11:30 AM - 11:45 AM
[SCG63-04] Tsunami Information and Applications of High-Frequncy Oceanographic Radar
★Invited Papers
Keywords:HR Oceannographic Radar, Tsunami Observation, Information for Evacuation
1. Oceangrapihc Radar
Oceanographic radar, which can measure the ocean surface current in coastal areas with high spatio-temporal resolution, is an extremely effective equipment of monitoring and predicting oceanographic conditions in coastal areas, for example, for detecting tsunamis, predicting the drift and spread of pollutants associated with oil spills and other accidents, and providing information on oceanographic conditions for the fisheries industry. For this reason, an increasing number of institutions in Japan have introduced oceanographic radars in recent years.
Oceanographic radar is a Doppler radar that propagates with ground-wave mode using High-Frequency radio-wave and receives signals scattered at sea surface beyond the horizon from a radar installed at the coast (Fig. 1). Surface current velocity and ocean wave information can be obtained from motion of the ocean wave that satisfies the Bragg resonance condition for the wavelength of the radio waves used. The accuracy of current measurement ranges from 5 to 10 cm/s, depending on the radar observation time. Oceanographic radar is also characterized by its ability to measure the current velocity field, wave height distribution, and other oceanographic information in real time over an area irradiated by radio waves. The observation range is up to 60 km for the 25 MHz band and up to 100 km for the 13 MHz band, which are commonly used in worldwide, depending on the sea surface conditions and the frequency used for propagation attenuation of the ground-wave mode.
2. Tsunami observation by oceanographic radar
Tsunami observations with oceanographic radar are made by capturing as the velocity changes of ocean waves on a tsunami in response to the motion of the tsunami waves. This possibility has been described since the early development of oceanographic radar (Barrick, 1979), and was demonstrated by observations of the velocity field of tsunamis generated by the Tohoku earthquake in Japan, the United States, and Chile, as shown in Fig. 2 (Hinata et al., 2011; Lipa et al. 2011; Dzvonkovskaya et al. 2012; Benjamin et al. 2016).
The velocity field of subsequent resonant response induced by tsunamis in the Kii Channel and Ise Bay was also revealed by taking advantage of the 2D observation features of the current velocity field with oceanographic radar (Hinata et al., 2012; Toguchi et al., 2018). Since the structure of subsequent resonant oscillations has complex aspects depending on the coastal geometry, and subsequent resonant oscillations may cause maximum wave heights, the 2D observation by oceanographic radar is useful for determining when to lift the tsunami warning.
Methods for tsunami height prediction based on data assimilation using current velocity data observed by oceanographic radar have been studied (Mulia et al., 2020; Wang et al., 2022). Furthermore, Sahana et al. (2024) improved the assimilation performance incorporating beam-angle dependent measurement error distribution using multiple radars (Fig. 3).
3. Advantage of oceanographic radar
In tsunami evacuation, only about 20-30% of all people evacuate when the situation is predetermined, such as “evacuate when large earthquake is felt” or “evacuate when a tsunami warning is announced,” and most people start evacuation intuitively, sensing the sense of urgency fostered in the community (Takahashi and Okumura, 2024). Tsunami observation with oceanographic radar allows monitoring of the gradual approach of a tsunami by the 2D-observations. If the situation can be appropriately communicated to people, it is expected to be a useful information that can effectively encourage evacuation behavior by creating a sense of urgency.
Offshore GPS buoy and ocean-bottom pressure gauges require huge costs for installation and maintenance. In contrast, land-based oceanographic radar can be deployed at 1/10 to 1/20 the cost of 150 to 200 units covering the entire coast of Japan (Fig. 4). The nationwide network of oceanographic radar can cover areas that are not covered by seafloor observation network for earthquakes and tsunami such as S-net, N-net, and DONET, as well as coastal areas closer than 50 km from the coast. Furthermore, oceanographic radar can be used for monitoring oceanographic conditions under normal conditions, and can be utilized for various purposes such as environmental protection, search-and- rescue, fisheries, and marine traffic, etc. Therefore, it is expected to reduce maintenance costs by sharing costs with these fields.
Oceanographic radar, which can measure the ocean surface current in coastal areas with high spatio-temporal resolution, is an extremely effective equipment of monitoring and predicting oceanographic conditions in coastal areas, for example, for detecting tsunamis, predicting the drift and spread of pollutants associated with oil spills and other accidents, and providing information on oceanographic conditions for the fisheries industry. For this reason, an increasing number of institutions in Japan have introduced oceanographic radars in recent years.
Oceanographic radar is a Doppler radar that propagates with ground-wave mode using High-Frequency radio-wave and receives signals scattered at sea surface beyond the horizon from a radar installed at the coast (Fig. 1). Surface current velocity and ocean wave information can be obtained from motion of the ocean wave that satisfies the Bragg resonance condition for the wavelength of the radio waves used. The accuracy of current measurement ranges from 5 to 10 cm/s, depending on the radar observation time. Oceanographic radar is also characterized by its ability to measure the current velocity field, wave height distribution, and other oceanographic information in real time over an area irradiated by radio waves. The observation range is up to 60 km for the 25 MHz band and up to 100 km for the 13 MHz band, which are commonly used in worldwide, depending on the sea surface conditions and the frequency used for propagation attenuation of the ground-wave mode.
2. Tsunami observation by oceanographic radar
Tsunami observations with oceanographic radar are made by capturing as the velocity changes of ocean waves on a tsunami in response to the motion of the tsunami waves. This possibility has been described since the early development of oceanographic radar (Barrick, 1979), and was demonstrated by observations of the velocity field of tsunamis generated by the Tohoku earthquake in Japan, the United States, and Chile, as shown in Fig. 2 (Hinata et al., 2011; Lipa et al. 2011; Dzvonkovskaya et al. 2012; Benjamin et al. 2016).
The velocity field of subsequent resonant response induced by tsunamis in the Kii Channel and Ise Bay was also revealed by taking advantage of the 2D observation features of the current velocity field with oceanographic radar (Hinata et al., 2012; Toguchi et al., 2018). Since the structure of subsequent resonant oscillations has complex aspects depending on the coastal geometry, and subsequent resonant oscillations may cause maximum wave heights, the 2D observation by oceanographic radar is useful for determining when to lift the tsunami warning.
Methods for tsunami height prediction based on data assimilation using current velocity data observed by oceanographic radar have been studied (Mulia et al., 2020; Wang et al., 2022). Furthermore, Sahana et al. (2024) improved the assimilation performance incorporating beam-angle dependent measurement error distribution using multiple radars (Fig. 3).
3. Advantage of oceanographic radar
In tsunami evacuation, only about 20-30% of all people evacuate when the situation is predetermined, such as “evacuate when large earthquake is felt” or “evacuate when a tsunami warning is announced,” and most people start evacuation intuitively, sensing the sense of urgency fostered in the community (Takahashi and Okumura, 2024). Tsunami observation with oceanographic radar allows monitoring of the gradual approach of a tsunami by the 2D-observations. If the situation can be appropriately communicated to people, it is expected to be a useful information that can effectively encourage evacuation behavior by creating a sense of urgency.
Offshore GPS buoy and ocean-bottom pressure gauges require huge costs for installation and maintenance. In contrast, land-based oceanographic radar can be deployed at 1/10 to 1/20 the cost of 150 to 200 units covering the entire coast of Japan (Fig. 4). The nationwide network of oceanographic radar can cover areas that are not covered by seafloor observation network for earthquakes and tsunami such as S-net, N-net, and DONET, as well as coastal areas closer than 50 km from the coast. Furthermore, oceanographic radar can be used for monitoring oceanographic conditions under normal conditions, and can be utilized for various purposes such as environmental protection, search-and- rescue, fisheries, and marine traffic, etc. Therefore, it is expected to reduce maintenance costs by sharing costs with these fields.