Influence of convective instability on canopy and roughness sublayer turbulence

Previous work shows that the inviscid instability of canopy-top shear creates turbulence structures within the roughness sublayer that consist of pairs of head-up and head-down vortices. These vortices are associated with ejection-sweep combinations and, in diabatically neutral flows, are responsible for a large fraction of the momentum and scalar exchange between the lower atmosphere and vegetation canopies. Here, we examine the influence of unstable stratification on such structures and on their spatial organization. A set of large-eddy simulations was completed, ranging from weakly unstable to strongly unstable. The computations took place using 2048×2048×1024 grid points corresponding to a 5120×5120×2048 m³ domain. The ten lower-most grid points resolve a 20 m tall forest, as specified by a height-dependent foliage area density and an element drag coefficient. A multi-level canopy model partitions incoming solar radiation into sensible and latent heat fluxes, and heat transfer to the soil. Convective boundary layers develop to heights of 700 to 1000 m. Changes in area-average statistics introduced by thermal instability include: (i) a reduction in the magnitude of the correlation between streamwise and vertical velocity, but not between vertical velocity and temperature, and (ii) a decrease in the spatial correlation between the flux of momentum and the flux of heat, implying that momentum and heat are transferred by different elements within the flow. The weakly unstable boundary layer exhibits longitudinal rolls, spanning the whole boundary layer, with diminished canopy-top streamwise velocity beneath regions of upwelling, and increased streamwise velocity beneath areas of downwelling. With increasing instability, the boundary-layer transitions toward an open cellular structure with discrete regions of relatively weak large-scale downdraft enclosed by narrow walls of strong updraft. Areas of enhanced canopy-top velocity occur in the downwind sections of the large-scale downdrafts and immediately ahead of the cell wall, while low speed areas appear within the upwind sections of the downdrafts. Consequently, areas of high canopy-top wind shear are controlled by convective cellular patterns, impacting the creation, spatial distribution, and character of roughness sublayer vortical structures. Areas of reduced canopy-top velocity occur in the upwind regions of the downdrafts, downwind of the cell walls, and in these regions canopy-scale plumes appear and coalesce to form the ascending walls of the large-scale cells. These structures appear to be responsible for the majority of the heat flux. Partitioning the fluxes of momentum and heat into quadrants (sweep, ejection etc.), we demonstrate an increasing dissimilarity between the fluxes resulting from their spatial distribution across the large-scale convective cellular patterns.