Geostrophic versus Wave Eddy Viscosities in Atmospheric Models

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  • 1 Recherche en Prévision numérique, Service de l’Environnement Atmosphérique, Dorval, Quebec, Canada
  • 2 LEGI-IMG, Institute National Polytechnique de Grenoble and Université Joseph Fourier, Grenoble, France
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Abstract

It is well established that at low Rossby and Froude numbers modes possessing potential vorticity behave differently from gavity-inertial wave modes. Wave energy cascades relatively more efficiently downscale to the dissipation, resulting in a geostrophic adjustment. For this reason, it has been suggested that wave energy be subjected to relatively stronger dissipation via “divergence damping.” This study reports separate measurements of the effective eddy damping acting on wave and rotational modes in simulations of nonhydrostatic Boussinesq flow. The method employs an arbitrary cutoff wavenumber, kc, within the simulation’s range of resolved scales, in order to calculate explicitly the effect of the smaller-scale motion on wavenumbers below kc. It is found that the rotational-mode eddy viscosity resembles that found in studies of 2D turbulence, with a significant negative range, while it is positive at all wavenumbers for the wave modes.

Abstract

It is well established that at low Rossby and Froude numbers modes possessing potential vorticity behave differently from gavity-inertial wave modes. Wave energy cascades relatively more efficiently downscale to the dissipation, resulting in a geostrophic adjustment. For this reason, it has been suggested that wave energy be subjected to relatively stronger dissipation via “divergence damping.” This study reports separate measurements of the effective eddy damping acting on wave and rotational modes in simulations of nonhydrostatic Boussinesq flow. The method employs an arbitrary cutoff wavenumber, kc, within the simulation’s range of resolved scales, in order to calculate explicitly the effect of the smaller-scale motion on wavenumbers below kc. It is found that the rotational-mode eddy viscosity resembles that found in studies of 2D turbulence, with a significant negative range, while it is positive at all wavenumbers for the wave modes.

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