## 4 Discussion  
From the results above, we can see that the upwind-downwind asymmetry could be well described by the modified TSM with the A spectrum input at different wind speeds and incidence angles. However, we find that the modified TSM performs poorly in predicting upwind/crosswind anisotropy. Here, we scale the anisotropy by the \(B\) parameter, defined as (\(\sigma^{0}\)(upwind) + \(\sigma^{0}\)(downwind)--2\(\sigma^{0}\)(crosswind))/2. Figure 11 shows estimations of \(B\) based on the modified TSM using different directional wave spectra, and their comparisons with the ASCAT data and CMOD5.n; Figure 11a-c as functions of incidence angle at wind speeds of 3 m/s, 9 m/s, and 16 m/s, and Figure 11d-f as functions of wind speed at incidence angles of 30\({}^{\circ}\), 40\({}^{\circ}\), and 50\({}^{\circ}\). As can be seen, the use of different wave spectra has little effect on the prediction of the \(B\) values, while the \(B\) values estimated with different spreading functions are rather different. We can see that none of the directional spreading functions provide reasonably good agreement with the ASCAT data and CMOD5.n in predicting upwind/crosswind anisotropy, though the Gaussian spreading function performs relatively better than the other two spreading functions. In other words, all the three directional spreading functions--the Cosine form, the Sech form, and the Gaussian form--cannot well describe the upwind/crosswind anisotropy of ocean backscatter. This might be due to the fact that most of the existing directional spreading functions cannot describe the directionality of the short gravity waves [48]. In [48], [PERSON] et al. constructed an improved spreading function based on the Cosine spreading function to describe the upwind-crosswind asymmetry of L-band ocean surface backscattering. That is to say, an improved spreading function also needs to be developed to better describe the C-band upwind/crosswind anisotropy. Furthermore, the applicability of the modified TSM at high wind speeds is discussed here. It can be seen from Figure 10 that the spreading function has little influence on the variation trend of NRCS and upwind-downwind asymmetry with wind speed. Therefore, the Gaussian distribution function, which has overall good performance, is selected here. Tables 3-5 show the \(\zeta\) values at different wind speeds and incidence angles of 30\({}^{\circ}\), 40\({}^{\circ}\), and 50\({}^{\circ}\) based on the A and AH18 wave spectra. It can be seen that the \(\zeta\) value first increases sharply and then decreases with wind speed increasing, and the transition wind speed is approximately 25 m/s. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**Wave**}} & \multicolumn{11}{c}{\multirow{2}{*}{**Wind Speed (m/s)**}} & \multicolumn{11}{c}{\multirow{2}{*}{**Wind Speed (m/s)**}} & \multicolumn{11}{c}{\multirow{2}{*}{**Wind Speed (m/s)**}} & \multicolumn{11}{c}{\multirow{2}{*}{**Wind Speed (m/s)**}} & \multicolumn{11}{c}{\multirow{2}{*}{**Wind Speed (m/s)**}} & \multicolumn{11}{c}{\multirow{2}{*}{**Wind Speed (m/s)**}} \\ \cline{2-2} \cline{11-17}  & **16** & **17** & **18** & **19** & **20** & **21** & **22** & **23** & **24** & **25** & **26** & **27** & **28** & **29** & **30** \\ \hline A & 0.84 & 1.00 & 1.40 & 1.80 & 2.20 & 3.20 & 4.80 & 8.00 & 27.00 & 24.00 & 9.00 & 5.00 & 4.00 & 3.00 & 2.50 \\ AEH18 & 0.84 & 1.00 & 1.40 & 1.80 & 2.20 & 3.20 & 4.80 & 8.00 & 27.00 & 24.00 & 9.00 & 5.00 & 4.00 & 3.00 & 2.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The \(\zeta\) values at different wind speeds under incidence angle of 40\({}^{\circ}\). Figure 11: Upwind–crosswind anisotropy predicted by the modified TSM with different directional wave spectra input, and their comparisons with the ASCAT data and CMODS.n. (**a–c**) Results plotted as functions of incidence angle for wind speeds of 3 m/s, 9 m/s, and 16 m/s.