A New Hybrid Fourier Ray Method for Computing Low Froude Number Stratified Flow over Topography in the Atmosphere

A new kind of forecast model is developed for the internal wavefield generated by stratified flow over three-dimensional topography. The new model is based on high-resolution nonlinear numerical simulation of the near-field flow around the topography combined with a ray-tracing of the far-field propagation of the internal waves generated by the near-field flow. The far-field model allows for the refraction and reflection of the waves by height-dependent winds and stratification. The far-field ray-tracing code is not in the usual spatial formulation. Instead the rays are traced in a Fourier-transform domain, and the resulting ray solution is mapped into a spatial solution by inverse Fourier transform. The use of a Fourier-transform domain simplifies all aspects of the ray calculation. The initialization of the rays from the numerical model is done at very close distances to the topography, in a way that is self-consistent with ray theory assumptions. This is a direct consequence of the use of a ray formulation in the Fourier transform domain, and it means that the numerical resolution for the near-field simulation can be concentrated near the topography. An example simulation and comparison with atmospheric observations will be shown.