Fine-scale Altimetry and the Future SWOT Mission
New Frontiers in Operational Oceanography
This chapter describes recent advances in improving altimetry observations over the ocean for the detection of fine-scale ocean dynamics. The first section gives an overview of the different satellite radar altimetry techniques being used today at high-resolution over the open and coastal oceans: from conventional alongtrack nadir altimetry to along-track Synthetic Aperture Radar (SAR) at nadir. We present the advantages of the measurement techniques in conventional Ku-band (Jason) and Ka-band
... Saral), and in global SAR mode . We show how the along-track errors are estimated, how they vary geographically and seasonally, and how they limit the sea surface height (SSH) scales resolved. We also address various mapping techniques being used to derive gridded SSH data and the issues for observing fine-scale ocean dynamics from altimeter data in the coastal zone. The second section addresses the future global SARinterferometry mission, Surface Water Ocean Topography (SWOT), which aims to measure terrestrial surface waters and ocean SSH over a wide swath. We concentrate on the ocean component of this mission, which will provide the first two-dimensional (2D) observations of SSH on a 1-2 km grid. The low noise level of the SWOT observations should allow us to observe physical processes in the open and coastal oceans with wavelength scales down to 15-20 km. We present the SWOT SAR-interferometry technique, as well as the mission's sampling characteristics and error budget. Of particular interest is the range of ocean dynamics that have a SSH signature in the wavelength scale of 15-200 km, including small mesoscale structures, larger submesoscale fronts and filaments, internal tides, and internal gravity waves. These are difficult to observe with the present altimeter constellation due to the along-track altimetric sampling and higher noise levels. The chapter addresses how these fine-scale dynamics will be observed with the future SWOT SARinterferometric altimetry technology, the challenges in mapping the SWOT swath SSH observations, and the preparation to assimilate SWOT 2D SSH images into operational ocean models.