Extraction of wave parameters from measured HF radar sea-echo Doppler spectra

Computerized techniques for extracting rms wave height and dominant wave period from HF radar sea-echo Doppler spectra are derived. Earlier theoretical models for first- and second-order sea backscatter (derived elsewhere) are employed to obtain the simple, closed-form inversion equations giving these two radar-deduced quantities. The results are general in that no specific models for the radial or azimuthal form of the wave height directional spectrum need be assumed; the resulting formulas are only weakly dependent upon the radar/wind direction. Approximations made in the derivation are stated, and they are ultimately tested by comparing wave heights and periods extracted from the inversion model with the actual values input to the original equations for scatter. The derived relationships are then tested against some eighty hours of radar measurements taken at a variety of frequencies and buoy-measured sea conditions. Theoretically predicted and empirically determined correction factors for wave height and period are in agreement. Finally, the measurements show that when sea wave height is greater than one twentieth of the radar wavelength, wave height extraction (rms) errors are less than 23%, and wave period extraction errors less than 12%.