A simple unified theory for flow in the canopy and roughness sublayer |
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Authors: | Ian N Harman John J Finnigan |
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Institution: | (1) CSIRO Marine and Atmospheric Research, Canberra, Australia |
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Abstract: | The mean flow profile within and above a tall canopy is well known to violate the standard boundary-layer flux–gradient relationships.
Here we present a theory for the flow profile that is comprised of a canopy model coupled to a modified surface-layer model.
The coupling between the two components and the modifications to the surface-layer profiles are formulated through the mixing
layer analogy for the flow at a canopy top. This analogy provides an additional length scale—the vorticity thickness—upon
which the flow just above the canopy, within the so-called roughness sublayer, depends. A natural form for the vertical profiles
within the roughness sublayer follows that overcomes problems with many earlier forms in the literature. Predictions of the
mean flow profiles are shown to match observations over a range of canopy types and stabilities.
The unified theory predicts that key parameters, such as the displacement height and roughness length, have a significant
dependence on the boundary-layer stability. Assuming one of these parameters a priori leads to the incorrect variation with stability of the others and incorrect predictions of the mean wind speed profile. The
roughness sublayer has a greater impact on the mean wind speed in stable than unstable conditions. The presence of a roughness
sublayer also allows the surface to exert a greater drag on the boundary layer for an equivalent value of the near-surface
wind speed than would otherwise occur. This characteristic would alter predictions of the evolution of the boundary layer
and surface states if included within numerical weather prediction models. |
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Keywords: | Canopy MOST Roughness sublayer Stability Wind profile |
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