Comparing and combining directional swell measurements from Sentinel-1, SWIM and SWOT

 

Various spaceborne Radar instruments are now operating with the common ability to observe long waves propagating across the oceans: the C-band Synthetic Aperture Radar (SAR) onboard Sentinel-1 (~23-36° of incidence), SWIM rotating Ku-band Radar onboard the Chinese-French CFOSAT satellite (~2 to 10° of incidence) and the Ka-band radar Interferometer (KaRIn) onboard the Surface Water Ocean Topography (SWOT) (1 to 5° of incidence).  

The directional swell measurements are investigated using model and in situ measurements but also co-locations between the spaceborne instruments (also known as cross-overs in altimetry). These latter co-locations involve both static and so-called “dynamic co-locations" where waves are propagated using a linear wave propagation model over a few hundred kilometers to maximize the number of co-locations between the sensors. Intercomparisons can therefore be performed either for specific case studies or for massive statistical comparisons. 

In this study, we use wave measurements from Sentinel-1A (S1-A) wave mode Level-2 Ocean (OCN) products (distributed by ESA) and SWIM L2P products from the new Near-Real Time (NRT) processing that extends the range of swell measurements up to 1200m wavelength (500m before) and better filter non-wave signatures (distributed by CNES/CLS, Q1 2024). We also investigate the possibility to use KaRIn SWOT spectral content of measurements from either the Sigma0 or the Sea Surface Height Anomaly (SSHA) observations to image so-called “forerunners”, the longest period swells that propagate ahead of the most energetic swell components (from Munk 1947). 

Intercomparisons show the promising synergies between these sensors and the potential to derive worldwide multi-sensor swell monitoring. First, SWIM shows great capabilities to image the shortest swell that S1 can totally miss or partially image because of its cutoff limitation. Then, S1 has shown capabilities to observe long swell (with peak wavelength larger than 800m), but their imaging mechanism is still limited by the reduced velocity bunching and tilt modulation of longest forerunners. On the other hand, comparisons with SWOT indicate that KaRIn unsmoothed (ocean data at their highest resolution) SSHA measurements can further extend the range of visible swell up to kilometric scales thanks to its interferometric capability. 

A significant work is still necessary to better understand, compare and inter-calibrate these directional swell measurements but they offer very promising perspectives for the exhaustive description of swell events, from the longest forerunners to the short wind sea.