Gaofen-3 (GF-3), the first Chinese spaceborne synthetic aperture radar (SAR) in C-band for civil applications, was launched on August 2016. Some studies have examined the use of GF-3 SAR data for ocean and coastal observations, but these studies generally focus on one particular application. As GF-3 has been in operation over two years, it is essential to evaluate its performance in ocean observation, a primary goal of the GF-3 launch. In this paper, we offer an overview demonstrating the capabilities of GF-3 SAR in ocean and coastal observations by presenting several representative cases, i.e., the monitoring of intertidal flats, offshore tidal turbulent wakes and oceanic internal waves, to highlight the GF-3’s full polarimetry, high spatial resolution and wide-swath imaging advantages. Moreover, we also present a detailed analysis of the use of GF-3 quad-polarization data for sea surface wind retrievals and wave mode data for sea surface wave retrievals. The case studies and statistical analysis suggest that GF-3 has good ocean and coastal monitoring capabilities, though further improvements are possible, particularly in radiometric calibration and stable image quality.

The accurate estimation of the upper layer thickness in a two-layer ocean is a crucial step in the retrieval of internal solitary wave (ISW) amplitude from synthetic aperture radar (SAR) data. In this paper, we present a method to derive the upper layer thickness and the consequent ISW amplitude by combining two consecutive SAR images with the extended Korteweg-de Vries (eKdV) equation. An ISW case observed twice by the Chinese C-band SAR GaoFen-3 (GF-3) and the German X-band SAR TerraSAR-X (TS-X) with a temporal interval of approximately 11 min in shallow water to the southeast of Hainan Island in the northwestern South China Sea was used to demonstrate the applicability of the method. Using the in situ measurements of temperature and salinity near the observed ISW, the proposed method yielded an ISW amplitude of −4.52 m, in close proximity to −5.66 ± 1.24 m derived by applying the classic Korteweg–de Vries (KdV) equation based on the continuously stratified theory. Moreover, the climatological dataset of the World Ocean Atlas 2013 (WOA13) was also used with the proposed method in the Hainan case, and the results showed that the method can still provide a reasonable estimate of ISW amplitude in shallow water even when in situ oceanic stratification measurements are absent. The application of our method to derive the ISW amplitude from consecutive SAR images seems highly promising with the increasing emergence of tandem satellites in orbits.

The generation of shoreward nonlinear internal waves (NLIWs) on the continental shelf south of the Hainan Island (SHI) is investigated based on spaceborne synthetic aperture radar (SAR) observations and numerical simulations. Two types of shoreward NLIWs are identified from SAR images according to their distinct geographic distribution. One type of NLIWs, named Type‐N NLIWs, is distributed in the northern part of SHI, and the other one is named Type‐S NLIWs, distributed in the southern part of SHI. The SAR‐observed wave occurrence frequency during the spring and neap tides, combined with the calculated body force, suggests that the Type‐N NLIWs originate from the Xisha Islands, whereas the Type‐S NLIWs originate from both the Xisha Islands and the continental shelf break, and the shelf break has a larger contribution. The synergistic analyses of the internal tidal raypath, gamma parameter and earliest SAR‐observed NLIWs suggest that the Type‐N NLIWs are caused by the impingement of the diurnal internal tidal beams emanating from the Xisha Islands on the near‐surface pycnocline close to the continental shelf. Based on the realistic shelf‐slope topography and tidal forcing, the two‐dimensional numerical simulations using the MITgcm suggest that the Type‐S NLIWs result from the nonlinear disintegration of a mode‐1 diurnal internal tide, which develops from a depression formed at the continental shelf break. Furthermore, numerical experiments show that the background current can greatly affect the nonlinear evolution of the internal waves generated at the shelf break.

A novel artificial neural network (ANN) model is introduced for the retrieval of tropical cyclone (TC) sea surface wind speed from the Cyclone Global Navigation Satellite System (CYGNSS) Level 1 data. WindSat TC wind data serves as the “truth” information for the ANN training. Compared to conventional machine learning approaches, the proposed model incorporates specialized information including the storm-centric coordinate information of CYGNSS observations and physical-guided scattering azimuth angle. Additionally, a feature selection process is employed, utilizing both XGBoost regressor and Pearson correlation coefficient, to identify the most pertinent input variables for wind speed retrieval. The results show that the proposed model with storm-centric coordinate information and first-order cosine form of scattering azimuth angle as additional inputs demonstrates good retrieval performance. It achieves a bias of -0.59 m/s and an RMSE of 3.43 m/s, corresponding to a decrease of 60.93% and 20.05% compared to the current CYGNSS baseline wind products for young seas with limited fetch (YSLF). Especially above 35 m/s, the proposed model outperforms the CYGNSS YSLF product, illustrating its advantages under high wind speeds. Moreover, the effects of two special inputs on the model performance are explored. It is found that the RMSEs are reduced by about 25.43% and 9.50%, respectively, after incorporating the two specific inputs, suggesting that considering TC-related inputs is more effective than the physics-guided initialization in improving model performance. Our retrieval results provide valuable guidance for improving the use of GNSS-R data for near real-time retrieval of TC winds.

The generation of shoreward nonlinear internal waves (NLIWs) on the continental shelf south of the Hainan Island (SHI) is investigated based on spaceborne synthetic aperture radar (SAR) observations and numerical simulations. Two types of shoreward NLIWs are identified from SAR images according to their distinct geographic distribution. One type of NLIWs, named Type-N NLIWs, is distributed on the northern SHI, and the other one is named Type-S NLIWs, distributed on the southern SHI. The SAR-observed wave occurrence frequency during the spring and neap tides, combined with the calculated body force, suggests that the Type-N NLIWs originate from the Xisha Islands, whereas the Type-S NLIWs originate from both the Xisha Islands and the continental shelf break, and the shelf break has a larger contribution. The synergistic analyses of the internal tidal ray path, gamma parameter and earliest SAR-observed NLIWs reveal that the Type-N NLIWs are excited by the impingement of the diurnal internal tidal beams emanating from the Xisha Islands on the near-surface pycnocline close to the continental shelf. Based on the realistic shelf-slope topography and tidal forcing, the two-dimensional numerical simulations using the MITgcm suggest that the Type-S NLIWs result from the nonlinear disintegration of a mode-1 diurnal internal tide which develops from a lee wave formed at the continental shelf break. Furthermore, the sensitive numerical experiments show that the background current can greatly affect the nonlinear evolution of the internal waves generated at the shelf break.