Assessing the impact of SAR altimetry for global ocean analysis and forecasting

Abstract

Satellite altimetry provides essential sea level observations to constrain ocean analysis and forecasting systems. New generation of nadir altimeters now provides enhanced capability thanks to a SAR mode that allows reducing the 1 Hz (7 km) measurement noise level from about 3 to 1 cm RMS. A first assessment of the impact of SAR altimetry for global ocean analysis and forecasting is carried out using Observing System Simulation Experiments (OSSEs) with the global Mercator Ocean high resolution 1/12° system. OSSEs are used to quantify the impact of assimilating multiple altimeter missions with and without a SAR mode. A simple twin experiment set up that only takes into account initialisation errors and impact of altimeter random noise is used. Results are analysed in high eddy energy regions where initialisation errors are the most important ones. Both sea surface height and surface velocity analyses and 7-day forecasts are improved. Compared to conventional altimetry, SAR altimetry sea surface height variance errors for both analyses and forecasts are typically reduced by 20% in western boundary currents. This suggests that use of SAR multiple altimeter missions with high-resolution models will significantly improve the capability of the ocean analysis and forecasting systems in the near future.