## 5 Conclusions  
Knowledge of the ocean backscatter at various azimuth angles is important for the interpretation of microwave remote sensing data. However, the azimuth angle dependence of ocean radar backscatter cannot be accurately described at present due to the lack of a clear understanding of the non-Gaussianity of the ocean surface. The numerical estimation of the ocean backscatter at different azimuth angles is affected by the selection of the directional wave spectra. In this paper, the effects of directional wave spectra on the prediction of ocean backscatter at various azimuth angles with the modified TSM improved based on CMOD5.n are investigated over the wind speed range of 3-16 m/s, the wind direction range of 0\({}^{\circ}\)-360\({}^{\circ}\), and the incidence angle range of 30\({}^{\circ}\)-50\({}^{\circ}\) by comparing the simulated results with the ASCAT data and CMOD5.n. At first, the skewness-related parameter \(\zeta\) in the modified TSM was estimated based on CMOD5.n under various incidence angles and wind speeds for different wave directional spectra. Results show that the magnitude of \(\zeta\) increases quadratically with wind speed increasing, and generally it first increases and then decreases with the incidence angle. The sensitivity of \(\zeta\) to the incidence angle is much weaker than to wind speed. The estimation of \(\zeta\) is slightly affected by the selection of the wave spectra, but is barely affected by the use of different spreading functions. Figure 12: Upwind–downwind asymmetry predicted by the modified TSM with A + Gaussian and AH18 + Gaussian input at high wind speeds, and their comparison with CMOD5.n as functions of wind speed (**a**) at incidence angle of 30\({}^{\circ}\), (**b**) at incidence angle of 40\({}^{\circ}\), and (**c**) at incidence angle of 50\({}^{\circ}\). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Wave**} & \multicolumn{10}{c}{**Wind Speed (m/s)**} \\ \cline{2-13}  & **16** & **17** & **18** & **19** & **20** & **21** & **22** & **23** & **24** & **25** & **26** & **27** & **28** & **29** & **30** \\ \hline A & 0.80 & 1.00 & 1.20 & 1.60 & 2.00 & 2.80 & 4.20 & 7.00 & 25.00 & 22.00 & 8.00 & 5.00 & 3.00 & 3.00 & 2.00 \\ AEH18 & 0.80 & 1.00 & 1.20 & 1.60 & 2.00 & 2.80 & 4.20 & 7.00 & 25.00 & 22.00 & 8.00 & 5.00 & 3.00 & 3.00 & 2.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The \(\zeta\) values at different wind speeds under incidence angle of 50\({}^{\circ}\). Then, the simulated results with the modified TSM were compared with CMOD5.n and the ASCAT measurements to discover the effects of different wave spectra and directional spreading functions on the prediction of azimuthal features of ocean radar backscatter. The results show that in upwind, downwind, and crosswind directions, the overall performance of the modified TSM under different wind speeds and incidence angles is highly dependent on the selection of the wave spectra. The AH18 spectrum has better performance at low and moderate wind speeds over an incidence angle range of 30\({}^{\circ}\)-50\({}^{\circ}\), while the A spectrum works better at a high wind speed. In addition, the difference in upwind-downwind NRCSs depends on the selection of the wave spectra as well, and the A spectrum generally works better. Nevertheless, the wave spectra have little effect on the azimuthal fluctuation of scattering. The azimuthal fluctuation of scattering is highly dependent on the type of directional spreading function. The relative patterns of azimuthal undulation produced by different spreading functions are rather different at different wind speeds, whereas they are mostly similar at different incidence angles. At a low wind speed, the scattering estimated with the Sech spreading function has greater ups and downs with wind direction, while at moderate and high wind speeds, the Gaussian spreading function produces the largest ups and downs.