Surface wind speed extraction from HF sky wave radar Doppler spectra

Abstract

The ability to obtain sea surface wind speeds from HF sky wave radar backscatter spectra is important if such a radar is to make a meaningful contribution to a meteorological or oceanographic observation network. Since the radar measures wave parameters only, wind vectors must be inferred from the wave measurements in the light of what knowledge we have concerning the processes of wave generation. Previous attempts to extract wind speeds have proved unsatisfactory for a variety of reasons. We present here a new technique to enable such a derivation, which uses sea state parameters obtained with an appropriate inversion technique, together with relevant empirical/theoretical relationships on the growth of a wind-generated sea. The procedure is illustrated with a Doppler spectrum obtained during the JASIN experiment, and some comparisons made with other JASIN surface data.Paper number IS 1924. 0048-6604/82/0506-1924508.00 ceases, this has both advantages and disadvantages for wind vector derivation: (1) It is advantageous for determining wind direction, since this direction will be virtually coincident with the mean wave propagation direction, which may be deduced (in principle) directly from first-order features of the backscatter spectrum.(2) It is disadvantageous for deriving wind speeds, since these wave components will saturate rapidly and thus offer no information on the magnitude of the excitation. Recourse must then be made to second-order Doppler spectral features [Barrick, 1972; Johnstone, 1975], with a variety of attendant difficulties. Some of these difficulties are discussed below, in conjunction with the presentation of a technique for inferring wind speeds, which forms the basis of this paper. Various techniques are currently available for extracting mean wave (and wind) direction from the first-order Doppler spectrum [e.g., Long and Trizna, 1973; Stewart and Barnum, 1975; Sandham, 1980]. All require essentially the ratio of the two first-order Bragg lines, combined with an assumed functional form for the directional distribution of the resonant ocean wave components about the mean. Although details of the techniques differ, the principle is now well established. The chief uncertainty lies in the dependence of most assumed wave directional distributions on wind speed, implying a dependence of measured mean wind direction on some prior knowledge of wind speed. Other extraction techniques for wind speeds have 643 644 DEXTERAND THEODORIDIS WIND SPEED EXTRACTION FROM RADAR SPECTRA 645

Wind-speed inversion from HF radar first-order backscatter signal

“…However, the least-squaresfit analysis suggests a better agreement in the range of 2.5 to 9 m/s wind speed, while at higher speeds a deviation occurs in accordance with the expected saturation of wave energy. Beyond this saturation limit, methods based on second-order sidebands (e.g., Dexter and Theodorides 1982;Green et al 2009) can be used if the signal-to-noise ratio allows. Figure 10d-f shows the ANN wind direction results for the same data set discussed above.…”

“…Dexter and Theodorides (1982) provided a method using wave height (H s ) and peak frequency (f p ) to estimate wind speed by relating these parameters to wind speeds required to generate waves with similar characteristics. Likewise, if we assume that the ocean waves are in equilibrium with the wind field, a PiersonMoskowitz type ocean wave spectrum (cf.…”

“…Most existing methods (cf. Dexter and Theodorides 1982;Green et al 2009) utilize the whole or part of the wave spectrum obtained from analysis of the second-order signal (see Section 2) to extract wind speed information and they assume that the ocean wave spectrum is in equilibrium with the prevailing wind. However, the use of the second-order signal suffers from reduced range coverage (when compared with the first-order signal) and also there might be parts of the ocean spectrum contributing to the second-order signal that are not related to local wind conditions (e.g., swell).…”

HF radar for coastal monitoring - a comparison of methods and measurements

“…Wind speed measurements have been made [14] but are not yet sufficiently reliable for operational use. In our work we are using an algorithm that assumes all wave energy is locally wind generated and can be described by a simple wind-wave model, the inversion of which provides a wind speed estimate [15]. This approach will fail in low wind speeds and would also be difficult to apply to a direction-finding system.…”

Sea Surface Echoes Observed with the MU Radar under Intense Sporadic E Conditions.

“…Introduction Crombie (1955) found experimentally Doppler frequency changes of HF radio signals at 13.56 MHz due to moving sea scatterers and identified the radio wave scattering mechanism responsible for the dominant nature of the observed Doppler spectra: Bragg backscatter from the gravity waves having wavelengths equal to half the radar wavelength. Since this discovery, a variety of HF and VHF radars have been used for remote sensing of sea surface state (e.g., Ward, 1969;Headrick and Skolnik, 1974;Lipa et al, 1981;Dexter and Theodoridis, 1982;Broche et al, 1987;Nadai et al, 1996). Most of these observations have used the HF band (10-30 MHz).…”