Transition in atmospheric boundary layer turbulence structure from neutral to convective, and large-scale rolls

The three-dimensional turbulence structure of the canonical daytime atmospheric boundary layer (ABL) reflects the balance between shear and buoyancy turbulence production characterized by , where is the boundary layer height and is the Obukhov length. In the shear-driven neutral ABL () the surface layer is characterized by coherent low-speed streaks, while the 'moderately convective' state () contains streamwise-elongated sheet-like updraughts. Using large-eddy simulation, we analyse the transition in ABL turbulence structure in response to systematic increases in surface heat flux and from neutral to moderately convective. We discover a sudden change in turbulence structure at the 'critical' state with nearly three-fold increase in the streamwise coherence length of 'streaks' at the upper surface layer and four-fold increase in the streamwise scale of updraughts in the mixed layer - as well as the initiation of spatial correlations between the two that contribute to the local generation of thermal updraughts as surface heating increases. At subcritical stability states, buoyancy impacts only the vertical thickness of the streaks. ABL receptivity to buoyancy changes when supercritical and updraughts now grow in the vertical with increasing surface heat flux; the previously amalgamated updraughts extend further in the streamwise direction. The post-critical regime is highlighted by a 'maximum coherence state' at and maximally coherent 'large-scale rolls'. At increasingly unstable states (), roll coherence decreases and the vertical scales asymptote to the canonical moderately convective ABL.