Measurement of ocean wave height and direction by means of HF radar: an empirical approach

High-frequency (HF) radars have been used since 20 years for remotely sensing ocean surface currents and ocean waves. Backscattered Doppler spectra contain two discrete lines, the frequencies of which (Bragg frequency) determine the current speed, and four continuous side bands, which allow to apply inversion techniques for retrieving ocean wave spectra. Recently, a new HF radar has been developed at the University of Hamburg (Germany). Data of a 34-days experiment reveal a high correlation between the standard deviation of the Bragg frequencies and the significant wave height weighted by an azimuthal function. Applying empirical regression curves it is possible to determine the significant wave height and the mean wave direction from intersecting beams of two radar stations. Compared with inversion techniques the new method is applicable to data with lower signal-to-noise ratio, i.e. allows larger ranges. For current measurements, two radar sites are necessary. The optimum distance between two 30 MHz radars is about 20 km and, with the new method, needs not to be reduced for the purpose of simultaneous wave measurements.