## 3 Results
### Effect of Swell on NRCS Simulation  
The swell can change the local height fluctuation and thus affect the patterns of the facet scattering [44]. However, research on the influence of swell on the distribution of facet scattering is lacking. In this section, we evaluate the effect of swell on the sea surface height distribution and TSM-simulated scattering distribution at the incidence angles of 30\({}^{\circ}\)-50\({}^{\circ}\) and wind speeds of 3-16 m/s under VV and HH polarizations. Figure 14 shows a comparison example of the distributions of the Tayfun sea surface without and with swell at the wind speed of 10 m/s and incidence angle of 32\({}^{\circ}\) in the upwind direction. Here, the significant wave height of swell = 4 m, swell wavelengths = 200 m, and the swell direction is upwind. Figure 14a shows that the introduction of swell has minimal impact on the mean sea surface height but widens the range of the elevation distribution of the sea surface; that is, the sea surface with swell has a lower probability near small heights and higher probability near large heights than that of the sea surface without swell. Figure 14b shows that the range of fluctuation of sea surface height with swell is substantially larger than that of the wind wave surface without swell. Figure 14c indicates that the probability of the sea surface slope with swell is higher at large slopes and lower at small slopes in comparison with that of the sea surface without swell. Additionally, we repeated the experiments at a low wind speed (i.e., 5 m/s) and found that the derived results are similar to those achieved at a wind speed of 10 m/s, while the difference between the sea surface slope with and without swell is more obvious at a low wind speed. Figure 13: Examples of the scattering profiles predicted based on the conventional TSM + Gaussian sea surface, the conventional TSM + Tayfun sea surface, and the modified TSM + Tayfun sea surface and their comparison with Sentinel-1 data at 32\({}^{\circ}\) incidence angle and 10 m/s wind speed (**a,b**) in the upwind direction under VV and HH polarizations and (**c,d**) in the downwind direction under VV and HH polarizations. Blue solid curve, conventional TSM + Gaussian sea surface; red dashed curve, conventional TSM + Tayfun sea surface; green solid curve, modified TSM + Tayfun sea surface; black solid curve, Sentinel-1 data. Figure 15 shows two examples of the scattering pdfs predicted by the modified TSM based on the Tayfun sea surface without and with swell and their comparison with the Sentinel-1 data. The detailed parameters of the examples are as follows: in Figure 15a, VV polarization, incidence angle of 35\({}^{\circ}\), wind speed of 13 m/s, wind direction of 240\({}^{\circ}\) relative to the range direction, significant wave height of swell = 4 m, swell wavelengths = 200 m, and the direction of the dominant wave is 180\({}^{\circ}\) relative to the range direction; in Figure 15b, HH polarization, incidence angle of 44\({}^{\circ}\), wind speed of 7.4 m/s, wind direction of 91\({}^{\circ}\) relative to the range direction, significant wave height of swell = 4 m, swell wavelengths = 196 m, and the direction of the dominant wave is 191\({}^{\circ}\) relative to the range direction. We can see from Figure 15 that the introduction of swell widens the range of the scattering distribution, and the peak value of the NRCS pdf from the sea surface with swell is lower than that without swell. All these changes diminished the gap between the simulated NRCS and the measured NRCS. Figure 16 shows the profile of NRCS in the range direction based on the same parameters in Figure 15. It can be seen from Figure 16 that the introduction of swell widens the range of fluctuation of the NRCS profile and that it is in better agreement with the Sentinel-1 data than that of the wind wave sea surface without swell. Figure 14: Comparison of Tayfun sea surface without and with swell at 10 m/s wind speed and 32\({}^{\circ}\) incidence angle in the upwind direction.