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

Calculation of fluxes from profile measurements above forest stands have fo r some time been considered unfeasible and profiles have primarily been use d to only characterise the lowest part of the planetary boundary layer. Ext ension of traditional theory to correctly relate fluxes to gradients would provide both new insight into transfer processes and a way to measure fluxe s. This paper presents measurements and analysis of profiles of wind speed, air temperature and humidity, and also eddy-correlation fluxes under unsta ble conditions. The measurements were carried out at the NOPEX Central Towe r Site 30 km north of Uppsala, Sweden. This site represented a mixed pine-s pruce forest of a height of about 24.5 m. The displacement height (d) was f ound to be 21.1 m. A roughness sublayer with enhanced eddy diffusivities an d decreased gradients was found above the forest. This was accounted for by multiplying the traditional non-dimensional gradients with a correction fu nction 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 exch ange and its extension were lower (n = 0.6, z(*) = 23.9 m). The fluxes calc ulated from profiles by a method which took into account the roughness subl ayer corrections were generally in good agreement with those measured by th e eddy-correlation method. (C) 1999 Elsevier Science B.V. All rights reserv ed.