The Role of Roughness Sublayer Dynamics Within Surface Exchange Schemes

• Published in: Boundary-layer meteorology Vol. 142; no. 1; pp. 1 - 20
• Main Author: Harman, Ian N.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 2012; Springer; Springer Nature B.V
• Subjects: Analysis; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; Boundary layer; Boundary layers; Canopies; Computational fluid dynamics; Convection, turbulence, diffusion. Boundary layer structure and dynamics; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Energy balance; Evolution; Exact sciences and technology; External geophysics; Fluctuations; Fluid flow; Mathematical models; Meteorology; Numerical analysis; Numerical weather forecasting; Roughness; Turbulence; Turbulent flow; Weather; Weather forecasting; Wind speed; Energy balance; Roughness sublayer; Numerical weather prediction; Canopy; Boundary layer; Weather forecast; Roughness length; General circulation models; Wind velocity; energy balance; digital simulation; Rough surface; turbulence; Displacement height; Momentum transfer; Drag; roughness; Coherent structures; Numerical forecast; Surface energy; stability; Canopy(vegetation)
• ISSN: 0006-8314; 1573-1472
• DOI: 10.1007/s10546-011-9651-z

Turbulence above and within canopies has characteristics distinct from that over rough surfaces. The vertical transport of momentum and scalars is dominated by coherent structures whose origin is now thought to be the result of the unstable inflexion in the profile of the mean wind speed established by the application of canopy drag. This distinctive property leads to the failure of the standard Monin–Obukhov flux–profile relationships over homogeneous canopies, relationships that are assumed in many surface exchange schemes within numerical weather prediction and general circulation models. A modification of the flux–profile relationships is presented that incorporates the effects of the canopy turbulence. The subsequent impacts on the evolution of the surface energy balance and boundary-layer state are investigated within a simple numerical model for the evolution of the boundary layer and canopy state. By comparing cases with and without the modification it is shown that canopy-generated turbulence can lead, not only to the alteration of the flux–profile relationships above the canopy, but also to a different evolution of the surface energy balance and differences in near-surface conditions that would be significant in numerical weather prediction. More fundamentally, the modifications to the flux–profile relationships imply that parameters such as the roughness length and displacement height for canopies should not be considered as invariant properties, but rather as properties that depend on the flow and hence vary systematically with the diabatic stability of the boundary layer.