Environmental Engineering Reference
In-Depth Information
As we approach the ground, wind speed decreases very rapidly to almost zero in contact
with the soil surface (Figure 8.4). This is largely due to the frictional drag exerted on the
air by the underlying rigid surface; the rougher the surface is, the more it slows the air
down (see Chapter 16). Over a soil surface the effect on the wind is fairly simple, but
when we are dealing with a vegetation layer or an urban area, interference is much
greater. In addition to friction, buoyancy in the lower layers has an effect on the details of
the profile. Rising air will assist mixing and reduce the gradient of wind speed.
The microclimate at a soil surface represents one of the simplest cases of energy
exchange at the ground surface. Both the inputs and the outputs of radiation are changed,
and that alters the way energy is used in terms of sensible and latent heat, and heat flow
or storage into the soil.
Figure 8.4
(left) (a) Wind speed profile near the ground. The
precise shape of the curve will depend upon the roughness of
the surface as well as any buoyancy. (b) Another profile
plotted in semi-logarithmic form, with the height axis
converted to the natural logarithm of the value. Both graphs
are shown dashed as the surface is approached because of
the difficulty of measuring speed close to the ground.
