## 3 Results
### Effect of Wind Wave Spectra on NRCS Simulation
That is, the A spectrum has a smaller underestimation than other three spectra in comparison with the Sentinel-1 data. At high wind speed (15 m/s), the A spectrum performs well at incidence angles of 30\({}^{\circ}\)-60\({}^{\circ}\) under VV and HH polarizations, whereas the H18 spectrum can better describe the variation trend of the scattering with the incidence angle. speeds at all incidence angles, and the A spectrum retains the best performance in NRCS simulation. Under VV polarization, the results are similar to those described above, except that the AEH18 composite spectrum has a better applicability for incidence angles larger than 35\({}^{\circ}\) under low and medium wind speeds, and the AE composite spectrum yields excellent results at high incidence angles (>44\({}^{\circ}\)) and high wind speed (15 m/s). Figure 5 shows the prediction of the conventional TSM using the four wind wave spectra and two composite spectra and their comparison with the reference data as a function of wind speed at the incidence angles of 32\({}^{\circ}\) and 45\({}^{\circ}\) in the upwind direction. The NRCS predictions based on the A spectrum have the largest level, whereas the results of the D spectrum are the smallest, similar to the conclusions derived from Figure 4. At a low incidence angle (32\({}^{\circ}\)) under VV polarization, the AEH18 combined spectrum works best when the wind speed is <5 m/s, while the A spectrum is in good agreement with the reference data at medium and high wind speeds (>5 m/s). For HH polarization, the error between the A spectrum and the reference data is the smallest, but it becomes larger in level as the incidence angle increases, even reaching 2 dB at a high wind speed (e.g., 16 m/s). The trend of the reference data with wind speed is most consistent with the H18 spectrum. At a high incidence angle (45\({}^{\circ}\)), the NRCS is well predicted by the AEH18 composite spectrum for the wind speed of <13 m/s and is consistent with that of the AE composite spectrum for the wind speed of >13 m/s. Under HH polarization, the A spectrum achieves a good agreement with the Sentinel-1 data at wind speeds of 3-16 m/s. The prediction by the empirical model is consistent with that of the AEH18 spectrum at low wind speeds (<7 m/s) but close to that of the A spectrum at high wind speeds (>12 m/s). When the wind speed is in the range of 7-12 m/s, the AE spectrum is in better agreement with the reference data. The above analysis reveals that the composite spectra have slightly better performance in NRCS prediction at high incidence angles in comparison with the four individual spectra. Generally, backscatter can be predicted accurately by the A spectrum, while the H18 spectrum effectively describes the trends of scattering with incidence angle and wind speed but with poor accuracy. Figure 5: Predictions of the conventional TSM based on the four wind wave spectra and two composite spectra and their comparison with the Sentinel-1 data, ASCAT data, and empirical model as functions of the wind speed at the incidence angles of **(a,b)** 32\({}^{\circ}\) and **(c,d)** 45\({}^{\circ}\) in the upwind direction under VV and HH polarizations. Curves and symbols are the same as those in Figure 4.  
Figure 6 shows examples of probability density functions (pdfs) of scattering and scattering profiles predicted by the conventional TSM with the input of the six wave spectra, and their comparison with Sentinel-1 data, for the wind speed of 10 m/s and incidence angles of 32\({}^{\circ}\) and 45\({}^{\circ}\) in the upwind direction. Considering that the sea surface resolution affects the scattering distribution (a detailed analysis is presented in Section 4), we selected the Sentinel-1 data in the IW mode, which have the same facet size as the simulation, as the reference data. Furthermore, we excluded the Sentinel-1 data including swell based on the method in [43] to eliminate the influence of swell on the scattering.