Evaluation and improvement of the Sentinel-3 Ku band backscatter coefficient atmospheric attenuation correction over the open ocean

The presence of water vapor and cloud liquid water in the Earth’s atmosphere induces an attenuation in satellite altimeters backscatter coefficient (σ 0 ) measurements. To account for this effect, an atmospheric attenuation correction, Δσ 0 , is added to the measured σ 0 . The focus of this study is the assessment and improvement over open ocean of the Sentinel-3 Synthetic Aperture Radar Altimeter (SRAL) Ku band Δσ 0 , operationally computed from Microwave Radiometer (MWR) measurements and the uncorrected SRAL σ 0 . In a first phase, comparisons are performed against the Jason-3 Ku band Δσ 0 and Δσ 0 computed from single-layer model grids from the ERA5 atmospheric model. Results show that the Sentinel-3A Δσ 0 lacks precision mainly over regions of high atmospheric water vapor content, where the standard deviation of the relative differences to the ERA5 model are 50% and higher. In a second phase, two new Sentinel-3A Δσ 0 algorithms are proposed, a new MWR-derived solution and one computed from the Global Navigation Satellite Systems-derived Path Delay Plus (GPD+) wet tropospheric correction. Results show that, w.r.t. the Sentinel-3A operational algorithm, the developed Δσ 0 approaches and the one derived from ERA5 are able to reduce the standard deviation of 10-meter surface wind speed (computed from σ 0 corrected for attenuation) differences to 10-meter equivalent neutral winds from the MetOp-B Advanced Scatterometer. The largest reductions are of the order of 0.3 m/s and higher and occur predominantly over regions of high atmospheric water vapor content. Additionally, the new MWR and GPD+ Δσ 0 algorithms present the most feasible solutions for operational applications.