Roughness lengths and roughness sublayer corrections in partly forested regions

Abstract: Questions related to water resources management and climate change at regional to global scales have inspired research about processes governing exchange of water, heat and carbon between land and atmosphere. The concept of roughness length is fundamental in deduction of bulk transfer relations from the generally accepted Monin-Obukhov similarity theory for this exchange. The theory has been questioned close to rough forest surfaces. A major problem has been to find effective roughness-length values for patchy boreal landscapes. This thesis contributes to these issues by theoretical analyses and measurements with focus on the roughness lengths, especially for temperature (zot) at various scales and on corrections of the Monin-Obukhov theory over forest surfaces.It was originally shown that the logarithmic quotient (kB-l) of roughness lengths for wind and temperature roughly equals 2 for barley. Analysis of pasture data showed that kB-1 depended on the roughness Reynolds number, Re0: kB-1=0.08Re00.5-1.9. A re-evaluation of barley data similarly gave: kB-1=0.348Re00.3-1.2.A measurement system was developed with a novel subsystem for humidity (and CO2) measurements based on an IR gasanalyser and a cascade filter. With this system, a roughness sublayer was detected extending to one tree height above the tree crowns. The eddy diffusivities for heat and humidity, but not for momentum, were much enhanced in the roughness sublayer. The multiplicative roughness sublayer corrections took the form: Φt,q=z/z' with z'=35.9 m andΦu= (z/z')0.6 with z'=23.9 m (above displacement height).kB-1 equalled -0.5 when forest surface temperature was given by the average radiation temperature over 20o-80o nadir angles at 4 azimuths. A negative value may be caused by eddy diffusivity for heat exceeding that for momentum in the roughness sublayer. The regional kB-1 factor was 1.5 for a 10 km upwind area. The regional zot was biased towards the forest-component zot.