Thermal roughness length of a boreal forest

The aim of this study was to determine the roughness length for temperature (z(ot)), which has rarely been done for forests. The analysis was based on measurements of profiles of wind speed, air temperature, and surface radia tion temperature in a forest at the NOPEX Central Tower Site, 30 km north o f Uppsala, Sweden. This site represented a mixed pine and spruce forest wit h a height of about 24.5 m. The analysis of the profiles took into account the roughness sublayer corrections. A single displacement height, equal to 21.1 m, was applied for both momentum and heat exchange. The roughness leng th for wind speed (z(ou)) was found to be 1.75 m. The surface radiation tem perature was measured with a fixed and a moving sensor and was studied in d etail. It was found that the inhomogeneities of the forest introduced large variations in the surface radiation temperature (up to 5 K). The temperatu re from the fixed sensor pointing under 45 degrees to the east was close to the average taken over all measured directions, near noon. Using the near- noon fixed-sensor data and assuming that it best represents the effective s urface temperature, the quantity In(z(ou)/z(ot)) was found to be slightly n egative (=-0.5) which implies that the roughness length for temperature is larger than that for wind speed. This is quite an unexpected result but it can be explained in terms of a deep roughness sublayer above the canopy, wh ere heat transfer is enhanced compared to momentum transfer, and to the fac t that the main roughness elements, small sized needles, have a thin bounda ry layer and therefore a small bluff-body effect. The analysis covered day- time unstable conditions only. (C) 1999 Elsevier Science B.V. All rights re served.