Multiscale Variability of Shear Stratified Turbulence Around a Jet Stream

Alex Mahalov, Departments of Mathematics and Mechanical & Aerospace Engineering, Arizona State University

JOINT CIMMS/CDS SEMINAR

There are few reliable results on the characterization of outer scales of turbulence in the remote atmosphere and their dependence on the background atmospheric conditions (stability and jet stream profile). Knowledge of the variability of turbulence outer scales and the refractive index structure parameter, Cn2, is of fundamental importance in understanding atmospheric processes as well as optical propagation in the atmosphere. In this talk, direct numerical simulations are performed to study the dynamics of an inhomogeneous stratified shear flow for a jet stream in the tropopause with a new spectral domain decomposition method that is particularly suitable for simulations of inhomogeneous stratified shear flows. The structure of the mean flow and turbulence fields are calculated, which are interpreted using relevant length scales (Ozmidov, Tatarski, buoyancy, shear, Ellison and Deardorff) and Richardson number profiles. The ratio of the Ellison to buoyancy scales also known as the temperature fluctuation parameter is 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 vertical profile of the ratio of Ozmidov and Tatarski scales is related to the gradient Richardson number and is in agreement with observations. 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 with increased values of Cn2, the refractive index structure parameter. 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.