The eddy, wave, and interface structure of turbulent shear layers below/above stably stratified regions. Issue 18 (23rd September 2015)

• Record Type: Journal Article
• Authors: Hunt, Julian C. R.; Moustaoui, Mohamed; Mahalov, Alex
• Is Part Of: Journal of geophysical research. Volume 120:Issue 18(2015:Oct.)
• Page Start: 9237
• Page End: 9257
• Publication Date: 2015-09-23
• Subjects: turbulence -- shear layers -- gravity waves; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1002/2015JD023067
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: High resolution three‐dimensional simulations are presented of the interactions between turbulent shear flows moving with mean relative velocity Δ U below a stably stratified region with buoyancy frequency ( N + ). An artificial forcing in the simulation, with a similar effect as a small negative eddy viscosity, leads to a steady state flow which models thin interfaces. Characteristic eddies of the turbulence have length scale L . If the bulk Richardson number R i b =( L N + /Δ U ) 2 lies between lower and upper critical values denoted as R i ∗ (<1/5) and R ~ i ( ∼ 1 ), a "detached" layer is formed in the stable region with thickness L + greater than L, in which rotational fluctuations and inhomogeneous turbulence are induced above an interface with large gradients of density/temperature. Comparisons are made with shear turbulent interfaces with no stratification. When R i b > R ~ i, vertical propagating waves are generated, with shear stresses carrying significant momentum flux and progressively less as R i b increases. Simulations for a jet and a turbulent mixing layer show similar results. A perturbation analysis, using inhomogeneous Rapid Distortion Theory, models the transition zone between shear eddies below the interface and the fluctuations in the stratified region, consistent with the simulations. It demonstrates how the wave‐momentum‐flux has a maximum when R i b ∼2 and then decreases as R i b increases. This coupling mechanism between eddies and waves, which is neglected in eddy viscosity models for shear layers, can drive flows in the stratosphere and the deeper ocean, with significant consequences for short‐ and long‐term flow phenomena. The "detached layer" is a mechanism that contributes to the formation of stratus clouds and polluted layers above the atmospheric boundary layer. Key Points: Finite range of Ri for which internal waves transport significant momentum from the shear layer In this range the induced waves lead to a smooth variation of mean and density gradients In the lowest range of Ri, the stable stratification has the sharpest gradients