Basil Nicolaenko
Departement of Mathematics and Center for Environmental Fluid Dynamics, Arizona State University, Tempe, USA

Quasi-equilibrium dynamics of stratified turbulence in a model tropospheric jet

Direct numerical simulations are performed to study the dynamics of an inhomogeneous stratified shear flow for a model atmospheric jet in the tropopause. The basic state is characterized by a jet centered at the tropopause in which the density stratification is vertically non-uniform. Small to moderate background stratifications are selected, a weak background rotation is imposed and simulations are conducted for a range of Reynolds and Froude numbers. A new spectral domain decomposition method that is particularly suitable for simulations of inhomogeneous stratified flows is developed to generate the desired turbulent jet, and quasi-equilibrium flow-fields are obtained fter long-time integration of the governing equations. The structure of the mean flow and turbulence fields are alculated, which are interpreted using relevant length scales (Ozmidov, buoyancy, shear, Ellison and Kolmogorov) and Richardson number profiles. The ratios of the Ellison to buoyancy scales are much smaller than unity at the jet core and approach unity at the edges, confirming that mechanical turbulence prevails in the jet core, whereas nonlinear waves and stratification effects are significant at the edges. The jet core is found to support sustained mechanical (active) turbulence, outside of which lay a region of intermittent turbulence and non-linear gravity wave activity characterized by spatially decaying velocity fluctuations and strong temperature fluctuations. Detailed energy budgets show how energy is partitioned within the flow, including the transport of energy from the jet to its immediate vicinity by non-linear gravity waves.