Contribution of breaking wave on the co-polarized backscattering measured by the Chinese Gaofen-3 SAR

ABSTRACT

In this study, the non-Bragg (NB) scattering due to breaking waves as measured by the C-band synthetic aperture radar (SAR) is investigated using more than 300 Gaofen-3 (GF-3) SAR images, which were acquired in quad-polarization stripmap (QPS) mode, that is, co-polarization [vertical–vertical (VV) and horizontal–horizontal (HH)] and cross-polarization [vertical–horizontal (VH) and horizontal–vertical (HV)]. First, the quality of SAR-based wind estimation is checked against the Haiyang-2B (HY-2B) scatterometer and European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-5), indicating a wind speed accuracy of 1.62 m s⁻¹ root-mean-square error (RMSE) and a 0.89 correlation. Then, the SAR-derived wind and HYbrid Coordinate Ocean Model (HYCOM) sea surface current are used to simulate Bragg resonant roughness. The non-polarized (NP) wave breaking contribution σ_wb on co-polarized SAR-measured normalized radar cross section (NRCS) σ₀ is studied, which is derived using two methods: an approach of the Bragg theory and empirical function. Numerical simulations are contrasted with actual SAR measurements: they show that the theoretical-based approach provides accurate enough simulations of the NP contribution, especially at the HH-polarization channel. To deeply understand the behavior of sea surface scattering under breaking conditions, the third-generation WAVEWATCH-III (WW3) model is used to simulate wake-breaking parameters, i.e. whitecap coverage (WCC), whitecap foam thickness (WCT) and whitecap breaking height (WCH), which are then collocated with SAR images. The difference between simulated co-polarized NRCS and the measured one versus sea surface dynamics parameters (i.e. SAR-derived wind speed, HYCOM sea surface speed, and WW3-simulated significant wave height) shows that NP enhances HH-polarized backscattering, while it damps the VV-polarized backscattering. In addition, the contribution of σ_wb could be ignored for WCC and WCT larger than 15 × 10⁻³ and 40 × 10⁻³ m, respectively. Moreover, the ratio reduces with the increasing WCH greater than 2.0 m; in particular, the ratio likely remains to be 0.1 as WCH is greater than 2.5 m. Generally, the HH-polarized backscattering is relatively sensitive with the wave-breaking parameters; however, this behavior has to be further studied utilizing buoy-measured wave breaking data.

KEYWORDS:

• wave breaking
• Gaofen-3
• synthetic aperture radar

Acknowledgments

We thank the National Satellite Ocean Application Service (NSOAS) for providing the Gaofen-3 (GF-3) synthetic aperture radar (SAR) images and the calibration document through an authorized account via https://osdds.nsoas.org.cn, as well as the Haiyang-2B (HY-2B) products. The WAVEWATCH-III (WW3) model was supplied by the National Centers for Environmental Prediction (NCEP) of the National Oceanic and Atmospheric Administration (NOAA) free of charge. The sea surface current and water level data from the HYbrid Coordinate Ocean Model (HYCOM) were obtained from https://www.hycom.org. The wind data from the ECMWF were freely downloaded from http://www.ecmwf.int.

Disclosure Statement

No potential conflict of interest was reported by the authors.

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This research was partially supported by the National Key Research and Development Program of China under contract No. 2016YFC1401100, the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) under contract No. GML2019ZD0302, the National Natural Science Foundation of China under contract Nos. 41806005 and 42076238, and the China Postdoctoral Science Foundation under contract No. 2020M670245; Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) [GML2019ZD0302]