Budgets of Lineal and Nonlineal Turbulent Kinetic Energy under Strong Shear Conditions

Abstract

Quasi-lineal “roll” coherent structures appear in a large eddy simulation of strong wind shear and moderate surface buoyancy flux. Unlike idealized two-dimensional roll vortices, these rolls are finite in length and vary in axial angle. Over a subdomain corresponding to an individual roll’s length and orientation, the flux due to the strictly two-dimensional “lineal” eddy component dominates the total vertical turbulent transport in the mid- and upper boundary layer. Separate analyses of turbulent kinetic energy budgets for the lineal (two dimensional) and nonlineal (three dimensional) eddies reveal differences in energy transfer between the large-scale eddies and the smaller-scale eddies that extract energy from them. For the transverse component, direct shear generation is small relative to pressure transfer from other components: for large (small) eddies the associated pressure transfer loss occurs primarily from the vertical (longitudinal) component and thus indirectly from buoyancy (shear) production of that component. The so-called return to isotropy pressure terms transfer energy from components with larger production terms to components with weaker production rather than from components of larger energy to components of weaker energy. Upscale transfer of vertical energy occurs in the upper boundary layer, suggesting the merger of small-scale thermal elements into the larger-scale roll. Roll energy results not from transverse shear, but instead from buoyancy being converted into horizontal energy through pressure forcing.