## 3 Results  
### Case Validation: Beibu Gulf  
The first case study is of the Beibu Gulf of the SCS, as illustrated in Figure 3. Figure 3a shows the surface wind speeds estimated from the Sentinel-1A VV-polarized NRCS using the CMOD5.N. A strong wind zone is observed south of Hainan Island, with maximum wind speeds reaching 16.1 m/s. Figure 3b presents the corresponding SWH derived from the WW3 model simulations. The alignment between the wind and wave propagation directions indicates that the wind waves dominant the sea state in this case. In the high-wind region south of Hainan Island, the maximum SWH reaches 4.7 m, consistent with the strongest wind conditions.  
By inputting the wind and wave fields from Figure 3a,b into the DPDop model, the Doppler shift induced by the wave motion is estimated. As shown in Figure 3c, following the correction procedure outlined in Figure 2, the radial OSC velocity at a spatial resolution of 1 km is presented. The strongest surface currents are observed in the high-wind region south of Hainan Island, with a maximum velocity of 0.95 m/s. The solid purple circle in Figure 3c marks the position of a collocated drifting buoy, which recorded a near-surface current speed of 0.59 m/s and a radial component of \(-0.03\) m/s. This observation closely matches the SAR-retrieved radial current velocity of \(-0.07\) m/s.  
Figure 2: Flowchart illustrating the procedure for retrieving and validating the radial OSC velocities using the SAR-measured Doppler shifts.  
Figure 3d displays the corresponding radial OSC velocities derived from the SMOC product. To enable its direct comparison with the SAR retrievals, the original SMOC data (1/12\({}^{\circ}\) resolution) were interpolated onto a 1 km grid. In Figure 3c,d, the black arrows indicate the direction of the surface currents. Overall, the SAR-retrieved radial OSC velocities exhibit spatial patterns consistent with those from the SMOC product. At the drifting buoy location, the SMOC product reported a current speed of 0.6 m/s and a radial component of \(-\)0.12 m/s, both of which are in reasonable agreement with the SAR retrievals and drifter observations. However, in regions with strong surface currents, the SAR-retrieved radial velocities tend to be slightly higher than those reported by the SMOC.  
Figure 4 presents a scatter plot comparing the SAR-retrieved and SMOC-derived radial OSC velocities. The comparison yields a bias of 0.07 m/s, a RMSE of 0.14 m/s, a MAE of 0.12 m/s, and a correlation coefficient (\(R\)) of 0.93. Although some discrepancies occur in areas of strong currents, the small bias, RMSE, and MAE, along with the high correlation coefficient, indicate strong overall agreement between the SAR retrievals and SMOC data. These results demonstrate the reliability and robustness of the SAR-derived radial OSC velocities obtained from Doppler shift measurements.  
Figure 3: **(a)** Sea surface wind field derived from a Sentinel-1A VV-polarization SAR scene acquired over the Beibu Gulf on 30 November 2020 at 10:56 UTC. The black arrows denote the wind directions from the ECMWF, and the colorbar represents the wind speed, in units of m/s. **(b)** The SWH simulated by the WW3 at 11:30 UTC on the same day. The black arrows denote the ECMWF wind directions, while the red arrows indicate the wave directions from the WW3. The colorbar shows the magnitude of the SWH, in units of m. **(c)** Radial OSC velocities (a negative value indicates that the surface current is directed toward the radar, while a positive value indicates flow away from the radar) retrieved from the SAR-measured Doppler shifts. **(d)** Radial OSC velocities from the SMOC product. Black arrows in **(c,d)** indicate SMOC current directions, and purple arrows represents the drifting buoy-measured current directions. The colorbars in **(c,d)** denote the radial OSC velocities, in units of m/s.