Atmosphere Ocean Science Colloquium

Stratified Turbulence at Laboratory and Atmospheric Scales

Speaker: Michael Waite, University of Waterloo

Location: Warren Weaver Hall 1302

Date: Wednesday, March 25, 2015, 3:30 p.m.


Stable density stratification has a profound effect on turbulence in the atmosphere and ocean:buoyancy forces inhibit vertical motion, leading to quasi-horizontal layerwise structures, interacting gravity waves, and intermittent bursts of instabilities and small-scale turbulence. For the last few decades, stratified turbulence has been proposed, with mixed success, as a mechanism to explain the shape of the kinetic energy spectrum in the atmosphere. The mesoscale portion of the spectrum has an approximately - 5/3 slope, which is reminiscent of the energy cascades of isotropic two-and three-dimensional turbulence. However, mesoscale turbulence is obviously not purely two-dimensional or isotropic in three dimensions. One possible explanation is that the mesoscale spectrum results from a stratified turbulence cascade. Stratified turbulence can be studied in an idealized computational framework (periodic boundary conditions, uniform stratification, etc.) and many such studies show broad consistency with the mesoscale cascade. In this talk, I will start with a review some of the basic properties of stratified turbulence, and give an overview of attempts to apply it to the atmospheric mesoscale. Second, I will present numerical simulations that explore the different behaviour of stratified turbulence at laboratory and atmospheric length scales. This work rules out some theories of mesoscale stratified turbulence, which are shown to be realizable only at laboratory length scales. Finally, I will discuss recent mesoscale model simulations that analyze the dynamics of the energy spectrum and cascade. It is argued that stratified turbulence alone cannot account for the mesoscale spectra in these simulations, and that other phenomena, especially the injection of kinetic energy by moist effects, are essential.