Assessing Earth’s sphericity effects in the specific case of geostationary satellites observations: focus on operational land/aerosol applications from Meteosat Third Generation-Imager

Geostationary satellites allow a continuous sub-hourly monitoring of the Earth including land surfaces and aerosols, which can now benefit from the advanced measuring performances of the new Meteosat Third Generation-Imager and its Flexible Combined Imager on board (FCI). In this study, we aim to improve our understanding of the impact of the Earth's sphericity on geostationary observations. Although sphericity effects in satellite data have been studied for many years, the curvature of our planet is still not accounted for in many operational radiative transfer-based retrieval algorithms due to the required increase in processing time, and therefore a plane-parallel atmosphere-surface system is assumed instead. While the limitations of this approximation have been widely assessed in the case of low Earth orbit satellites, they must be reevaluated with regard to geostationary satellites, which have a broader range of observing and illumination geometries. Furthermore, we currently lack precise benchmarking of the errors caused by neglecting the Earth's sphericity in the case of land surface and aerosol applications, which show significant differences with respect to the commonly considered ocean color applications. For example, surface/aerosol algorithms use instrument channels in the red and near-infrared spectral ranges where there is a growing impact of molecular absorption compared to the ocean color-sensitive blue channels where Rayleigh scattering predominates. In this context, we perform quantitative analyses of the impact of ignoring the Earth's curvature on FCI-like top-of-atmosphere reflectance calculations using the accurate Monte Carlo radiative transfer code SMART-G. Results enable quantification of important biases introduced by the plane-parallel assumption, with a strong dependency on the satellite acquisition geometry and, to a lesser extent, the measuring wavelength, but without significant dependency on surface and aerosol properties. We also find that 36 % of FCI observations are significantly affected by sphericity effects, in particular in the channels centered at short visible wavelengths (i.e., 444 and 510 nm for FCI). Based on these results, this study makes recommendations on the development of methods to correct geostationary data for sphericity effects so that one can keep using plane-parallel radiative transfer codes for near-real-time operational applications.