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

Abstract

The presence of water vapor and cloud liquid water in the Earth’s atmosphere induces an attenuation in satellite altimeters backscatter coefficient (σ₀) measurements. To account for this effect, an atmospheric attenuation correction, Δσ₀, is added to the measured σ₀. The focus of this study is the assessment and improvement over open ocean of the Sentinel-3 Synthetic Aperture Radar Altimeter (SRAL) Ku band Δσ₀, operationally computed from Microwave Radiometer (MWR) measurements and the uncorrected SRAL σ₀. In a first phase, comparisons are performed against the Jason-3 Ku band Δσ₀ and Δσ₀ computed from single-layer model grids from the ERA5 atmospheric model. Results show that the Sentinel-3A Δσ₀ 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 Δσ₀ 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 Δσ₀ approaches and the one derived from ERA5 are able to reduce the standard deviation of 10-meter surface wind speed (computed from σ₀ 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+ Δσ₀ algorithms present the most feasible solutions for operational applications.