Vertical Length Scale of Transporting Eddies for Sensible Heat in the Unstable Roughness Sublayer Over a Forest Canopy

Abstract

This study investigated the vertical length scales of transporting eddies calculated from multi-level turbulence measurement in the unstable roughness sublayer over a flat pine forest by using the surface renewal method as a description of heat transport. It was observed that the vertical integral scale exhibited a increasing tendency with thermal instability, whereas the shear length scale of Raupach et al. (1996, Boundary-Layer Meteorol., 78, 351-382) was relatively stability-insensitive. These length scales were used as the height of the transporting eddies in the surface renewal method in order to examine the relevance of the length scales to the turbulent transfer. Comparisons with the eddy covariance measurement showed that variation in the performances of the surface renewal method with the different length scales was most noticeable under unstable conditions; the use of the vertical integral scale leads to better accuracy in flux computation. The results indicated the validity of the vertical integral scale as a direct measure of the transporting eddy scale in the surface renewal context. It was also suggested that the shear length scale may have limitations in terms of describing the vertical scale of the eddies in the unstable roughness sublayer when the buoyancy has a considerable effect on turbulence production.