UHF Radar signature of a tsunami approaching coastal area: modelling, experiments and application to tsunami warning
Résumé
Perhaps for the first time, satellite altimeters provided transects of the 12/26/04 tsunami elevation across the Indian Ocean, while the event unfolded. Here we 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 greatly increases in speed (maybe up to 10-20 cm/s) and may induce significant Doppler shifts in ocean surface waves, particularly for those of smaller wavelength (high frequency). Given proper processing, such shifts could be identified by shore-based UHF radars as a tsunami signature and trigger a warning. Due to the tsunami slowing down with decreasing water depth, from the shelf break to shore, warning times of 5-15 minutes could be conceivable, depending on the shelf width, which could be sufficient to proceed with vertical evacuation in exposed areas. 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/refraction caused by slowly varying depth uniform currents. Such currents can be obtained for selected case studies, e.g., from tsunami propagation modeling, using a standard long wave model. UHF radar backscattering is modeled by a Boundary Element Method, solving Maxwell's equations, developed and validated in earlier work. We present initial results of this modeling study, in terms of spatio-temporal UHF radar signatures and their sentsitivity to governing physical parameters. We also present large-scale laboratory results for radar backscattering spectra measured during wave-current interaction experiments performed in the FIRST basin (La Seyne, France). We intend to apply our detection methodology to Southern Thailand, based on earlier modeling work we performed for the 12/26/04 tsunami propagation.