Momentum Fluxes of Gravity Waves Generated by Variable Froude Number Flow Over Three-Dimensional Obstacles

摘要

Fully nonlinear mesoscale model simulations are used to investigate the momentum fluxes of gravity waves that emerge at a 'far-field' height of 6 km from steady unsheared flow over both an axisymmetric and elliptical obstacle for nondimensional mountain heights h(sub m) = 1/Fr in the range 0.1-5, where Fr is the surface Froude number. Fourier- and Hilbert-transform diagnostics of model output yield local estimates of phase-averaged momentum flux, while area integrals of momentum flux quantify the amount of surface pressure drag that translates into far-field gravity waves, referred to here as the 'wave drag' component. Estimates of surface and wave drag are compared to parameterization predictions and theory. Surface dynamics transition from linear to high-drag (wave-breaking) states at critical inverse Froude numbers 1/Fr(sub c) predicted to within 10% by the transform relations of Smith (1989b). Wave drag peaks at 1/Fr(sub c).