by aerosol presence (first indirect effect). In particular, there is a decrease in cloud droplet effective radius and liquid water path with aerosol enhancement. On the other hand, we could not evidence any significant impact on the cloud reflectance.

We also analyzed the aerosol impact on precipitation (second indirect effect). In polluted low clouds over the ocean, we found evidence of precipitation suppression and cloud cover increase with increasing aerosol concentration. On the other hand, cloud fraction is shown to be affected by aerosol presence, even if pollution particles are located above cloud top, without physical interaction. This observation is interpreted as a consequence of the aerosol radiative effect.

Aerosol shortwave direct (DRF) and indirect (IRF) radiative forcing at TOA has been quantified, with the use of a radiative transfer model constrained by satellite observations. For the direct effect, there is a competition between cooling (negative, due to light scattering by the aerosols) and warming (positive, due to the absorption by the same particles). The regional six year (2005-2010), the spatial mean and the standard deviation is equal to -0.07±8.03 W/m² for DRF and -0.05±0.54 W/m² for IRF. Indirect forcing results from the balance of cloud albedo effect (-0.07±0.55 W/m²) and life time effect (0.02±0.12 W/m²). Total aerosol forcing is then negative (cooling) and equal to -0.12±8.02 W/m².