X-band radar observation of 2025 Kamchatka tsunami at the surf zone

X-band marine radar is a shore-based remote sensing system used to detect sea-surface roughness and to monitor coastal morphological changes. In Taiwan, an X-band radar network has been operated by the Central Weather Administration since 2018, with five radar stations deployed along the northern and northeastern coasts to observe sea-surface waves and currents. During the 2025 Kamchatka tsunami event, observations within the surf zone confirmed that tsunami-induced breaking waves can be detected in variance (SIGMA) radar images, as enhanced surface turbulence produces strong backscatter signals. A 6-min time window (288 images) was applied to derive time-dependent parameters. The results indicate that wave breaking persisted for nearly one day, spanning from the flood tide to the ebb tide. High-frequency fluctuations in radar backscatter intensity were observed in the surf zone and closely followed tsunami oscillations, with a maximum height of approximately 0.4 m recorded in the harbor.

In addition, the estimated width of the breaking zone reached approximately 20 m, suggesting that shallow bathymetry plays a significant role in enhancing wave breaking and energy dissipation. These processes likely reduce the momentum of tsunami runup as it propagates toward a sloping beach. Consequently, this study primarily focuses on surf-zone observations. In contrast, the tsunami runup edge exhibits weaker signatures in SIGMA images and may be smoothed out by averaging over a 6-min time window. To better capture individual runup events, shorter time windows are required, and the optimization of temporal processing for runup detection warrants separate investigation.

For tsunami-related features within the surf zone, our results demonstrate the feasibility of X-band radar to characterize wave-breaking processes. Notably, rip currents were also identified in time-averaged (TIMEX) radar images during the tsunami impact period. These features may be associated with rip currents generated by meteotsunami-induced drawdown, as proposed by Linares et al. (2019).

Linares, Á., Wu, C.H., Bechle, A.J. et al. Unexpected rip currents induced by a meteotsunami. Sci Rep 9, 2105 (2019). https://doi.org/10.1038/s41598-019-38716-2

Figure 1. Time series of (a) cross-shore backscatter intensity, (b) estimated surf zone width, and (c) sea level. All timestamps are displayed in UTC+8.