310 P. Zeng, H. Takahashi Agricultural and Forest Meteorology 103 2000 301–313
Fig. 4. Profile of the computed Reynolds stress line and the measured data closed circles for the corn canopy.
R
l
. Meyers and Baldocchi 1991 have pointed out that the shear production, which is generated by the
interaction between the turbulent field and the mean velocity gradient, is small and turbulence imported
from above the canopy is a strong source for the turbulent kinetic energy in the lower canopy. Thus,
the present model can account for the phenomena of counter-gradient momentum transport and secondary
wind maxima that occurs in the lower portions of veg- etation canopies. R
l
peaks at about 0.8h and decreases above and below this height, and this distribution
pattern is similar to those of the measured hw
′
u
′
w
′
i
Fig. 5. Profiles of two components of the Reynolds stress in the corn canopy: small-eddy diffusion R
s
solid line and non-local transfer R
l
dash line.
non-local transport of the Reynolds stress in many canopies e.g. Shaw and Seginer, 1987; Baldocchi and
Meyers, 1988a, b; Amiro, 1990a. Above the canopy, R
s
is more than three times larger than R
l
and is the main source of the Reynolds stress, implying that the
predicted mean wind profile above the canopy is ap- proximately logarithmic. For the layer above 2h, the
non-local transfer momentum is parameterized to be zero in the model. Though only the profiles for the
corn canopy are shown, those for the other canopies are qualitatively the same.
5. Conclusions
Taking into account the non-local turbulent trans- port, we have developed a first-order closure model
for predicting the wind flow within and above vege- tation canopies. The high accuracy and the universal
utility of the model were verified by comparisons of modeled results and measured data in six types of
vegetation canopy and a rubber tree plantation dur- ing fully leafed, partially leafed and leafless periods.
The root-mean-square errors in the predicted wind speeds were about 0.2 or less for all of the canopies;
these errors are smaller than those results from a higher-order closure model. The wind speeds in the
lower canopies, which appeared to be almost constant or reversed in gradient, were also correctly predicted.
In addition to its high accuracy and universal utility, the present model costs little computation time due to
its simplicity. The model would be very useful for the applications for predicting vegetation wind flows or
scalar fluxes e.g. heat and water vapor between the atmosphere and vegetated surfaces when it is coupled
with other models.
A simulation study on the influence of foliage den- sity on the wind profiles within and above a vegetation
canopy was performed for a rubber tree plantation during fully leafed, partially leafed and leafless peri-
ods. The simulated wind profiles within the canopy changed little in form during the three periods but
that the normalized wind speed normalized by the friction velocity above the canopy within the canopy
increased as the foliage density decreased. The slope of the wind profile above the fully leafed canopy was
larger than that above the leafless canopy.
P. Zeng, H. Takahashi Agricultural and Forest Meteorology 103 2000 301–313 311
Based on the modeled wind speeds using the present model, we were able to clarify the effects of canopy
density, structure and effective drag coefficient on the bulk momentum transfer coefficient and the coefficient
λ, and we were also able to determine the correlations between C
M
and C
F
and between λ and C
F
. The Reynolds stress in the present model was
parameterized by two terms: one representing the small-eddy diffusion, and one representing non-local
transport through large-scale turbulent eddies. The modeled results showed that the non-local transfer
component was small above the canopy but large, and the main source of the Reynolds stress, in the lower
portion of the canopy. The model can also account for counter-gradient momentum transport occurs in the
lower portion of a vegetation canopy. A parameteri- zation scheme was developed for the mixing length
within a canopy.
6. Nomenclature
