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

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Abstract

Observations from two towers situated in flat, tree-covered terrain (z0 lying between 0.4 and 0.9 m) have been used to investigate the flux-profile relations in the height range z/z0 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 ϕM, H(z/L) in unstable conditions and, by implication, of the logarithmic wind law in neutral conditions. We find z*/z0 ≃ 35 and 150 for wind at the denser and less dense (lower z0) surfaces, whilst for temperature z*/z0 ≃ 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 zs, related to surface wake generation. On the assumption that z* α zs, 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(0.7z/z*)

Both for wind and temperature. For wind at both surfaces, the depth z* is approximately constant in unstable conditions and equal to 3δ, δ being the tree spacing. We tentatively conclude that δ is the relevant surface length scale zs characterizing the wake field and depth of penetration z*.

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