Validation of Surface Waves Investigation and Monitoring Data against Simulation by Simulating Waves Nearshore and Wave Retrieval from Gaofen-3 Synthetic Aperture Radar Image

Abstract

The Chinese-French Oceanography SATellite (CFOSAT) jointly developed by the Chinese National Space Agency (CNSA) and the Centre National d’Etudes Spatiales (CNES) of France carries a wave spectrometer (Surface Waves Investigation and Monitoring, SWIM). SWIM has one nadir and five off-nadir beams to measure ocean surface waves. These near-nadir beams range from 0° to 10° at an interval of 2°. In this work, we investigated the performance of wave parameters derived from wave spectra measured by SWIM at off-nadir beams during the period 2020 to December 2022, e.g., incidence angles of 6°, 8° and 10°, which were collocated with the wave simulated by Simulating Waves Nearshore (SWAN). The validation of SWAN-simulated significant wave heights (SWHs) against National Data Buoy Center (NDBC) buoys of National Oceanic and Atmospheric Administration (NOAA) exhibited a 0.42 m root mean square error (RMSE) in the SWH. Our results revealed a RMSE of 1.02 m for the SWIM-measured SWH in the East Pacific Ocean compared with the SWH simulated by SWAN, as well as a 0.79 correlation coefficient (Cor) and a 1.17 squared error (Err) for the wave spectrum at an incidence angle of 10°, which are better than those (i.e., the RMSEs were > 1.1 m with Cors < 0.76 and Errs > 1.2) achieved at other incidence angles of SWH up to 14 m. This analysis indicates that the SWIM product is a relevant resource for wave monitoring over global seas. The collocated wave retrievals for more than 300 cases from Gaofen-3 (GF-3) synthetic aperture radar (SAR) images in China Seas were also used to verify the accuracy of SWIM-measured wave spectra. The energy of the SWIM-measured wave spectra represented by SWH was found to decrease with an increasing incidence angle in a case study. Moreover, the SWIM-measured wave spectra were most consistent with the SAR-derived wave spectra at an incidence angle of 10°, yielding a 0.77 Cor and 1.98 Err between SAR-derived and SWIM wave spectra under regular sea state conditions (SWH < 2 m). The error analysis indicates that the difference in SWH between SWIM at an incidence angle of 10° and SWAN has an increasing tendency with the growth in sea surface wind and sea state and it stabilizes to be 0.6 m at SWH > 4 m; however, the current and sea level have less influence on the uncertainties of the SWIM product.

Feasibility of Wave Simulation in Typhoon Using WAVEWATCH-III Forced by Remote-Sensed Wind

“…The wind vectors measured from the HY-2B scatterometer and radiometer [29] on 26 September 2021 are presented in Figure 2a and 2b, respectively. At present, the wave spectrum can be inferred from SAR [30] and SWIM, especially in tropical cyclones [31,32]. Unfortunately, the accuracy of wave retrieval from typhoon SAR images necessitates further improvements [33], and there are few SAR images of the above typhoons.…”

“…Moreover, the SWIM-measured wave spectrum is transformed to the wave number space and then compared with WW3 simulations. (e) At present, the wave spectrum can be inferred from SAR [30] and SWIM, especially in tropical cyclones [31,32]. Unfortunately, the accuracy of wave retrieval from typhoon SAR images necessitates further improvements [33], and there are few SAR images of the above typhoons.…”

A Technique for SAR Significant Wave Height Retrieval Using Azimuthal Cut-Off Wavelength Based on Machine Learning

“…Monitoring the sea surface in near real-time has become a crucial area of research thanks to the increasing popularity of remote-sensing technology for earth observation since the 1970s. Currently, the remote-sensed products are officially released, including sea surface wind from a scatterometer [2], significant wave height (SWH) from an altimeter [3], and wave spectrum from Surface Wave Investigation and Monitoring (SWIM) onboard Chinese-French Oceanography SATellite (CFOSAT) [4,5]. However, monitoring nearshore waves with high precision remains a challenge for the remote sensing community, although two high-frequency phased-array radars [6] can detect swell and current in coastal areas.…”