## 3 Results
### The Influence of Directional Wave Spectra on NRCS Simulation at Different Azimuth Angles  
Figure 7 gives the modified-TSM NRCSs simulated with A + Cosine, A + Sech, and A + Gaussian, and their comparisons with the ASCAT data and CMOD5.n as functions of  
Figure 5: Estimations of the \(\zeta\) parameter with the use of different directional wave spectra of D + Cosine, D + Sech, D + Gaussian, A + Cosine, A + Sech, A + Gaussian, E + Cosine, E + Sech, E + Gaussian, H18 + Cosine, H18 + Sech, and H18 + Gaussian as functions of wind speed (**a–c**) at incidence angle of 30\({}^{\circ}\), (**d–f**) at incidence angle of 40\({}^{\circ}\), (**g-i**) at incidence angle of 50\({}^{\circ}\). Figure 6: Example of comparison of the NRCSs predicted by the modified TSM (solid curves) and the conventional TSM (dashed curves) using different wave spectra and the Cosine spreading function, and their comparisons with CMOD5.n at wind speed of 16 m/s and incidence angle of 50\({}^{\circ}\). (**a**) Results obtained with the D and A spectra. (**b**) Results obtained with the E and H18 spectra. incidence angle at wind speeds of 3 m/s, 9 m/s, and 16 m/s in upwind, downwind, and crosswind directions. The ASCAT NRCS is averaged within \(\pm\)0.5 m/s wind speed, 6\({}^{\circ}\) wind direction, and \(\pm\)0.5\({}^{\circ}\) incidence angle bins. The mean values and standard deviations of every bin are shown in Figure 7. We can see that the CMOD5.n NRCSs exhibit reasonably good agreement with the ASCAT NRCS measurements. For a certain incidence angle, wind speed, and wind direction, the properties of three spreading functions to underestimate or overestimate the reference data are almost the same. For different azimuthal directions, different spreading functions yield rather similar incidence angles and wind speed dependences. Figure 8 shows the simulated NRCSs based on the D + Cosine, A + Cosine, E + Cosine, H18 + Cosine, AH18 + Cosine, and AEH18 + Cosine, and their comparisons with the ASCAT data and CMOD5.n as functions of incidence angle at wind speeds of 3 m/s, 9 m/s, and 16 m/s along upwind, downwind, and crosswind directions. The results show that the incidence angle and wind speed dependences of scattering produced by the different wave spectra are rather different. For the upwind direction, at a low wind speed, the ASCAT data and CMOD5.n are consistent with the A and E spectra at low incidence angles (\(<\)37\({}^{\circ}\)), but closer to the H18 spectrum when the incidence angle is larger than 37\({}^{\circ}\). At a moderate wind speed, compared with the ASCAT data, the A spectrum works better at low incidence angles (\(<\)34\({}^{\circ}\)), and the E spectrum has better performance at incidence angles of 34\({}^{\circ}\)-44\({}^{\circ}\). When the incidence angle is larger than 44\({}^{\circ}\), the E and H18 spectra achieve relatively good agreement with the ASCAT data. Compared with CMOD5.n, the A spectrum works well when the incidence angle is smaller than 37\({}^{\circ}\), and the E spectrum  
Figure 7: Simulated NRCSs based on A + Cosine, A + Sech, and A + Gaussian, and their comparison with the ASCAT data and CMOD5.n as functions of incidence angle at wind speeds of 3 m/s, 9 m/s, and 16 m/s in the (**a**–**c**) upwind direction, (**d**–**f**) downwind direction, and (**g**–**i**) crosswind direction. performs better for incidence angles larger than 37\({}^{\circ}\). At a high wind speed, the A spectrum always performs better than the others compared with the ASCAT data and COMD5.n for incidence angles of 30\({}^{\circ}\)-50\({}^{\circ}\). For the downwind direction, the results are similar to those along the upwind direction, except for the high wind speed, where all four spectra have good agreement with the ASCAT data when the incidence angle is larger than 42\({}^{\circ}\). For the crosswind direction, the results at a low wind speed are similar to those in the upwind and downwind directions.