Energy anisotropy, turbulence organization and the role of pressure-redistribution in near-surface unstably-stratified turbulence

Reynolds stress anisotropy is one of the fundamental characteristics of all wall bounded turbulent flows, especially those in the atmosphere. In canonical (flat and horizontally homogeneous) boundary layers, the anisotropy presents a balance between the processes that generate it (shear, buoyancy and wall blocking) and the pressure-redistribution terms that act to redistribute turbulent kinetic energy towards the non-energetic velocity components. This pressure redistribution remains an area of active research, especially in stratified conditions, dominating the atmospheric surface layer.

Here we use observations from four turbulence towers in flat and horizontally homogeneous terrain to explore the evolution of anisotropy as the stratification becomes increasingly unstable. We identify three regions that can be roughly related to dynamic, convective-dynamic, and convection regimes, and show which processes, including turbulence organization into coherent structures, dominate anisotropy in each region, and particularly what is the role of rapid pressure-redistribution terms in driving the decrease of wall-normal and increasing spanwise variance in the dynamic-convective region.