Séminaire

Improvements in both algorithms and high performance computing resources now permit global atmospheric simulations to be performed at resolutions at which non-hydrostatic effects become significant. As such, we consider HOMME-NH, a variable-resolution, efficient and architecture-aware, non-hydrostatic dynamical core that has been developed under the US DOE’s earth system modeling initiative. We study the nonlinear evolution of an unstable baroclinic wave in both the hydrostatic and non-hydrostatic settings with an aim of understanding how attendant dynamical interactions across scales differ in the two settings. Our aim in this study is to try to anticipate how biases in climate models may change when non-hydrostatic dynamics are resolved in the atmosphere.
Linkages between the larger scale balanced modes and smaller scale, imbalanced modes play an important role in ocean circulation as well. For example, the question of how ocean circulation equilibrates in the presence of continuous large-scale forcing and a tendency of geostrophic turbulence to confine energy to large and intermediate scales is related to such linkages. Again, by considering the nonlinear evolution of an unstable baroclinic wave at small Rossby and Froude numbers (and in a small aspect ratio domain) at high resolutions, we show that submesoscale instabilities provide an interior pathway between the energetic oceanic mesoscales and smaller unbalanced scales. Phenomenology-wise, mesoscale shear and strain resulting from the primary baroclinic instability drive frontogenesis; fronts in turn support ageostrophic secondary circulation and instabilities. These two processes together lead to a quick rise in dissipation rate which then reaches a peak and begins to fall as frontogenesis slows down; eventually balanced and imbalanced modes decouple. Dissipation of balanced energy by imbalanced processes is shown to scale exponentially with Rossby number of the base flow. Further, a break is seen in the total energy (TE) spectrum at small scales with a transition from −3 to −5/3 reminiscent of the atmospheric spectra of Nastrom and Gage.

bruno.deremble@ens.fr