Flux-profile relationships over a boreal forest -: roughness sublayer corrections

Calculation of fluxes from profile measurements above forest stands have for some time been considered unfeasible and profiles have primarily been used to only characterise the lowest part of the planetary boundary layer. Extension of traditional theory to correctly relate fluxes to gradients would provide both new insight into transfer processes and a way to measure fluxes. This paper presents measurements and analysis of profiles of wind speed, air temperature and humidity, and also eddy-correlation fluxes under unstable conditions. The measurements were carried out at the NOPEX Central Tower Site 30 km north of Uppsala, Sweden. This site represented a mixed pine-spruce forest of a height of about 24.5 m. The displacement height (d) was found to be 21.1 m. A roughness sublayer with enhanced eddy diffusivities and decreased gradients was found above the forest. This was accounted for by multiplying the traditional non-dimensional gradients with a correction function of the form phi(u,t,q) = (z/z(*))(n) (z denotes height above d). phi was a linear function of height (n = 1) for temperature and humidity with their roughness sublayer height z(*) = 35.9 m. Enhancement of momentum exchange and its extension were lower (n = 0.6, z(*) = 23.9 m). The fluxes calculated from profiles by a method which took into account the roughness sublayer corrections were generally in good agreement with those measured by the eddy-correlation method. (C) 1999 Elsevier Science B.V. All rights reserved.