Agriculture Reference
In-Depth Information
zd
z
−
u
(
z
)= 2.5
u
ln
(
)
(6.1)
*
0
The constant
u
*
, the friction velocity, scales the wind speed and defines the amount
of turbulence;
z
0
,
the roughness length, scales the height and
d
is a datum level less
than the crop height called the zero plane displacement. Equation 6.1 predicts that
u
= 0 at height
d
but the equation is not valid within a crop. For most crops,
z
0
is an
order of magnitude smaller than the height of the crop
h,
and
d
is between 0.6 and
0.8
h
(Thom, 1975). On sunny days when there is convective activity (unstable
temperature lapse rate) or in the evening when atmospheric mixing is suppressed
(stable temperature lapse rate), the wind profile deviates from equation 6.1 (Thom,
1975; Grace, 1977; McCartney and Fitt, 1985). The wind profile is logarithmic only
with well formed surface boundary layers over large uniform areas. Wind profiles
near obstructions such as hedges or near changes in terrain, for example woodland
boundaries, may be more complex than suggested by equation 6.1. However, the
general form of the profile is usually similar and the equation can be used as an
approximation to estimate wind speeds at crop height from local synoptic
measurements (usually made at heights of 10 m). The vertical turbulence mixing
scales (the dispersive ability) above crops also increase with height (Thom, 1975).
This can be parameterised as a diffusion coefficient
K
that increases with height.
For a neutral atmosphere,
K
is proportional to the friction velocity
u
*
, but the
relationships are more complex for stable and unstable conditions (Thom, 1975).
Mean wind profiles within crops depend greatly on crop architecture, particularly
the vertical distribution of foliage and the size, shape and density of leaves. For
crops where the leaves are relatively uniformly distributed with height, such as
cereals, wind speed profiles can often be estimated using the equation (Cionco
et al.
,
1963):
u
(
z
)=
u
(
h
)exp
[
a
(
1
)]
z
h
−
(6.2)
where
h
is the crop height and the attenuation coefficient
a
has a value between 0.3
and 3, depending upon crop type and leaf area density. In crops with a 'canopy and
stem' structure (e.g. orchards), the wind speed profile may be S-shaped, with wind
speeds greater in the 'stem' layer than the 'canopy' layer (Grace, 1977). It has been
suggested that turbulent diffusivity is more or less constant within the upper two-
thirds of crop canopies (Thom, 1975) and decreases towards the ground level.
However, wind speeds within crops are highly intermittent (Aylor, 1990), with air
flow at relatively low speeds interspersed with sporadic bursts at high speeds with
increased turbulence. This produces highly skewed distributions of wind speed, with
a long 'tail' of high wind speeds (gusts) occurring at low frequencies (Shaw
et al.
,
1979; Shaw and McCartney, 1985). These high wind speeds are caused by relatively
large eddies which penetrate the canopy (Aylor, 1990). The corollary of gust
penetration is the gradual ejection of air from the canopy after gusts, which can be
