## 3 Results
### Effect of Non-Gaussianity on NRCS Simulation
Blue solid curve, the conventional TSM + Gaussian sea surface; red dashed curve, the conventional TSM + Tayfun sea surface; green solid curve, the modified TSM + Tayfun sea surface; red star, the Sentinel-1 data; blue triangle, the ASCAT; magenta solid curve, the GMF. Figure 10: The average NRCSs based on conventional TSM + Gaussian sea surface, conventional TSM + Tayfun sea surface, and modified TSM + Tayfun sea surface and their comparison with reference data as functions of the wind direction at a 10 m/s wind speed and 32\({}^{\circ}\) incidence angle under VV and HH polarizations. Curves and symbols are the same as those in Figure 8. (**a**) The average NRCSs as functions of the wind direction at a 10 m/s wind speed and 32\({}^{\circ}\) incidence angle under VV and HH polarizations. Curves and symbols are the same as those in Figure 8. (**a**) The average NRCSs as functions of the wind direction at a 10 m/s wind speed and 32\({}^{\circ}\) incidence angle under HH polarization; (**b**) the average NRCSs as functions of the wind direction at a 10 m/s wind speed and 32\({}^{\circ}\) incidence angle under HH polarization. Figure 9: The simulated average NRCSs based on conventional TSM + Gaussian sea surface, conventional TSM + Tayfun sea surface, and modified TSM + Tayfun sea surface, and their comparison with the reference data at a 32\({}^{\circ}\) incidence angle as functions of the 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. Curves and symbols are the same as those in Figure 8.  
Examples of the pdfs for scattering predicted based on the conventional TSM + Gaussian sea surface, conventional TSM + Tayfun sea surface, and the modified TSM + Tayfun sea surface and their comparison with Sentinel-1 data and Gaussian fitting to Sentinel-1 data at the incidence angle of 32\({}^{\circ}\) and wind speed of 10 m/s in the upwind direction under VV and HH polarizations are shown in Figure 12a,b, and those in the downwind direction under VV and HH polarizations are shown in Figure 12c,d. The Sentinel-1 data in the IW mode from the wind wave sea surface were selected as the reference data, as shown in Figure 12. Notably, the IW Sentinel-1 data from the wind wave sea surface are minimal, especially in the downwind direction for VV polarization. Therefore, we appropriately widened the averaged bins of the Sentinel-1 data; that is, the Sentinel-1 data are averaged within \(\pm\)0.7 m/s wind speed, \(\pm\)7\({}^{\circ}\) wind direction, and \(\pm\)0.7\({}^{\circ}\) incidence angle bins in the downwind direction under VV polarization. Figure 12 shows that the Sentinel-1 NRCS deviates from the Gaussian distribution, and the mean value shifts to a larger value in comparison with that of the Gaussian distribution. The introduction of a non-Gaussian large-scale sea surface based on the Tayfun model can make the scattering distribution deviate from the Gaussian distribution, but the deviation is very weak. The introduction of the bispectrum correction makes the scattering distribution deviate from the Gaussian distribution obviously, which is consistent with the Sentinel-1 data. As shown in Figure 12, the scattering predicted by the modified model shifts to a larger value compared with the conventional TSM in the upwind direction because of the positive correction introduced by the bispectrum. The results are opposite in the downwind direction. Figure 12 also shows that the extent of the Sentinel-1 NRCS distribution is always larger than that of the  
Figure 11: The upwind-downwind asymmetries predicted based on conventional TSM + Gaussian sea surface, conventional TSM + Tayfun sea surface, and modified TSM + Tayfun sea surface and their comparison with the reference data (**a**,**b**) as functions of the incidence angle at the wind speed of 10 m/s under VV and HH polarizations and (**c**,**d**) as functions of the wind speed at the incidence angle of 32\({}^{\circ}\) under VV and HH polarization.