The influence of large coherent structures on near-surface and entrainment zone properties in convective boundary layers

Turbulent flows exhibit not only chaotic motion, but also coherent structures. In the convective boundary layer (CBL), these structures take the form of circulation cells or rolls on the scale of the boundary layer depth (referred to here as “large scale"). The influence of these large-scale structures on the small-scale eddies embedded within them challenges our understanding of the surface layer and of the entrainment process. Using conditional analysis to isolate the behaviour within different parts of the large-scale structures and direct numerical simulation to accurately represent the full range of turbulence scales, we address three open questions regarding the influence of large coherent structures on the surface layer and entrainment zone of the CBL. The first question tackles the failure of Monin–Obukhov similarity theory (MOST) in free convection. Surface-layer statistics have been found to deviate from the scaling laws predicted by MOST and various hypotheses have been proposed to explain this. We test the hypothesis that large-scale downdrafts are the primary culprit, as they introduce non-local properties to the surface layer and thereby violate the assumption of no interaction with the outer layer. We find that the buoyancy and vertical velocity variance do not follow MOST within large-scale downdraft regions, but neither do they in large-scale updraft regions. The updraft regions are at least as important as the downdraft regions, if not more so, for determining the near-surface behaviour and hence the cause of departures from MOST is not as straightforward as has been hypothesised. The second question is whether more idealised flows with similar large coherent structures to the CBL exhibit similar near-surface behaviour. In particular, we compare surface-layer properties between the shear-free CBL and Rayleigh–Bénard convection. We find that statistical properties in the near-surface region of Rayleigh–Bénard convection have substantially different behaviour to those in the CBL. This is due to a difference of sign and magnitude of the buoyancy flux at the top of the fluid layer: the small, negative buoyancy flux at the top of the CBL generates weak, warm downdrafts, whereas the large, positive buoyancy flux at the upper plate of Rayleigh–Bénard convection generates strong, cold downdrafts that modify the near-surface region. However, we also find that only a small change to the classical setup of Rayleigh–Bénard convection is needed for surface-layer properties to behave in a similar way to the CBL. By substituting the cooled upper plate for an adiabatic one, the downdrafts are weaker and warmer, resulting in surface-layer statistics that are almost indistinguishable from those in the CBL. Finally, we reconsider the causes of wind shear enhancement of the entrainment buoyancy flux. The entrainment zone consists partly of non-turbulent, entrained air, and partly of turbulent plumes, associated with the ascending branch of the large-scale circulations, penetrating into the overlying stratification. Our third question asks how wind shear modifies properties within these different regions and how important each of these changes are for increasing the magnitude of the entrainment buoyancy flux. We find that in both the shear-free and the sheared CBL, the buoyancy flux is strongest within turbulent regions. However, wind shear does not increase the magnitude of the flux itself within those regions, but can even weaken it. The important effect of wind shear is that it substantially increases the turbulent area fraction in the entrainment zone, thereby reducing the non-turbulent area where entrainment is much less efficient.