IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS002 Poster presentation 1793
Modeling of the UHF Radar signature of a tsunami approaching coastal
areas: application to tsunami warning
Prof. Stephan Grilli
Department of Ocean Engineering University of Rhode Island
Marc Saillard, Sara Dubosq
During the catastrophic 12/26/04 tsunami, perhaps for the first time, satellite remote sensing (radar
altimetry) was able to provide transects of tsunami elevation across the Indian Ocean while the event
unfolded (e.g., Jason I). Here we similarly use well established Ultra High Frequency (UHF) radar
technology, to develop a method that could provide warning of an incoming tsunami to coastal
populations. When a tsunami reaches the continental shelf, the mostly depth-uniform current it induces,
indeed greatly increases in speed (maybe up to 10-20 cm/s) and starts causing significant Doppler shifts
in ocean surface waves, particularly those of smaller wavelength (high frequency). Such shifts may be
recognized as a typical signature, by a shore-based UHF radar, and thus trigger a warning, provided
proper numerical simulations and sensitivity analyses have been performed ahead of time. Since
tsunamis very much slow down due to decreasing water depth, from the shelf break to shore, warning
times 5-15 minutes could be conceivable, depending on the shelf width. Here, we use a Higher Order
Spectral (HOS) Method to model fully nonlinear sea states caused by wind, down to typical UHF
wavelength of order 10 cm, as well as Doppler shifts and wave shoaling and refraction caused by a
slowly varying depth uniform current. The latter current is obtained from tsunami propagation modeling,
using a standard long wave model, for some selected case studies. UHF radar backscattering is modeled
by a Boundary Element Method solving Maxwells equations, developed and validated in earlier work. We
present initial results of this modeling study, in terms of spatio-temporal UHF radar signatures
corresponding to some tsunami case studies. In particular, we apply this methodology to southern
Thailand, based on earlier modeling work we performed for tsunami runup caused in Thailand for the
12/26/04 tsunami.
Keywords: tsunam modeling, remote sensing, warning systems