## 4 Results
### Wind Field Retrieval  
To verify the effectiveness of the direct inversion and three different VAMs, this study conducted wind field retrieval experiments using 1803 scenes of Sentinel-1 IW mode OCN data. The results were compared with 27 NDBC in situ measurements, as shown in Figure 5. The results demonstrate that in the wind speed range of up to 23.2 m/s, VAM3 (\(J_{\text{N}RCS+DCA}\)) exhibits optimal performance with a root mean square error (RMSE) of 1.42 m/s and a correlation coefficient (R) of 0.92. This is closely followed by VAM1 (\(J_{\text{N}RCS}\)) with an RMSE of 1.43 m/s and R of 0.92. While the direct inversion method demonstrates minimal bias  
Figure 4: Observed Doppler centroid frequency (**a**), predicted Doppler centroid frequency (**b**), uncorrected DCA (**c**), simulation of scalloping effect in different sub-swaths (**d**), DCA corrected by scallop effect term (**e**), and DCA corrected by land (**f**). (\(-\)0.03 m/s), it exhibits poorer performance in terms of RMSE (1.51 m/s) and correlation coefficient (R = 0.89) compared to the VAM1 and VAM3. For the same study area, [PERSON] et al. [29] obtained similar wind speed results using direct inversion, corroborating these findings. Within a wind speed range of up to 12 m/s, the scatter plot distributions show that the results from all methods are relatively concentrated and exhibit a strong correlation with in situ measurements. However, as the wind speed increases, the scatter plot distribution gradually widens. This may be due to a limited sample size at higher wind speeds, as well as the influence of complex scattering mechanisms on the sea surface under high-wind-speed conditions. At the same time, the accuracy of the wind speed inversion is degraded due to the reduced sensitivity of the VV-polarized signal at high wind speeds [15]. Furthermore, with increasing wind speed, particularly above 12 m/s, all VAM approaches exhibit a significant underestimation trend, which leads to greater bias in VAM approaches compared to the direct inversion method. This discrepancy likely occurs because the direct inversion method solves for wind speed using CMOD5.N GMF directly, thereby avoiding systematic biases potentially introduced by background wind speed. Conversely, VAM incorporates both background wind direction and speed as constraint conditions, making it susceptible to systematic biases from background wind speed. Regarding wind direction retrieval, as illustrated in Figure 6, VAM3 (\(J_{NRCS+DCA}\)) and VAM2 (\(J_{DCA}\)), which incorporate DCA information, demonstrate notably superior performance with RMSEs of 26.00\({}^{\circ}\) and 26.05\({}^{\circ}\), and correlation coefficients of 0.85 and 0.83, respectively, compared to VAM1 (\(J_{NRCS}\)), which relies solely on NRCS information, with RMSE of 27.66\({}^{\circ}\) and R of 0.82. (RMSE of 27.66\({}^{\circ}\), R of 0.82), as shown in Figure 6. This indicates that DCA information provides significant constraints for wind direction retrieval, whereas NRCS data alone cannot provide sufficient directional information. Figure 5: Wind speeds retrieved by direct inversion method (**a**), VAM1 (\(J_{NRCS}\)) (**b**), VAM2 (\(J_{DCA}\)) (**c**), and VAM3 (\(J_{NRCS+DCA}\)) (**d**), respectively, all validated against buoy-measured wind speeds. In terms of wind direction retrieval, as illustrated in Figure 6, the VAM3 (\(J_{NRCS+DCA}\)) and VAM2 (\(J_{DCA}\)) methods, both incorporating DCA information, demonstrate notably superior performance compared to VAM1 (\(J_{NRCS}\)). Specifically, VAM3 (\(J_{NRCS+DCA}\)) achieves an RMSE of 26.00\({}^{\circ}\) with an R of 0.85, while VAM2 (\(J_{DCA}\)) shows an RMSE of 26.05\({}^{\circ}\) with an R of 0.83. In contrast, VAM1, which relies exclusively on NRCS information, yields an RMSE of 27.66\({}^{\circ}\) and an R of 0.82, indicating relatively inferior performance. To further assess the performance of wind field inversion methods, Figure 7a analyzes the variation in RMSEs and biases of retrieved wind speeds across different wind speed ranges.