Numerical Simulations of the Vertical Structure of Quasi-Geostrophic Turbulence.
Résumé
Direct simulations of turbulent quasi-geostrophic flow on a βplane have been performed with an emphasis on three-dimensional resolution [128 × 128] in the horizontal and up to 6 vertical modes). The statistically equilibrated turbulence is forced by baroclinic instability of a mean shear, and dissipation occurs through friction in a bottom Ekman layer. Parameters which control the vertical structure of the variability and the energy partition between the vertical modes are identified. We verity Charney's prediction of three-dimensional isotropization of quasi-geostrophic flows in the case of a constant Brunt-Väisälä profile; for a variable Brunt- Väisälä stratification spectral shapes are preserved but isotropization is lost. All baroclinic modes show a tendency for a direct energy cascade, as opposed to the red cascade of purely barotropic flow. The coalescence of potential vorticity into three-dimensionally isolated structures is observed to occur; however, their vertical scale is highly dependent on whether they appear in a forced turbulent flow or in freely evolving turbulence, the latter having a larger vertical scale.