Effects of Directional Wave Spectra on the Modeling of Ocean Radar Backscatter at Various Azimuth Angles by a Modified Two-Scale Method  
[PERSON]  
1 College of Oceanography and Space Informatics, China University of Petroleum, Qingdao 266580, China; [EMAIL_ADDRESS] (Q.Y. ); [EMAIL_ADDRESS] (T.S.) [PERSON]  
1 College of Oceanography and Space Informatics, China University of Petroleum, Qingdao 266580, China; [EMAIL_ADDRESS] (Q.Y. ); [EMAIL_ADDRESS] (T.S.) Chenqing Fan  
2 Technology Innovation Center for Ocean Telemetry, Ministry of Natural Resources, Qingdao 266061, China 23 First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China 34 Frankfurt University of Technology Innovation Center for Ocean Telemetry, Ministry of Natural Resources, Qingdao 266061, China 45 Frankfurt University of Technology Innovation Center for Ocean Telemetry, Ministry of Natural Resources, Qingdao 266061, China 5 Frank  
However, to date, most scattering studies have focused on the Gaussian case and ignored the non-Gaussianity effects of the sea surface, and thus cannot finely describe the azimuthal behavior, especially the upwind-downwind asymmetry of backscatter from the ocean. The hydrodynamic modulation of short waves induced by long waves gives rise to the skewness of ocean surface waves [18]. The skewness is believed to be the cause for the upwind-downwind asymmetry of ocean backscatter [19]. The skewness function or its Fourier transform, the bispectral function, has been used in several studies to interpret the difference between the backscatter in upwind and downwind directions [19; 20; 21; 22; 23]. [PERSON] [19] and [PERSON] [20] systematically explored the effects of bispectrum on radar backscatter predicted with the integral equation method and found that the IEM can simulate the upwind-downwind difference in the ocean backscattering normalized radar cross section (NRCS) when the bispectrum part comes into the model. [PERSON] [21; 22] showed that the first-order small-slope approximation scattering model (SSA-1) can predict the upwind-downwind asymmetry by introducing the skewness function into the model. [PERSON] et al. [23] found that the TSM modified by adding a correction term could describe the upwind and downwind difference as well. The modified TSM adds a correction term into the conventional TSM. It is a good choice for simulating the upwind-downwind difference in the ocean backscatter. Furthermore, the impacts of sea surface morphology and distribution characteristics are very significant for the sea surface backscatter calculation [23]. It is of great necessity to analyze the effects of directional wave spectra on the performance of the scattering model in predicting ocean backscatter. There have been several studies focusing on the influences of wave spectra on backscatter prediction in recent years [24; 25; 26; 27]. [PERSON] [24] systematically explored the effects of four wind wave spectra on the scattering prediction of the multiscale model. [PERSON] [25] evaluated the effects of six wind spectra on the prediction of NRCS by SSA. [PERSON] [26] analyzed the influences of five single-wave spectra and three combined-wave spectra on the performance of the advanced IEM in predicting NRCS. [PERSON] [27] combined three wave spectra and two spreading functions to estimate the effects of directional wave spectra on the prediction of wind-wave-induced Doppler shift. In general, the influences of wind wave spectra on IEM and SSA have been analyzed systematically. However, there are few studies in regard to the impact of spectra on TSM, which is commonly used in SAR imaging of the sea surface. Additionally, the effect of the directional spreading function on the estimation of azimuthal fluctuation of ocean backscatter has hardly been analyzed. In this paper, the empirical parameter in the modified TSM is first determined by fitting the modified model to the empirical ocean backscatter geophysical model function (GMF), CMOD5.n.