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A promising new remote sensing technique for early detection of tsunamis
Recent observations of "tsunami shadows," i.e.,
extended darker strips on the ocean surface along
a front of a weak tsunami off Oahu, Hawaii, suggest
that tsunamis in the deep ocean may be
remotely detected through changes in ocean surface
roughness. An investigation was done to
identify physical mechanisms responsible for the
formation of "tsunami shadows." It has been
demonstrated that the change in surface roughness
is due to air-sea interaction. Assuming a
neutrally-stable atmosphere, using the eddy-viscosity
model for average Reynolds stresses in turbulent
flow, and taking advantage of the very
high velocities and very large wavelengths characteristic
of tsunamis, an asymptotic theory has
been developed that describes tsunami induced
perturbations to the mean wind
velocity. The asymptotic theory has been verified
against numerical simulations.
It has been demonstrated that, in the lowest
tens of centimeters of the atmosphere, tsunami-
induced perturbations of the mean wind
velocity are much greater than current velocities
in the water and can be comparable to
unperturbed wind velocity. The results are
shown to be robust with respect to a choice of
a closure model for Reynolds stresses.
Theoretically predicted features of the "tsunami
shadows" are in agreement with observations
in visible light. For potentially destructive
tsunamis, expected changes in surface
roughness are such that the "tsunami shadows"
should be observable also on the high seas
with airborne and satellite-based microwave
radars and radiometers. The spatial structure of
the "tsunami shadows," namely, their length of
thousands of kilometers along the tsunami wave
front and width of a few tens of kilometers
across the wave front, are conducive to tsunami
detection from space. Of particular significance
in application to tsunami warning is the fact that
the "tsunami shadows" propagate at a known
and very distinct speed, which allows for their
unambiguous differentiation from other features
on the ocean surface.
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CIRES Research Theme
Advanced Modeling and Observing Systems
Project Personnel
O. A. Godin
Funding Source(s)
DoD
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