Inertia-gravity-wave scattering by geostrophic turbulence
Orateur : Jacques Vanneste / Univ. Edinburgh
Abstract :
Inertia-gravity waves (IGWs) propagating in the atmosphere and ocean are scattered through advection and refraction by the geostrophically balanced background flow. Under a realistic assumption of a weak flow represented as a random field, we derive a kinetic equation that captures this scattering and is determined by the flow’s kinetic energy spectrum. This equation quantifies IGW energy transfers catalyzed by the flow and restricted to constant-frequency cones in wavenumber space. It predicts the horizontal isotropization of the IGW field and a cascade to small scales, making it possible to estimate the time scales of these processes. For short-wavelenth IGWs, the kinetic equation reduces to a diffusion equation with a wavenumber dependent diffusivity. In the stationary limit, this yields IGW energy spectra that scale with the wavenumber k as k^-2. This scaling is consistent with atmospheric and oceanic observations in the range of scales where the assumptions underlying the diffusive approximation are expected to hold. In the atmospheric case, this supports the interpretation of the shallow part of the Nastrom—Gage spectrum as associated with almost linear IGWs. Our theoretical predictions are tested against high-resolution simulations of the non-hydrostatic Boussinesq equations. (Joint with M A C Savva and H A Kafiabad)
Date et lieu : le vendredi 29 janvier 2021 à 11h00 (visio)
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