Validation of the Surface Water and Ocean Topography (SWOT) KaRIn Sea Level Data

Abstract

The Ka-band interferometer (KaRIn) altimeter onboard U.S.-French Surface Water and Ocean Topography (SWOT) satellite is the first swath altimeter in operation. Its main innovation is its 2-D view of the Earth's surface, widening the ocean and land applications of satellite altimetry. The main objective of our work was to validate the sea level derived from KaRIn with the water levels from three tide gauges located around Spanish coasts at stations in Huelva, Barcelona, and Bilbao. To do this, we estimated the standard deviation of the differences (SDDs) between the altimeter and the ground-truth data. The accuracy of the Sentinel-3A (S3A) satellite was also computed for comparison. We analyzed two/one track at each location for SWOT/S3A. From the options available to compute the sea level anomaly (SLA) from SWOT, we tested the wet tropospheric correction (WTC), the mean sea surface (MSS), and the tidal model. The comparisons with the tide gauges revealed that the ECMWF model for the WTC gave a higher number of valid data and the same level of accuracy as the correction derived from the advanced microwave radiometer (AMR) instrument. The same accuracy was observed regardless of the MSS used (CNES and DTU). Finally, the FES tidal model improved the accuracy with respect to the DTU model. The average SDD of SWOT oscillates between 7:4 ± 0:8 cm (Bilbao) and 12:2 ± 2:3 cm (Barcelona). A more in-depth analysis of the SWOT data at the Barcelona station revealed a cycle with anomalous sea level anomalies not detected in the data screening. Removing that cycle, the average SDD dropped to 8:7 ± 1:7 cm. S3A showed 4%-24% better accuracy than SWOT.