Cause of Double-Peak Spectrum in Hyuga-Nada by the HF Radar

High-frequency (HF) radar is a land-based instrument that can observe the radial distribution of oceanographic information, such as surface currents, wave heights, and wind directions. The first-order echo peak of the Doppler spectrum is often splits in this observation, and the levels of the split peaks are of approximately the same magnitude. We call this the double-peak spectrum. Because such a spectral shape may affect the derivation of oceanographic information, we examined the factors that cause double-peak spectra. The Kuroshio Current flows near the observation area of this study's radar, which may have caused the double-peak spectrum. By mapping the spatial distribution of the double-peak spectra, we found areas in which the double-peak spectra occur relatively often at each radar station. The area of high vorticity ahead of the northeastward Kuroshio Current coincided with the area in which the double-peak spectra occurred, indicating the complexity of the currents occurring there; furthermore, more than two current velocity patterns can be mapped. These results show that the distance and directional continuity of the double-peak spectra can be used to calculate the flow-changing region with better spatial resolution.