Assessing the impact of along-track altimetry data on ocean state estimation in the SWOT Cal/Val region off the California coast via data assimilation

A nadir altimeter provides sea surface height (SSH) measurements that can resolve spatial scales down to 40 km to 70 km along-track, depending on altimeter design and ocean surface conditions. The two-dimensional resolution of SSH mapping is limited by the distance between satellite tracks and repeat time. During the time when SWOT is in orbit, there may be more than 5 nadir altimeters. The measurements from these nadir altimeters constitute an important asset of measurements for SWOT Calibration and Validation and also may be combined with SWOT measurements to spatially and temporally resolve submesoscales of order of 20 km. We examine the impact of assimilating five-satellite altimetry data into a model that resolves submesoscale ocean circulation and internal tides. To maximize the impact of the satellite altimetry data on SSH mapping, we applied a multiscale three-dimensional variational data assimilation (MS-3DVAR) approach to achieve this objective. This approach allows assimilating altimetry measurements within a long-time window, effectively retaining the high resolution along-track information and spreading it across tracks, and dealing with internal tide and gravity wave signals. Results from the MS-3DVAR system for the SWOT Cal/Val Region off the California Coast are systematically evaluated to assess the scales that are resolved.