Geoscience Reference
In-Depth Information
Table 3.3
Typical values for aerodynamic roughness length and displacement
height for natural surfaces
Surface
Remark
z
0
(
m
)
d (
m
)
Water
Still - open
10
-4
-10
-3
—
Ice
Smooth sea ice
10
-5
—
Ice
Rough sea ice
10
-3
-10
-2
—
Snow
10
-4
-10
-3
—
Soils
0.002
—
Short grass, moss
h
c
: 0.02-0.05 m
0.01
—
Long grass, heather
h
c
: 0.2-0.6 m
0.04
0.2
Low mature crops
h
c
: 0.3-1 m
0.07
0.5
High mature crops
h
c
: 1-2.6 m
0.15
1
Continuous bushland
h
c
: 2.3-3 m
0.3
2
Mature pine forest
h
c
: 10-27 m
1.2
14
Tropical forest
h
c
: 27-31 m
2
30
Deciduous forest
h
c
: 10 m
0.8
7
Dense low buildings
Suburb
0.6
3
Regularly built town
1.2
10
After Wieringa (1992). Note that the roughness for water and all vegetations may depend on
the wind speed.
u
z
z
−
z
L
z
L
u
uz
()
=
*
ln
Ψ
u
+
Ψ
0
u
m
m
κ
0
(3.42)
z
z
θ
−
z
L
z
L
θ
θ
(
z
)
−=
θ
*
ln
Ψ
+
Ψ
θ
0
h
θ
s
h
h
κ
0
h
where
z
u
is the observation height of the wind speed and
z
θ
is the height of the temper-
ature observation. The Ψ-functions involving
z
0
and
z
0h
are generally small and often
neglected in practice. Provided that the relevant roughness lengths are known as well
as the surface temperature, then an iteration similar to that given in
Section 3.6.1
can
be used to obtain
u
*
and
θ
*
, and hence the turbulent luxes of momentum and heat.
The determination of luxes from single level atmospheric data can also be described
in terms of resistances (see Eq. (
3.24
)). Equation (
3.24
) remains valid, but the deini-
tion of the resistance has to be adapted. Apart from the transition from neutral to non-
neutral conditions (going from Eq. (
3.25
) to Eq. (
3.31
)), now the lower level becomes
special. For momentum the lower level is located at
z
0
whereas for temperature it will
be located at
z
0h
:
uz
()
ρ
θ θ
()
z
−
2
τ
≡
ρ
u
=
ρ
u
,
H
=−
c
θ
s
(3.43)
*
p
r
r
am
ah
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