Geoscience Reference
In-Depth Information
R
n
uT e
c
Z
h
1
0
Figure 22.1
Vertical profiles of
net radiation, (
R
n
), wind speed
(
u
), temperature (
T
), vapor
pressure (
e
) and CO
2
concentration (
c
) that are typical
of those observed in a uniform
stand of cereal during the day
and at night.
Z
h
1
0
Whole-canopy aerodynamics and canopy structure
Chapter 20 described the aerodynamic behavior and aerodynamic resistance
of whole canopies when viewed from above, i.e., on the basis of measurements of
profiles of atmospheric variables made above the canopy. Chapter 21 then
discussed detailed studies of in-canopy exchanges with individual vegetation
elements (especially leaves). One result of these studies is the estimated order of
magnitude for the drag coefficient,
c
d
, of a typical leaf. The model leaf described
in Fig. 21.2, for example, when aligned at an average angle
45° to the wind with
the drag coefficient reduced by a 'shelter factor' of order two, suggests
c
d
≈ 0.2. As a
result, the question arises, is a drag coefficient of this order consistent with typical
values of
d
and
z
o
found for whole canopies and with their observed variation
with crop height and canopy density?
Modeling studies have been used to investigate how the effective values
d
and
z
o
for a vegetation canopy vary with canopy density and the vertical distribution of
leaves. Shaw and Pereira (1982) used a second order closure model to describe
vertical transfer within and above a modeled canopy assuming that the momen-
tum divergence at height
z
in the canopy is proportional to a drag coefficient
c
d
∼
0.2 multiplied by the plant (mainly leaf ) area present at each height. They
simulated the resulting wind speed profile above the model canopy and from this
calculated the values of
d
and
z
o
that best described the shape of the modeled
profile assuming this had a logarithmic height dependency, compare Equation
(19.22). In the model the simple height dependent leaf area distribution
L
(
z
)
shown in Fig. 22.2 was used, with
L
(
z
) normalized such that:
=
h
∫
Lz dz LAI
().
=
(22.1)
0
