Student Seminar: Olivier Asselin
In this talk, I will spend some time introducing a few turbulence results that could potentially better our understanding of the atmosphere and oceans. Using those results, I will try to shed some light on the controversial interpretation of the atmospheric energy spectrum. I am making a particular effort to keep the presentation as accessible as possible, so students new to the department are particularly welcome! Now, here's the more technical abstract:
On near-tropopause turbulence
The Global Atmospheric Sampling Program (GASP) provided data on the horizontal kinetic and potential energy spectra near the tropopause. While the steep synoptic-scale part of the energy spectrum agrees well with Charney's theory of geostrophic turbulence, its break to a shallower -5/3 slope in the mesoscale is still a subject of controversy.聽
Several homogeneous turbulence studies have addressed the issue. In particular, Bartello (2010) presented numerical simulations of decaying triply-periodic rotating-stratified Boussinesq turbulence. It was found that the steep geostrophic spectrum eventually crosses a shallower ageostrophic spectrum at large-enough Rossby numbers. The total energy spectrum 聽thus exhibits a slope break similar to that observed in the GASP data.
A similar slope break can also occur within the quasigeostrophic (QG) framework when vertical boundaries are taken into account (e.g. Tulloch and Smith 2006). In this surface quasigeostrophic (SQG) model, the stratification jump at the tropopause plays a crucial role in the slope break and ageostrophic motion is ignored. In the homogeneous turbulence model, it is the other way round. There is thus a need to reconcile these mutually exclusive perspectives. Numerical simulations will be presented and discussed.