The 1-day fast-sampling orbit phase of the Surface Water Ocean Topography (SWOT) satellite mission provides a unique opportunity to analyze high-frequency sea-state variability and its implications for altimeter sea state bias (SSB) model development. Time series with 1-day repeat sampling of sea-level anomaly (SLA) and SSB input parameters—comprising the significant wave height (SWH), wind speed (WS), and mean wave period (MWP)—are constructed using SWOT’s nadir altimeter data. The analyses corroborate the following key SSB modelling assumption central to empirical developments: the SLA noise due to all factors, aside from sea state change, is zero-mean. Global variance reduction tests on the SSB model’s performance using corrected SLA differences show that correction skill estimation using a specific (1D, 2D, or 3D) SSB model is unstable when using short time difference intervals ranging from 1 to 5 days, reaching a stable asymptotic limit after 5 days. It is proposed that this result is related to the temporal auto- and cross-correlations associated with the SSB model’s input parameters; the present study shows that SSB wind-wave input measurements take time (typically 1–4 days) to decorrelate in any given region. The latter finding, obtained using unprecedented high-frequency satellite data from multiple ocean basins, is shown to be consistent with estimates from an ocean wave model. The results also imply that optimal time-differencing (i.e., >4 days) should be considered when building SSB model data training sets. The SWOT altimeter data analysis of the temporal cross-correlations also permits an evaluation of the relationships between the SSB input parameters (SWH, WS, and MWP), where distinct behaviors are found in the swell- and wind-sea-dominated areas, and associated time scales are less than or on the order of 1 day. Finally, it is demonstrated that computing cross-correlations between the SLA (with and without SSB correction) and the SSB input parameters offers an additional tool for evaluating the relevance of candidate SSB input parameters, as well as for assessing the performance of SSB correction models, which, so far, mainly rely on the reduction in the variance of the differences in the SLA at crossover points.

A key correction in altimetry measurements is the wet tropospheric correction (WTC), which accounts for the delay on the measured range predominantly caused by water vapor in the troposphere. The WTC is currently one of the main sources of error affecting the altimeter range over ocean surfaces, with direct impact on estimates of sea surface height and the global mean sea level. Most current altimetry missions rely on onboard passive microwave radiometers that measure the brightness temperature at key frequencies for the spatio-temporal collocated estimation of the WTC. For such missions, the current operational algorithms for the estimation of the WTC can be placed in two categories: those using a neural network and those using a log-linear equation dependent on classes of WTC and windspeed. These two methodologies both follow a semi-empirical approach, but differ significantly in the inputs used, the training databases on which they are developed, and the mathematical approach itself. The objective of this study is to conduct a comprehensive comparison between these two algorithms, focusing solely on the mathematical aspect, in order to determine if the differences in the approaches of the two algorithms have a significant effect on the resulting WTC products, their performance and their limitations. To ensure the identical conditions required to achieve this, we develop the two algorithms so that they take the same inputs, have the same training database, and then we apply them to identical simulated and observed datasets to determine their performance. We use the three-frequency configuration (18.7 GHz−23.8 GHz−34 GHz) of the Advanced Microwave Radiometer for Climate (AMR-C) on Copernicus Sentinel-6 Michael Freilich (Sentinel-6MF), and radiative transfer modelling using the Radiative Transfer for TOVS (RTTOV), to create a simulated database that will serve for both development of the algorithms and performance evaluation. Both algorithms are then applied to the same observations from Copernicus Sentinel-6MF to characterise their observational performance in comparison to both model and operational products, as well as to each other. We find that, despite the significant differences in the two methodologies, when the algorithms are developed and applied to the same databases, they deliver very similar performances both on simulated and observed datasets and face similar limitations. In addition, for both algorithms we find performances similar to or slightly better than that of the operational WTC product for Copernicus Sentinel-6MF.