Observation of the direct effect of aerosols in the South-East Atlantic at high temporal resolution from MSG/SEVIRI

The direct radiative forcing due to aerosols remains highly uncertain in the South-East Atlantic. During the fire season, absorbing aerosols from the African biomass burning are transported towards the ocean where they are frequently observed above the semi-permanent layer of stratocumulus. Biomass burning aerosols above an ocean surface will exert a negative radiative forcing, while over a highly reflectant cloud, will exert a positive radiative forcing. As a result, the direct forcing of aerosols is expected to be highly influenced by the strong diurnal cycle of the clouds in this region.

Until recently, passive satellite observations of aerosols were limited to cloud-free scenes as aerosol and cloud retrievals used to be mutually exclusive. Also, cloud properties retrieved from shortwave passive sensors are expected to be biased in the South-East Atlantic because operational retrievals do not account for the impact of above-cloud aerosol absorption on the measured signal. In the last decade, methods have been developed to retrieve simultaneously above-cloud aerosol and corrected-cloud properties. However, these methods have been mostly applied to polar orbiting instruments, which provide one or two observations a day. Consequently, the study of aerosol-radiation interaction in the South-East Atlantic would benefit from the high-temporal-resolution observations provided by geostationary satellite platforms.

In this study, we develop a novel and comprehensive approach to estimate the direct forcing of aerosols in the South-East Atlantic by combining cloudy and cloud-free retrievals at high temporal resolution from the geostationnary satellite MSG/SEVIRI. These observations allows to properly evaluate climate models participating in CMIP-6 and AeroCom phase III. The reasons for the differences between the observed and the modelled direct effects will be discussed.