Steady State Two Dimensional Air Flow In Forests And The Disturbance Of Surface Layer Flow By A Forest Wall

New semi-empirical models are obtained of the mean momentum transport processes in and above forests for two contrasting micrometeorological problems: (1) the equilibrium air flow in forests far upwind of any inhomogeneity in the fetch, and (2) the nonequilibrium air flow in the transition region of the surface layer flow disturbed by the entrance into a forest. The study is confined to neutral stability conditions. Experimental data are from studies in eight forests and a wind tunnel simulation. The equilibrium mean velocity profiles in forest canopies are shown to be two-dimensional. A lateral component of flow increases downward from the top of the canopy. A semi-empirical model of mean velocity components is derived and verified for the upper 90% of the forest canopy depth. In the nonequilibrium flow in the transition region downwind of a forest wall: (a) The logarithmic velocity profile is found to be an empirical approximation for the lower part of the boundary layer, assuming that only the friction velocity varies with downstream distance. An empirical relation for the horizontal variation of the friction velocity is obtained. (b) The mean velocity profiles in a mixing zone located at midcanopy level are shown to have a low-level velocity maximum and horizontal similarity. (Author).

Effort has been expended in modeling air flow within and above simple roughness elements for neutral steady-state conditions. Application of the ideal canopy flow concept suggested by Cionco, Ohmstede and Appleby is now extended to various natural and artificial canopies of simple or complex structure. Properties and characteristics considered were: Shape of the unit canopy wind profile, intensity of turbulence magnitudes, an index of canopy flow, and the effects of density and flexibility variations upon the flow. The profile shape is characteristic and best described by an exponential relationship within simple-structured elements. In complex structures, the profiles exhibited low-level maximums and no-gradient layers. Above the vegetation, the logarithmic wind relation was valid for each canopy type. Intensity of turbulence was relatively uniform with height for simple canopies and significantly non-uniform with height in complex structures, with large leaf area maximums and very low wind speeds. The ideal canopy flow concept can potentially serve many disciplines and efforts besides micrometeorology and diffusion research. If to serve no other purpose, it can be used to establish the initial formulation of a boundary condition or minor portion of a larger problem. The mathematical simplicity of the concept may be its most important quality when it is to be used as part of a larger system of equations.