Wind
0
1
2
3
4
5
6
7
Wind Speed (mls)
FIGURE 5.4. Wind profiles from Fig. 5.3 plotted on a logarithmic scale.
of the crop surface has an effect, however. An equation which accounts
for this offset is
where u* is called the friction velocity, d is the zero plane displacement,
and zm is the momentum roughness parameter. Equation (5.1) is only valid
for z 2 z, + d. The constant 0.4 is called the vonKannan constant. Figure
5.4 showsthe windprofiles fromFig. 5.3 plotted withln[(z -d)/z,] as the
vertical axis. Both profiles plot as straight lines which extrapolate to zero
wind at = d + zm (when = d + zm, ln[(z - d)/zm] = ~(z,/z,) =
ln(1) = 0). This log-profile expression (eq. 5.1) is appropriate for the
lowest 50 to 100 m of the atmosphere, which is called the atmospheric
surface layer. Above this height is the planetary boundary layer, which
extends to a height of 1 to 3 km during daytime. The friction velocity, u*
is constant in this surface layer.
5.4 Finding the Zero Plane Displacement and
the Roughness Length
The height d' = d + z,, where the wind profile extrapolates to zero,
is an actual measurable height within the canopy. Its meaning can be
understood as follows. For smooth surfaces, the drag of the surface on