The most common models of the wind wave spectrum are reviewed, and the compliance of the studied spectra with several fundamental criteria is estimated. These criteria are the ability to simulate diverse wave climate and agreement between model‐based calculations of the mean square slope and experimental data. The spreading function of the spectrum should also correspond to the experimentally measured Doppler spectrum, while the dependence of the radar backscatter cross section should conform to geophysical model functions for various wavelength ranges of incident electromagnetic radiation at moderate incidence angles (20–60°). An analysis has shown that none of the considered spectrum models fully satisfies all the criteria; thus, a new spectrum model for wind waves was developed. Boundary wave numbers for various wavelength ranges of incident electromagnetic radiation within the framework of a two‐scale surface model were determined for the new model. The spectrum model can be used to simulate ripple attenuation in oil slicks and to calculate the radar backscatter cross section inside slick.

By using ten years of collocated Precipitation Radar and buoy data, the relationships between Ku-band normalized radar cross section (σ • ) and integrated wind and wave parameters (e.g., significant wave height, wave period, wave steepness, and wave age) at low incidence angles are analyzed for different sea states using correlation and dependence analysis. The results show that the relationships are significantly different for different sea states. Second, the potential for inverting these parameters directly from a single σ • is investigated. The results reveal that the retrieval of wind speed above 10 m (U 10 ) is most suitable for a nonpure-swell sea and that the performance of significant wave height (H s ) retrieval decreases with the ratio of swell to wind waves. For wave period and wavelength, a feasible inversion can only be performed for a wind-wave-dominated sea. Wave steepness (δ a ) is strongly correlated with σ • in all sea states and can even be higher than that for U 10 in pure-swell seas. It is suggested from the present data set that real wave age (β) may be retrieved from a swell-dominated sea and, therefore, has a low-value cutoff. Furthermore, the extent to which U 10 retrieval depends on sea state is examined by calculating the correlation coefficient between σ • residuals and various standard wave parameters and, then, by determining the partial correlation coefficient between σ • and each wave parameter controlled by U 10 . Both results indicate that wind direction is better taken into account above 13.5 • . Moreover, an auxiliary parameter related to wave slope, such as wave steepness, would help to improve retrieval performance. Significant wave height data are of secondary use, but an implementation with real wave age information might not significantly improve wind speed retrieval. To improve the performance of H s and wave period retrieval, auxiliary information, such as wind speed, wave steepness, or wave age, is needed. Finally, multivariable empirical models are proposed.Index Terms-Integral wave parameter, low-incidence-angle remote sensing, radar cross section, radar signal analysis.

The influence of a surface-active substance (SAS) film on the Doppler spectrum width at small incidence angles is theoretically investigated for the first time for microwave radars with narrow-beam and knife-beam antenna patterns. It is shown that the requirements specified for the antenna system depend on the radar motion velocity. A narrow-beam antenna pattern should be used to detect slicks by an immobile radar, whereas radar with a knife-beam antenna pattern is needed for diagnostics from a moving platform. The study has revealed that the slick contrast in the Doppler spectrum width increases as the radar wavelength diminishes, thus it is preferable to utilize wavelengths not larger than 2 cm for solving diagnostic problems. The contrast in the Doppler spectrum width is generally weaker than that in the radar backscattering cross section; however, spatial and temporal fluctuations of the Doppler spectrum width are much weaker than those of the reflected signal power. This enables one to consider the Doppler spectrum as a promising indicator of slicks on water surface.

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