Aerospace and Electronic Systems Magazine March 2018 - 52

HF Surface Wave Radar for Tsunami Alerting

Figure 3.

Top: range-time image of a residual radial current velocity (meter per second) estimated along a radar look in the northwest direction from Rumena,
Chile, on March 12, 2011. Bottom: tsunami current velocity data from the NOAA tsunami propagation model (data courtesy of D. Figueroa, University
of Concepcion, Chile, and C. Moore, NOAA, United States).

analyze how HF radar is able to meet the challenges associated
with early tsunami detection.

HF RADAR MEASUREMENTS OF GENUINE TSUNAMI
EVENTS
A 9.0-magnitude undersea earthquake occurred near the coast of
Japan at 05:46 Coordinated Universal Time (UTC) on March 11,
2011, and generated a powerful tsunami, which devastated parts
of the Japanese coastline and propagated across the Pacific Ocean
reaching both the North and South American continents. Several
HF ocean radar systems worldwide were able to capture this event
and provided postprocessed estimates of tsunami features.
One of the WERA ocean radar systems was installed in Rumena, Chile. The radar was operated by the University of Concepción when the 2011 Japan tsunami waves encountered the Chilean
coast after propagating nearly 17,000 km within 22 hours across
the Pacific Ocean. After the earthquake in Japan and before the
arrival of the tsunami in Chile, the Rumena radar was reconfigured
to record the time series of the complex-valued signals from the
receive antennas in successive 5-minute intervals. The FMCW radar operated at a center frequency of 22 MHz and a full bandwidth
of 500 kHz (emergency case), corresponding to a range resolution
of about 300 m. The eight-element linear receive antenna array
enabled software beamforming up to ±45° from boresight. The
unique opportunity to observe a natural tsunami event between
03:00 and 07:00 UTC on March 12, 2011, by using an HF radar
showed that such radar systems may be used to measure tsunami
surface current velocity [20]; nevertheless, the observations indicated that the measurement update rate needed to be increased.
Postprocessing of measurements was done on a windowed
133-second time series, sliding in 33-second steps across each
52

5-minute block of coherently measured data. Intense tsunami signatures of changing surface current velocities were observed by
the HF radar system (see Figure 3, top). Large deviations up to 50
cm/s in ocean current measurements were obtained after detrending the natural tidal component from measured velocities. The
tsunami wave train can be clearly seen already tens of kilometers
offshore in the radar measurements. The current velocity becomes
stronger closer to the coast. Due to the narrow shelf (10-20 km)
covered by radar, the first appearance of current deviation occurred
about 7 minutes before the waves reached the coast.
The current velocities were found to be significantly correlated with the water level measurements from the tide gauge in Lebu,
Chile, located 50 km south of the radar site [20]. The water level
periodically increased up to 1-2 m for several hours, although
the first tsunami wave height was only 40 cm (see Figure 4). The
tsunami wave periodicity was estimated from the radar and tide
gauge data. It has values of 14 and 32 minutes, the same for both
instruments. Note that the first wave was not the strongest wave,
as is usually thought. This indicates a distinguished and important
property of HF radar, i.e., it offers the possibility of not only being able to detect the first tsunami wave, but it can also be used to
continuously monitor the full tsunami wave train; hence, an alert
message may be issued each time when the next wave approaches
the coast.
The estimated radial current residuals were compared with
modeled zonal and meridional velocity components calculated
specifically for the HF radar coverage in Chile by using the National Oceanic and Atmospheric Administration (NOAA) Tsunami
Forecast model [21]. Figure 3 shows a comparison between modeled radial velocities and those obtained from field data. An additional model comparison with tide gauge sea-level measurements
is presented in Figure 4 and exhibits that the model estimated the
first wave arrival about 20 minutes earlier than what actually hap-

IEEE A&E SYSTEMS MAGAZINE

MARCH 2018



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