Large-scale coherent structures associated with forward and backward scatter energy transfer in a strongly convective planetary boundary layer flow (CBL) are studied using large-eddy simulation (LES). It has been shown that in the laboratory-scale shear-driven turbulent boundary layer and channel flows, forward and backward scatter energy transfer are strongly correlated with ejection and sweep eddies, respectively, and statistically appear side by side around vortical structures. In the CBL, under mild or strong wind conditions, the dominant coherent structures consist of convective rolls, thermals, and vortices. We aim to study the relationship between coherent structures and interscale energy transfer in the CBL.
In our LES, the SGS energy content is about 26% at the surface, drops to 17% at the top of the surface layer and remains less than 15% in the mixed layer. In this study, we examine the energy transfer between the resolved large scale with wavenumber k £ ko/2 and the near-grid scale with wavenumber ko/2 £ k £ ko, where ko is the cutoff wavenumber. The box filter is adopted for the calculation of the resolved SGS stress t ij and the resolved strain-rate tensor Sij. The backward scatter event occurs at about 30% of the grid points. Conditional sampling at three different heights indicates that strong forward and backward scatter events are associated with particular coherent motions in the flow. Near the surface, strong forward scatter events are physically correlated with the convergence lines of the strong updraft and are dominated by the normal component t 22S22. The strong off-diagonal component t 13S13 is found at locations where downdrafts collide with the surface. Away from the surface, the vertical normal component t 33S33 becomes dominant in the inter-scale energy transfer. The forward scatter events are found at the top of the updrafts, while the backward scatter events are associated with local acceleration of vertical velocities of vortices embedded in the updrafts.