Measuring rms wave height and the scalar ocean wave spectrum with HF skywave radar

Abstract

Estimates of rms wave height and the scalar ocean wave frequency spectrum were made by inverting high‐frequency (HF) skywave radar‐measured sea‐echo Doppler spectra. Whereas low‐power surface‐wave radars can make these measurements out to approximately 100 km from the radar, coverage out to 3000 km can be obtained with skywave radars that illuminate the sea via a single ionospheric reflection. To demonstrate this capability, we used the Wide Aperture Research Faculty (WARF) HF skywave radar to measure Pacific Ocean sea backscatter near NOAA data buoy EB 20 during the passage of an at mospheric cold front. The height of the wind‐generated waves measured at the buoy doubled within a 6‐h period. Two new analysis techniques were used to derive rms wave height and the scalar ocean wave frequency spectrum from radar echoes. Estimates of rms wave height were made east and west of the front by using a power‐law relation that was derived from theoretical simulations of the sea‐echo Doppler spectrum for a wide range of wave conditions. Two of the rms wave height estimates were compared with the estimates made at EB 20 at two different times and are in agreement to within 7 and 17%, respectively. Scalar wave spectrum and rms wave height estimates were made by matching a theoretical Doppler spectrum derived in terms of a five‐parameter model of the wind‐wave spectrum to the measured Doppler spectrum. The radar and buoy estimates of the rms wave height agree within 7%. Agreement between the WARF‐derived and buoy‐derived rms wave height and wave spectra is within the combined experimental error of the buoy and the radar measurements.