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

Abstract

<p>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.</p><p>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.</p><p>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.</p>