Wind and Current Dependence of the First-Order Bragg Scattering Power in High-Frequency Radar Sea Echoes

In addition to being able to extract current velocities and wind directions, the first-order Bragg peaks in high-frequency (HF) radar sea echoes also have the potential to independently map wind speeds. During a two-month experiment with the HF-radar ocean state measuring and analyzing radar, model S in the Taiwan Strait in 2013, the dependence of the first-order spectral power (FSP) on wind was found to be deterministic. An empirical model was thereafter proposed to estimate the wind speed, which gave a performance that was comparable with the popular second-order inversion method. Strong oscillations (up to ±3 dB) in the FSP caused by underlying currents were observed and cannot be readily explained by the classic wave-current interaction theory with the assumption of wave-action conservation. When we matched the local maximum and minimum values of the FSP with the local extreme and zero values of the radial-current velocities, we found that most of the extremes of the FSP occurred when the radial-current velocity was 0. To account for this phenomenon, additional smoothing with a 12.4-h moving window was applied, which led to a significant improvement in the wind-speed estimates. This knowledge of the FSP's wind and current dependence helps us make another step toward the operational estimation of wind speeds using the first-order Bragg peaks.