Evaluating the use of high-frequency radar coastal currents to correct satellite altimetry

Abstract

Coastal altimeter waveforms may differ from the ones in the open ocean, either from rapid changes in the sea state or the presence of land within the satellite altimeter footprint. The optimal retracking method for an individual track may turn out to be a combination of several retrackers and may depend on the sea state. The coastal high-frequency radar (HFR) ocean surface currents, hourly interpolated with a resolution up to 2 km and an offshore range up to 150 km, are evaluated to validate the altimeter sea surface height (SSH) measurements. A method to derive HFR SSH mapped, with a varying spatial-scale optimal interpolation, from the HFR velocities has been implemented. Evaluated mainly in the regions farther than 25 km off the U.S. West Coast, the HFR SSH shows good agreement with Jason-1-2 altimetry products over the years 2008 and 2009. Three Jason-2 PISTACH retrackers and one generic open ocean retracker have been analyzed using the traditional 1 Hz sampling rate. Nearshore, an experimental reprocessing of the 20 Hz range measurements is also tested to check for a gain in along-track spatial resolution. Referencing to the HFR SSH indicate the need to have several retrackers available, even over the continental shelf, with Ice3 fitting better during Bloom events and MLE-4 (or Red3) for high sea states. These studies demonstrate the value of HFR as a potential tool to correct coastal altimeter SSH, refine their spatial resolution and provide some insight into the altimeter behavior as a function of ocean conditions.