CoAuthors

Grégoire Mureau (IRSI - IFREMER, France); Mickaël ACCENSI (LOPS - IFREMER, France)

Satellite altimeter records of significant wave heights (SWH) represent the largest source of sea state information available to date. Sea state information is critical for a broad range of human activities (shipping, marine energy resource assessment, marine engineering) most of them being concentrated along the coastal zone. However, the quality of altimeter observations is downgraded in the coastal zone due to surface heterogeneity within the radar signal footprint (Vignudelli et al. 2019). In the last decades, increasing efforts have been devoted to exploit altimeter observations closer to the coast, using innovative sensor technologies (e.g. Ka-band or Delay-Doppler radar altimeters), improved waveform retracking algorithms (e.g. Tourain et al. 2021) or sophisticated post processing filtering techniques (e.g. Quilfen and Chapron, 2020). One major difficulty that remains to assess the performance of coastal altimetry is the limited availability of ground truth measurements to compare with. Indeed, along the coast, sea states exhibit a much stronger variability than in offshore waters, and the collocation criteria for satellite-buoy crossover detection need to account for this variability, which depends on a number of environmental factors (Nencioli and Quartly, 2019).
In this study, we aims at assessing the quality of SWH measurements from the nadir beam of the SWIM instrument on-board CFOSAT (Hauser et al., 2021). For this respect, we use a WW3 high-resolution numerical wave model implemented over the European coastal waters in order to characterize the spatial variability of sea states in the proximity of coastal in situ buoys. Areas of sea state similarity are defined from the computation of systematic and random errors between the time series simulated at the station and those of neighboring nodes. These areas are then used to compute buoy – altimeter matchup statistics and estimate altimeter errors with respect to the buoy data. The validation of the 5Hz SWIM nadir SWH is presented and its advantage with respect to conventional 1Hz measurements is discussed.

References

Hauser, D., et al. 2021. New Observations From the SWIM Radar On-Board CFOSAT: Instrument Validation and Ocean Wave Measurement Assessment. IEEE Transactions on Geoscience and Remote Sensing 59, 5–26. https://doi.org/10.1109/TGRS.2020.2994372
Nencioli, F., Quartly, G.D., 2019. Evaluation of Sentinel-3A Wave Height Observations Near the Coast of Southwest England. Remote Sensing 11, 2998. https://doi.org/10.3390/rs11242998
Quilfen, Y., Chapron, B., 2020. On denoising satellite altimeter measurements for high-resolution geophysical signal analysis. Advances in Space Research. https://doi.org/10.1016/j.asr.2020.01.005
Tourain, C., Piras, F., Ollivier, A., Hauser, D., Poisson, J.C., Boy, F., Thibaut, P., Hermozo, L., Tison, C., 2021. Benefits of the Adaptive Algorithm for Retracking Altimeter Nadir Echoes: Results From Simulations and CFOSAT/SWIM Observations. IEEE Transactions on Geoscience and Remote Sensing 1–14. https://doi.org/10.1109/TGRS.2021.3064236
Vignudelli, S., Birol, F., Benveniste, J., Fu, L.-L., Picot, N., Raynal, M., Roinard, H., 2019. Satellite Altimetry Measurements of Sea Level in the Coastal Zone. Surv Geophys 40, 1319–1349. https://doi.org/10.1007/s10712-019-09569-1