## 3 Results
### Effect of Swell on NRCS Simulation
The significant wave height of swell = 4 m, swell wavelengths = 20 m, and the swell direction is upwind. (**a**) Pdfs for surface elevations; (**b**) profiles of surface elevations along range direction; (**c**) pdfs for slopes along range direction. Blue solid curve, sea surface without swell; red dashed curve, sea surface with swell. Figure 15: Two examples of scattering pdfs predicted by the modified TSM from Tayfun sea surface without and with swell and their comparison with the Sentinel-1 data. (**a**) Incidence angle is 35\({}^{\circ}\), wind speed is 13 m/s, wind direction is 240\({}^{\circ}\) relative to range direction, VV polarization, significant wave height of swell is 4 m, swell wavelength is 200 m, and direction of the dominant wave is 180\({}^{\circ}\) relative to range direction; (**b**) incidence angle is 44\({}^{\circ}\), wind speed is 7.4 m/s, wind direction is 91\({}^{\circ}\) relative to range direction, HH polarization, significant wave height of swell is 4 m, swell wavelength is 196 m, and direction of the dominant wave is 191\({}^{\circ}\) relative to range direction. To further analyze the impact of swell, we used three examples of simulated scattering based on the modified TSM + Tayfun sea surface without and with swell and their comparison with the Sentinel-1 data at low, medium, and high wind speeds in two respects: the scattering image and the scattering image spectrum [46]. The detailed parameters of three examples are shown in Table 1. Here, the size and resolution of the sea surface were set as 5 km \(\times\) 5 km and 10 m \(\times\) 10 m, respectively. Figure 17 shows the scattering images corresponding to ID 1 of (a) the Sentinel-1 data, (b) simulated NRCS based on modified TSM + Tayfun sea surface without swell, (c) the area enclosed by the red solid line in (b), and (d) simulated NRCS based on modified TSM + Tayfun sea surface with swell. We can see that, at a low wind speed (4.7 m/s), the simulated NRCS based on the wind wave ocean surface without swell shows only the textural features of the wind waves, and the patterns of the swell are not displayed. A comparison of the scattering patterns with swell (i.e., Figure 17d) with the Sentinel-1 data (i.e., Figure 17a) revealed that the textural features in the actual image were simulated successfully when swell is considered. To quantitatively compare the consistency of the simulated textural features with the Sentinel-1 data, Figure 18 presents the image spectra of the scattering image corresponding to ID 1 of (a) Sentinel-1 data, (b) simulation based on the modified TSM + Tayfun sea surface without swell, and (c) simulation based on the modified TSM + Tayfun sea surface with swell. Furthermore, the wavelengths of the dominant wave and the directions of the dominant wave obtained from the image spectra in Figure 18 are listed in Table 2. The dominant wave wavelength and dominant wave direction of the simulation without swell are different from those of the Sentinel-1 data, and the spectral density of the simulation without swell is much smaller than that of the Sentinel-1 data. This is because only wind \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**ID**} & **Incidence Angle** & **Wind Speed** & **Wind Direction** & **Significant Wave** & **Wavelength of** & **Direction of the Dominant Wave** \\  & **(degree)** & **(m/s)** & **(degree)** & **Height (m)** & **Wave (m)** & **(degree)** \\ \hline
1 & 32.2 & 4.7 & 260.0 & 2.0 & 171.5 & 149.0 \\
2 & 41.5 & 7.1 & 137.7 & 2.6 & 185.7 & 158.2 \\
3 & 35.0 & 13.0 & 240.0 & 4.1 & 200.0 & 180.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The parameters of examples.