Extrapolating Offshore Tsunami Model Results to the Coast

The tsunami forecast system in use at NOAA's Tsunami Warning Centers takes advantage of the linear nature of tsunami propagation in deep water by pre-computing basin scale wave results and using a deep-ocean data-model inversion technique to scale/combine them in real time during an event [Wei, et al., (2008), Geophys. Res. Lett 35, L04609]. The resulting offshore model is used as forcing for fully nonlinear high-resolution inundation models at the community scale. NOAA's forecast system relies on real-time computations of these high-resolution models using a telescoping grid technique in which only the inner-most grid provides guidance. This study looks at the possibility of using offshore model results from either the pre-computed propagation database or intermediate-scale inundation models to provide coastal wave heights as guidance in areas outside the inner-most grids, and to prioritize the order in which the forecast system should allocate computer resources. This coastal guidance would derive coastal wave heights from three difference approaches: 1) Green's law scaling, 2) the runup formula for a solitary wave on a sloping plane beach [Synolakis, (1987), J. Fluid Mech. 185, 523-545], and 3) a leading-depression N-wave runup formula for a sloping plane beach [Tadepalli & Synolakis, (1994), Proc. R. Soc. Lond. A 445, 99-112]. The accuracy of these three schemes is investigated by comparing extrapolated heights with high-resolution inundation model wave heights for specific events including the 11 March 2011 Tohoku, Japan tsunami.