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
A 30-MHz ground-wave ocean surface radar has been deployed inside the Great Barrier Reef where the water is sheltered from ocean swell. The spatial resolution of the radar is 3 km radially and 3.5� in azimuth. In each cell a 102.4-s time series is used to determine radial surface currents, wind directions, root- mean-square wave heights and wind speeds. Coincident observations of sea-wave spectra, surface currents and boundary-layer winds are used to evaluate the radar performance and to modify some of the methods of data analysis to suit these conditions. Surface current values are observed by the radar to an accuracy of �0.05 m s-1, wind directions to �10� , root-mean-square wave heights to 0.15 m and wind speeds to �3 m s-1. In some spectra, the peak in the second-order continuum caused by the non-directional sea- wave spectrum is not resolved from a second-order resonance line. This disallows the derivation of the period of the dominant sea wave on a routine basis.
An HF Doppler radar, designed for use at long range via an ionospheric propagation mode, has been developed primarily for the determination of wave states over large ocean areas. The operating frequency is 21 �840. MHz, and the array is physically rotatable through a full 3600 of azimuth, thus allowing for great flexibility in the choice of target area. The experimental technique utilizes a well-known resonance interaction mechanism for electromagnetic waves backscattered from a moving sea wave surface to derive sea state parameters in the scattering region for input to oceanographic and meteorological synoptic data networks. An ultimate angular resolution of less than 10 of azimuth, coupled with high operational flexibility, suggest possible utilization of the aerial array for tracking and interrogating free-floating ocean buoys, tracking radio noise associated with tropical cyclones and investigating aspects of ionospheric dynamics.
An experiment is described in which measurements are made of the Doppler frequency shift imposed on an acoustic signal by resonant backscatter from a wind-generated rough water surface. In particular, for the case where the water waves have plane wave fronts and move in a single direction across the surface, the effects on the Doppler shift of varying the horizontal angle of incidence of the acoustic beam with respect to this direction of movement are studied. Some simple theoretical concepts are invoked in an attempt to explain the apparent dependence of the Doppler frequency shift on the azimuth angle measured from the acoustic beam radial direction. Because of the analogy which exists between the scattering of acoustic and electromagnetic waves from the sea surface, it is proposed that a model employing a procedure similar to that described here would be of use in interpreting data gained in large-scale ocean backscatter experiments.
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