Influence of stability on roughness sublayer turbulence

Analysis of field observations and turbulence/canopy-resolving numerical simulations reveals that the three-dimensional space-time structure of the dominant canopy eddies are fundamentally linked to the inviscid hydrodynamic instability resulting from the inflected mean velocity profile generated by canopy-induced distributed pressure drag. In this sense, canopies generate near-wall physics distinct from that of smooth-wall boundary layers.

The impact of diabatic stability on turbulence dynamics in the canopy and roughness sublayer has been noted in a qualitative way for at least two decades but we still lack a proper understanding of the flow regimes's controlling physics under such conditions. In fact, the usual way of classifying stability effects in canopies by using as a parameter h/L -- the ratio of the canopy height h to the Obukhov length L -- lacks any physical basis.

Recently generated high-resolution simulations of the full planetary boundary layer (PBL) responding to variations in the magnitude of the geostrophic wind and a fully interactive vegetation canopy offer a unique venue to analyze the coupling between PBL-scale motions (and their variation with diabatic stability) and canopy exchange. Analysis of these simulations will be presented offering guidance toward the mechanisms controlling canopy exchange under the influence of diabatic stability.