Sea state frequency features observed by ground wave HF Doppler radar

Abstract

This paper investigates the relationship between the frequency features of the directional spectrum of ocean waves (cutoff frequency and peak frequency of wind waves, frequency of quasi‐monochromatic swell) and the properties of the spectrum of the sea echo obtained by HF Doppler radar. A theoretical analysis is performed to check the validity of a simple result that states that at upper HF, a long‐wave spectral feature at frequency F 0 affects the radar echo at frequencies ±ƒ B ± F 0 on either side of the Bragg lines at ±ƒ B corresponding to resonant backscattering of radio waves with a given wavelength by half‐wavelength ocean waves. An experiment using both radar and standard in situ measurements for several weeks at different times of the year shows that this relationship can be used to formulate a method for estimating the frequency features of the sea with suitable accuracy.