Measuring Significant Wave Height Fields in Two Dimensions at Kilometric Scales With SWOT

We demonstrate that spatial maps of significant wave height (SWH) with kilometric resolutions can be derived from the data acquired by the Ka-band radar interferometer (KaRIn) instrument onboard the surface water ocean topography (SWOT) mission by exploiting the measured interferometric decorrelation. We discuss the sensitivity to errors in the volumetric decorrelation estimates and show that a successful inversion of SWH, particularly in the outer part of KaRIn’s swath and for low values of SWH, requires factoring out all sources of decorrelation of instrumental origin to an exquisite precision. We then validate KaRIn’s SWH measurement against independent data, namely, GPS buoys, airborne LiDAR, Sentinel3, SWOT’s nadir altimeter, and the ECMWF global wave model. We show that biases between KaRIn and the other sensors are centimetric and that KaRIn is able to capture features in the 2-D SWH field of only a few kilometers. While KaRIn’s SWH measurement error is difficult to fully characterize due to the absence of 2-D ground-truth data valid at such fine spatial scales and spanning a wide range of sea states, we argue that the retrieved fields are dominated by signal rather than noise, except possibly in the last few kilometers of the swath at low SWH. We briefly discuss the implications in terms of advancing our understanding of the phenomena that shape the wave fields at small scales. The algorithm and calibration described in this article will be the basis for version D of the operational SWOT products.