Observations of the Second-Harmonic Peaks From the Sea Surface With High-Frequency Radars

High-frequency (HF) radars have achieved great success in the remote sensing of sea states. Most operational methods for wave extraction resort to the second-order Doppler spectral continuum based on Barrick's formulas. However, accurate division of the first-order Bragg peak and the second-order continuum is often unavailable due to spectral broadening and splitting when the radar beam is not sufficiently narrow. The second-order peaks, particularly the second-harmonic peaks (SHPs), can also introduce large errors to the wave height estimates. In this letter, we describe some properties of the SHP that are directly learned from the experimental observations of sea echoes by HF radars in both the 25- and 13-MHz bands. The SHPs exist throughout two radar experiments with comparable strengths to the maximum of the second-order continuum. They have similar spectral structures as the Bragg peaks in both the Doppler and spatial domains, showing that these peaks are representations of some particular sea waves. The power ratios of the SHPs to the Bragg peaks (RSBs) in decibels have strong correlations with the wave heights recorded by an in situ buoy or a known model, and the linear regression models between the RSB and the wave height can be determined by data fitting. These findings give new insights into wave height estimation from the spectral peaks only, which can provide supplemental information to the conventional results from the second-order continuum to improve both the accuracy and robustness of the wave extraction by HF radars.