Towards an Improved Reconciliation of High- and Low-Resolution Ocean Altimeter Measurements Under Changing Surface Wave Structure Conditions

Author:

Alejandro Egido - (Global Science & Technology, Inc.)

Co-Investigator(s):

Bertrand Chapron (IFREMER)
Christopher Buchhaupt (University of Maryland, College Park)
Douglas Vandemark (University of New Hampshire, Durham)
Frederic Nouguier (IFREMER)

Collaborator(s):

Claire Maraldi (CNES)
Francois Boy (CNES)
Thomas Moreau (Collecte Localisation Satellites)

Abstract:

Since its conception, satellite altimetry has been a major breakthrough in the field of ocean surface topography observations, enabling an improved understanding of oceanographic processes at global and regional scales. Starting with GEOS-3 in 1975, most satellite radar altimeters have used the same measurement and data processing scheme that the community now refers to as conventional or low-resolution mode (LRM) altimetry.

The introduction of the delay/Doppler (D/D) altimetry concept, posed a new paradigm in the field of satellite radar altimetry. In a process resembling an unfocused synthetic aperture radar (SAR) calculation, the D/D concept improves the spatial resolution along the flight direction and achieves a significant increase on the effective number of looks (ENL) of the surface, thus reducing the noise in the retrieved geophysical parameters.

During the past decade, scientist have been able to demonstrate the enhanced performance of SAR altimeters over the open ocean. However, despite the advances in the processing and understanding of SAR altimetry data, discrepancies still exist in the determination of ocean geophysical parameters between the high resolution (HR), and low resolution (LR) measurement modes. Specifically, we observe sea-state dependent biases in the determination of both significant wave height (SWH) and sea surface height (SSH), which prevents a precise reconciliation of the SAR and LRM data records.

These discrepancies arise from the wave motion and horizontal structure effects on the SAR altimetry processing. In this project we will focus on developing understanding of critical aspects pertaining to the interaction of nadir-looking radar altimeter pulses with the ocean surface, with the ultimate objective of improving the consistency of conventional and SAR altimetry measurements of the open ocean.

Through simulations of realistic ocean surfaces and the collocation of altimeter measurements with ancillary data, we seek to determine how vertical velocity variance, horizontal motion, and/or wavefield steepness affect SAR altimeter measurements and how well these effects correlate with observed variations of SWH. Once the effects of surface wave motion are understood, we will strive to incorporate those in the SAR waveform model to account for them during the geophysical parameter retrieval process, with the ultimate objective of developing a fully-consistent LRM and SAR retracking library. The project will conclude with the in-depth assessment of the updated models and processing methods over extensive datasets of Sentinel-6/Jason-CS HR and LR data. The consistency of the SAR and LRM data records will be evaluated at regional and global scales.

The objectives of this project are of utmost relevance for this solicitation, and particularly for the future reference altimeter mission, Sentinel-6/Jason-CS, as ensuring its measurement consistency within the geodetic data record is imperative for the mission success. The HR/LR consistent datasets that we will generate in this project are unprecedented in the field of radar altimetry. We expect these data and our refined geophysical models of SAR altimeter measurements will contribute to further scientific progress in physical oceanography.

Supported by NOAA