Round Robin Assessment of altimetry algorithms for coastal sea surface height data

Multidecadal efforts have made possible the current record of ocean sea surface height variations, 30-year long, provided by high-precision altimeter satellites. These observations have greatly improved our knowledge of the open ocean and are now an essential component of many operational marine systems and climate studies. It is not the case in the coastal ocean, where satellite altimetry encounters different issues that make it more difficult to derive accurate geophysical data. However, monitoring coastal sea level changes at global scale is now a critical need that the too rare in-situ data cannot fulfill. This has motivated many efforts to bring the sea level retrieval by satellite altimetry as close as possible to the shore. The result of these efforts is now the availability of many new algorithms for retracking radar altimeter data, correcting sea surface heights and finally deriving sea level variations. The main objective of this study, initiated and funded by the French space agency (CNES), is to objectively define the best set of altimetry algorithms for computing sea level anomalies (SLA) in coastal zones. We focus on Low Resolution Mode altimetry and aim for the creation of long-term time series spanning different missions. For the different processing components (retrackers, geophysical corrections, and auxiliary parameters, for a total of 21 algorithms), the relative quality of the different solutions is assessed with clearly defined metrics for variability, data availability, and impact on SLA as a function of distance to the nearest coast. The analysis is made at both global and regional scales for 3 specific zones: the Mediterranean Sea, the North East Atlantic and the East Australian coasts. In the regions considered, a comparison against available tide gauge data is also performed. This methodology is applied to Jason-2 and Jason-3 missions, and we determine which algorithms are the most limiting, and how results vary from one mission to the other. The set of best performing algorithms near the coast will finally be presented, with the criteria used for the selection. This set will constitute the baseline algorithms for the generation of a new Level-3 SLA product.