Relationships Between Ku-Band Radar Backscatter and Integrated Wind and Wave Parameters at Low Incidence Angles

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 (U₁₀) 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 U₁₀ 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 U₁₀ 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 U₁₀. 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 i- prove 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.