Geoscience Reference
In-Depth Information
21
Canopy Processes
and Canopy
Resistances
Introduction
As discussed in Chapter 17, within the ABL itself the Reynolds number is such
that movement of energy, water, and atmospheric constituents is primarily by tur-
bulent transport rather than molecular transport because of the physical scale of
the turbulent eddies involved. Approaching the surface, the efficiency of turbulent
transport reduces as the scale of the turbulence reduces and turbulent transport no
longer dominates very near the ground. If the surface is bare soil, or very near the
components that make up the canopy when the surface is vegetation-covered, the
resistances to flow are primarily determined by molecular diffusion processes; in
these cases transfers are through non-turbulent boundary layers close to the
sources or through pores in the soil or vegetation.
If the surface is vegetation-covered, as is often the case, flux exchange is complex
within the canopy and for some distance above it. Over a height range on the order
of ten times the aerodynamic roughness above the vegetation, surface-related
features influence the nature of the turbulent regime. While within the vegetation
canopy itself, flux exchange involves interplay between still turbulent vertical
diffusion of fluxes and the divergence of these fluxes through interaction with the
vegetation elements (leaves, twigs and branches) that make up the canopy. In
effect, molecular diffusion through non-turbulent boundary layers around
vegetation and (in the case of water vapor) through stomata in the leaves, act like
resistances which control transfer of those portions of the fluxes that are dissipated
or generated at each level in the canopy.
Further complications arise. The exchange of momentum between moving air
and a body is more efficient than the exchange of other entities such as heat, water
vapor and carbon dioxide. This is because momentum can be transferred not only
by molecular diffusion through the boundary layer surrounding the body, but also
