Surface influence upon vertical profiles in the atmospheric near‐surface layer

Observations from two towers situated in flat, tree‐covered terrain ( z 0 lying between 0.4 and 0.9 m) have been used to investigate theflux‐profile relations in the height range z / z 0 from 5 to 85, where z is the height above the zero‐plane displacement. The analysis confirms a lower height limit (at z = z * ) to the validity of the Monin and Obukhov functions ϕ M , H ( z / L ) in unstable conditions and, by implication, of the logarithmic wind law in neutral conditions. We find z * / z 0 ≃ 35 and 150 for wind at the denser and less dense (lower z 0 ) surfaces, whilst for temperature z * / z 0 ≃ 100. The level z * corresponds with the top of the transition layer, within which it is assumed the profiles depend additionally upon a length scale z s , related to surface wake generation. On the assumption that z * α z s , modification of the profiles in the transition layer is then described through a function ø( z / z * ) whose explicit form is derived from length‐scale considerations in a region of wake‐shear interaction. The observed non‐dimensional profiles ϕ° are well represented by ϕ° ≃ 0.5ϕ( z / L )exp(0.7 z / z * ) Both for wind and temperature. For wind at both surfaces, the depth z * is approximately constant in unstable conditions and equal to 3 δ, δ being the tree spacing. We tentatively conclude that δ is the relevant surface length scale z s characterizing the wake field and depth of penetration z * .