Effects of swell on the altimeter-derived estimates: analyzing real and simulated data

Thomas Moreau (CLS, France)


Ngan Tran (CLS, France); Laiba Amarouche (CLS, France); Matthias Raynal (CLS, France); Sylvie Labroue (CLS, France); Romain Husson (CLS, France); Céline Tison (CNES, France); Sophie Le Gac (CNES, France); Francois Boy (CNES, France); Alexis Mouche (IFREMER, France); Lotfi Aouf (Meteo France, France)

Event: 2016 Ocean Surface Topography Science Team Meeting

Session: Quantifying Errors and Uncertainties in Altimetry data

Presentation type: Type Oral

A first analysis determining the impact of swell on the radar altimeter waveforms, for both LRM and SAR modes, has been recently carried out with Jason-2 and Cryosat-2 data [Moreau et al., 2016]. This study showed evidence that the effect of sea state condition (swell and wind-sea) on altimetric measurements is important at high wave heights and wavelengths. In SAR mode, the impacts are even more important than for conventional altimetry, and additionally depend on the wave direction (different sensitivities are observed for along- or across-track directions).
Complementary to the analyses performed with Cryosat-2 data (over reduced geographical SAR-mode areas), several months of Sentinel-3 data obtained under different wave conditions over the global ocean, have been recently analyzed and compared to reliable co-located data. Thanks to this additional dataset, these effects are better characterized thus improving our understanding of potentially induced altimetric errors.
CLS, Ifremer and Meteo France partners have been conducting a study, under CNES funding, to characterize these effects using both Cryosat-2 flight data and simulations. An important component of this study is the joint use of collocated wave data from independent sources (wave buoys, satellite data such as altimeter-derived significant wave height or SAR wavenumber spectra) and wave model. Such comparison is necessary to understand possible errors in the wave altimeter data processing when inconsistencies are found with trustworthy data with respect to the sea state condition.
First, this paper presents the main results of this quantitative collocated-data analysis. Second, this paper deals with a numerical study based on the use of an end-to-end simulation tool including a SAR mode simulator capable of generating realistic models of long ocean surface waves and a SAR retracking algorithm. In particular, we simulate the effects of different swell spectrum characteristics (in both direction and frequency) on the surface geophysical estimated parameters and finally we assess the consistency of these results with those obtained on real data.
We foresee that these results will be of great importance in the near future to develop specific processing algorithms and improve the current ones for the specific cases where SAR mode altimeter data are affected by swell. This is of high importance for the current Sentinel-3 topography mission and the further Sentinel-6 mission, both embarking a SAR-mode radar altimeter. Similar questions are likely to come up with the LRM mode data.

Oral presentation show times:

RoomStart DateEnd Date
Auditorium Thu, Nov 03 2016,10:00 Thu, Nov 03 2016,10:15
Thomas Moreau