Aerospace and Electronic Systems Magazine March 2018 - 51

Dzvonkovskaya

NECESSARY CRITERIA TO SUPPORT TSUNAMI DETECTION
HF ocean radar may provide valuable information to aid in increasing the reliability of TEWS under fulfillment of certain conditions:
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The ocean bathymetric data within the radar coverage has to
be known in detail to plan an ocean radar installation having
a maximum effectiveness for tsunami monitoring.
The width of the shelf is sufficiently extended to allow time
for issuing and transmitting a tsunami alert.
The spatial resolution of a radar mapping must be high
enough to resolve the tsunami current signatures and must
thus have high signal-to-noise performance. A narrow-beam
phased array radar system can be implemented to meet such
requirements.
The temporal resolution of the radar system must be high
enough to detect the rapidly changing surface velocity with
periods of several minutes. The potential tsunami-affected
areas should be monitored in a fast acquisition mode by using a rapid update (e.g., at 30-second intervals) of the ocean
surface current information. On the basis of such a mode, an
algorithm for real-time detection of a tsunami signature in
ocean radar data is developed.
The radar system should be equipped with an additional uninterrupted power supply unit to account for the possibility
that a power outage can happen before the tsunami waves
reach the coast.
The transmission link between a radar site and the central
server of TEWS should be stable and independent of local
communication networks, because, for example, during and
after strong earthquakes, mobile communication networks
may fail in the region.

Beyond any doubt, an optimization process is necessary for
each radar site individually due to different geometries of the width
and gradient of the continental shelf and radar operating frequency. Nevertheless, by measuring only surface current velocities, HF
ocean radar systems are able to contribute to the development and
improvement of TEWS.

SIMULATIONS OF TSUNAMI INFLUENCE ON HF RADAR
SPECTRA
The basic idea of ocean remote sensing by HF surface wave radar
finds its roots in the work of Crombie [17] who suggested that the
dominant contribution of the HF radar backscatter from the sea
surface can be explained by resonant scattering from ocean waves,
which, for grazing incidence, have a wavelength (from crest to
crest) equal to half of the radar wavelength and move radially toward or away from the radar site. This is an example of Bragg
scattering, and the phenomenon leads to two strong resonant peaks
at the so-called Bragg-resonant frequencies in the processed HF
radar Doppler spectrum. Because the phase velocity of the ocean
waves responsible for these resonant peaks is easily determined
MARCH 2018

by invoking the water wave dispersion relationship [11], their
theoretical corresponding spectral peak positions in the Doppler
spectrum are well established. Thus, in an actual Doppler spectrum
obtained from the radar echo, any frequency shift in the Bragg
peaks from their theoretical still-water values in deep water can
be related directly to an underlying ocean surface current velocity.
Similarly, surface velocities induced by tsunami waves would appear in the radar spectra as additional frequency deviations of the
Bragg peaks. At the shelf edge, the tsunami current velocity begins
to increase and produces Doppler frequency deviations of less than
1 Hz in the spectra.
An idealized scenario of tsunami-induced current velocity was
described in [12], where the simulations were done by using the
hydrodynamic HAMburg Shelf Ocean Model, which was specially adapted to represent not only water elevation caused by tsunami
waves but also corresponding surface currents. The shelf edge
was modeled as being approximately 100 km from the coastline.
Thus, the radar system was simulated to detect the tsunami about
45 minutes before hitting the coast, leaving enough time to issue a
tsunami alert. The simulated tsunami-induced currents were converted into frequency-modulating signals and superimposed on
the measured radar backscatter signals from each antenna of the
phased array HF radar system. Hence, the radar spectra included
the simulated tsunami wave train in the radar range-angle-Doppler frequency space. The potential tsunami currents provided an
additional Doppler shift to the first-order radar backscatter. Moreover, the altered current pattern in the beamformed radar spectra
showed the spatial scale of the tsunami wavelength to be tens of
kilometers on the shelf. Further details of this approach can be
found in [12], [16].
This idea was further implemented in more realistic simulations of tsunami scenarios for HF radar detection presented in [18],
[19]. The study in [18] examines the possibility of detecting the
first wave of a tsunami by an ocean radar installed in Mihama,
Japan, on the basis of a virtual tsunami observation experiment
assuming a Nankai Trough underwater earthquake with a magnitude of 8.4 Mw. The purpose of this research was to test the possibility of the radar system contributing to the prevention of underestimation of a tsunami warning, to identify severely affected
areas, and to plan further multiple HF radar system deployments.
The tsunami propagation toward the Japanese coast was assumed
to be along the radar boresight. The study in [19] was based on
simulated tsunami currents corresponding to the arrival of a tsunami generated by a virtual 9.1-Mw seismic source in the Semidi
Subduction Zone near Alaska. Simulations of tsunami propagation
were performed with FUNWAVE-TVD, a Boussinesq long-wave
model. A case of tsunami-induced water elevation near Vancouver
Island, Canada, was considered in [19]. The region of observation
was chosen specifically to overlap with the coverage of a phased
array ocean radar for an installation located in British Columbia,
Canada. Unlike the previously mentioned scenarios, the simulated
tsunami wave front was not assigned to the radar boresight, which
means that the expected radial tsunami current velocities estimated
from the radar spectra would be even smaller than the ones generated by the FUNWAVE-TVD model. The presented tsunami scenarios indicate that a multidisciplinary approach is necessary to

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