Roughness layer corrections with emphasis on SVAT model applications

Implementation and application of soil vegetation atmosphere transport (SVAT) models require knowledge of aerodynamic properties of the exchanging surface. Therefore, one needs to specify several parameters such as the zero plane displacement (d), the roughness length for momentum (z0,m) and additional parameters describing the roughness layer just above the canopy as the height of the roughness layer for momentum and scalars (View the MathML source, View the MathML source), and parameters of the modified diffusivity profile functions. Several data summaries from the literature on aerodynamic properties over a broad range of plant canopies are used to develop and test predictive models for a number of needed aerodynamic parameters. A new predictive equation for d is presented as a function of canopy height (hc) and plant area index (PAI). A semi-empirical equation for the canopy length scale Ls=uh/(du/dz) (where u is the horizontal wind speed) is derived from hc, PAI, fractional crown height (fcr) and inter-element spacing of roughness elements (D). Furthermore, for tall canopies (hc>1.7 m) View the MathML source is well described by the simple relationship: View the MathML source and was also valid for sparse canopies from savannah sites with high D. For short canopies the relation View the MathML source described data reasonably. Having estimates of d, Ls, View the MathML source and other inputs one can derive predictive equations for z0,m with choice of an appropriate profile function of momentum exchange enhancement in the roughness layer. Using observed z0,m as evaluation criteria, the hyperbolic diffusivity profile function performed better than the exponential type. Moreover using the hyperbolic profile function, predictions of z0,m were improved compared to a previous similar approach. Assuming independence of buoyancy and roughness effects, the integrated stability/roughness layer functions were derived, and an approximate integration of the momentum function for unstable atmospheric conditions is proposed. Closed form analytical expressions are given for the friction velocity (u∗), the horizontal wind speed profile (u(z)) and the aerodynamic resistance (ra), which account for both stability and roughness layer effects.