Comparison of aerodynamic and energy budget estimates of fluxes over a pine forest

Abstract

Values of the total vertical flux of sensible and latent heat over a level forested region, obtained from aerodynamic (profile-gradient) formulae appropriate to airflow over relatively smooth surfaces, are found to fall consistently short of independent energy-balance estimates by a factor of 2 to 3 in unstable and near-neutral conditions (Richardson number, Ri, in the range −0·4 to +0·01), whereas for Ri > + 0·02 no similar discrepancy is detected. These results, based on tangents drawn to (semi-logarithmic) profiles at a height of about nine aerodynamic roughness parameters (z0) above the zero plane displacement level (d) of the forest, rely on the basic assumption that the value of d established in very nearly neutral conditions (|Ri| < 0·003), namely 0·76 mean tree heights, holds under all conditions of thermal stability. Wake diffusion and thermal seeding effects are discussed as possible additional transfer mechanisms acting to reduce profile gradients immediately over aerodynamically rough surfaces. In terms of the former mechanism (assumed to operate below d + 25z0, or so), approximate empirical formulae are derived which attempt to quantify the observed discrepancy in terms of Ri and the proximity of the surface. It is concluded that aerodynamic equations ought not to be used to give independent flux estimates close to aerodynamically rough surfaces.