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 o affects the radar echo at frequencies +-fB +-Fo on either side of the Bragg lines at +_fB 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. 1. INTRODUCTION More than 20 years ago, the special spectral properties of high-frequency electromagnetic waves backscattered by the surface of the sea suggested that it might be possible to use HF radar to investigate wave fields on the surface of the ocean [Crombie, 1955]. There is indeed a correlation, because of the interaction of electromagnetic waves and ocean surface waves, between the Doppler spectrum of the radar echo and the directional spectrum of the sea. This directional spectrum can be fully extracted by the complete inversion of the radar echo, although with some difficulty [Lipa, 1978]. Nevertheless, many users who cannot operate standard in situ sensors (buoys, etc.) because of technical or economic constraints need some overall parameters of the sea state, and therefore it is of interest to study specific ways of estimating these parameters: wind direction [Long and Trizna, 1973; Stewart and Barnum, 1975; Broche, 1979], significant wave height and dominant period [Barrick, 1977a], swell characteristics [Tyler et al., 1972; Lipa and Barrick, 1980], and surface currems [Stewart and Joy, 1974; Barrick et al., 1977]. This paper deals with estimating the main fre-Copyright ¸ 1981 by the American Geophysical Union. quency-characteristic features of the sea spectrum, i.e., the cutoff and dominant frequency of wind waves and the frequency of swell. The general appearance of the Doppler spectra observed (Figure 3) is correctly described by current models of the interaction between the sea surface and HF electromagnetic waves, and especially by the models developed by Barrick [1972]. This appearance can be expressed by E(fa) = E(')(fa) + E(2)(fa) in which fa is the Doppler shift between the frequency of the radio wave received and that of the wave transmitted. E(I) (fa) represents the so-called 'first-order' echo which occurs in the form of two quasi-monochromatic lines ('Bragg lines') whose positions, which are symmetrical in relation to zero frequency, are given by fa = ---fB = -+(2gki)•/2/2• where g is the acceleration of gravity and k i the wave number of the incident electromagnetic waves. E(2)(fa) represents the so-called '...

Observations of the sea surface by coherent L band radar at low grazing angles in a nearshore environment

“…[21] The dominant features of Doppler spectra (Figures 5a and 5b) are in this case similar to the general features which have been often described in the literature and which apply to a broad interval of the electromagnetic spectrum: HF band [Barrick, 1972;Forget et al, 1981], VHF band [Broche et al, 1987], L and Ku bands [Plant and Keller, Figure 1).…”

Sea surface probing with L-band Doppler radar: experiment and theory

“…2) are similar to what could be observed in HF [11,12], VHF [13] and X-band [14], with the same coarse resolution. The energy is concentrated in the vicinity of Bragg frequencies, ±f B , with f B given by 2πf B = (2gk) 1/2 , g the gravity acceleration and k the electromagnetic wavenumber.…”

Short‐wave directional properties in the vicinity of atmospheric and oceanic fronts

“…With regard to ocean surface waves, HF ocean radar can be used to measure wave heights [e.g., Barrick , 1977], dominant wave periods [e.g., Forget et al , 1981], and wave directional spectra [e.g., Hisaki , 1996] from first‐ and second‐order scattering. On the other hand, short‐wave directional properties [e.g., Heron , 1987; Wyatt et al , 1997] are estimated from first‐order scattering.…”