High-resolution Gravity Anomalies in the South China Sea from the SWOT SAR Altimeter

Abstract:

Surface Water and Ocean Topography (SWOT) is a joint US and France mission that will be launched in September 2021. SWOT’s main payload is Ka-band Radar Interferometer (KaRIN), which can measure elevations over a 120-km wide swath centered at the sub-satellite track, with a 20-km gap around the nadir. The resolution of the wide swath is 50 m over land surface water and 1 km over the oceans. SWOT’s high spatial resolution enables high-precision and high-accuracy mapping of surface elevations for hydrological, oceanographic and geophysical studies. Sea surface heights (SSHs) over a swath of oceanic area measured by SWOT can be used for deriving marine gravity. In theory, such areal SSHs can produce marine gravity fields far better than those produced by along-track SSHs delivered by conventional altimeters, including recent SAR-based altimeters such as Sentinel-3A and Cryosat-2. To investigate the potential of SWOT in deriving high-frequency marine gravity and how errors in SWOT’s measured SSHs should be treated to optimize the accuracy of gravity anomaly from SWOT, we simulate the high-resolution SWOT-observed SSHs and use two methods to derive gravity anomalies. The multi-beam depths in the area south of Dongsha Island are used to generate high-frequency SSHs on the concept of residual depth model. The residual depths are the differences between the true multi-beam depths and the reference depths, which are obtained by smoothing the true depths. Then the residual depths are used to determine the geoidal undulations, which are then combined with mean sea surface heights from DTU18 to simulate SSHs as observed by SWOT. We use realistic error models of SWOT SSH measurements to examine how such errors impact the recovered marine gravity in northern South China Sea. We also compare two data types (SSHs gradient and SSH) and two computational methods (inverse Vening-Meinesz and inverse Stokes formulae) to search for the best data processing strategy for recovering a best SCS gravity field from SWOT. The assessments using dense shipborne gravity anomalies show that SWOT can deliver high-quality marine gravity anomalies in the South China Sea.