Correction for above-clouds aerosols in geostationary satellite cloud property retrievals

Abstract. The high-temporal-resolution observations from Spinning Enhanced Visible and InfraRed Imagers (SEVIRI) and its successor Flexible Combined Imager (FCI) of cloud optical thickness (COT) and cloud particle effective radius (CER), suffer from biases due to absorption by aerosols, when smoke is overlying the cloud scene. This is often the case over the south-east Atlantic Ocean (SEAO), which is an important part of the field of view of these instruments, which are onboard the geostationary Meteosat satellites series, stationed around 0° latitude and longitude. The SEAO is an important area to study the impact of smoke on clouds, because the semi-permanent stratocumulus cloud decks over the dark ocean effectively reflect solar radiation and have a strong cooling effect on the Earth climate system. During the annual dry seasons in Africa, smoke from vegetation fires is advected over the clouds over the SEAO and mixes with them, making this region a natural laboratory to study aerosol-cloud-radiation interactions. In this paper, the biases in the COT and CER are quantified for typical smoke over cloud scenes, using an adaptation of the traditional cloud retrieval algorithms, including an extra channel in the usual bispectral minimisation procedure. This allows the simultaneous retrieval of COT, CER and above-cloud aerosol optical thickness (ACA AOT), which has been shown in several papers. The impact of smoke on the SEVIRI retrievals was investigated during July – October 2017 for a small region of the SEAO in the satellite field of view. The SEVIRI COT and CER were increased by a factor of 1.9 and 1.4, respectively, as a result of accounting for overlying smoke during two days with an average ACA AOT (550 nm) of 0.82. A similar event in 2025 during one day saw an increase in FCI COT and CER of 2.2 and 1.3, respectively, for an average ACA AOT of 0.95. Furthermore, the distribution of COT changed significantly when accounting for the overlying aerosols, showing a much wider distribution of COT values.