Tsunami Early Warning Using High-Frequency Ocean Radar System in the Kii Channel, Japan

Abstract The tsunami of the 2011 Tohoku earthquake was monitored by the high-frequency (HF) ocean radar system in the Kii Channel, Japan. In this study, we conducted a retrospective analysis of tsunami early warning based on HF radar observations. First, we synthesized the surface current velocity using observation data of two radar land stations: Minato and Saikazaki. After vector synthesis, the time series of current velocity showed high consistency with the results of forward simulation. Then, we used the time series starting from 08:00 (UTC), 11 March 2011, as the input for data assimilation, reconstructing the tsunami wavefield in the Kii Channel. Comparison with actual observations at Kobe showed that our approach can accurately forecast coastal tsunami waveforms at least 50 min prior to tsunami arrival. The normalized prediction error decreased over time, indicating a progressively improving forecast accuracy. This is the first study on tsunami data assimilation using HF radar for the early warning of an actual seismogenic tsunami. We also discussed the limitations of the current HF radar system in the Kii Channel and made suggestions regarding the deployment of future radar land stations.