A Controlled Intercomparison of Altimeter Wet Tropospheric Correction Retrieval Algorithms Using Sentinel-6MF Observations

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.