Geoscience Reference
In-Depth Information
Figure 12.5 A schematic of the nocturnal oscillation of the horizontal wind vec-
tor that can exist when turbulence above the SBL has decayed. The evolving
wind vector, with initial value
,
traces a clockwise (in the north-
ern hemisphere) path at angular frequency
f
on the circle of squared radius
R
2
[
U(t
0
), V (t
0
)
]
2
2
.
=[
U(t
0
)
−
U
g
]
+[
V(t
0
)
−
V
g
]
and the stability-depressed mean wind speeds in the surface layer, within a few
hours this can create a substantial difference between the mean wind speeds at the
surface and at a few hundred meters height in the nocturnal boundary layer.
As discussed by
Stull
(
1988
), low-level jets are quite common and originate from
several types of forcing in addition to the one we discussed. Such jets can lead to
strong differences in nocturnal dispersion of effluents from surface and elevated
sources, and create both opportunities and hazards for wind-power generation.
Even though over land there is apt to be an SBL every night in clear weather, its
structure and dynamics are less well understood than those of the convective case.
We'll cover some of the reasons for this.
12.2.3 The effects of sloping terrain
The downslope buoyancy forces experienced by cooler, and therefore denser, air
over sloping terrain can be quite important in the mean momentum balance in the
SBL. We'll analyze flow over flat terrain having a downslope of small angle
β
to
the horizontal by tilting our coordinates by
β
so the
x
and
y
axes are parallel to
the surface. With this tilting operation the horizontally homogeneous momentum
equations parallel to the surface (
Caughey
et al
.
,
1979
)are
∂U
∂t
+
∂uw
∂z
=
g
θ
0
θ
β
x
,
f(V
−
V
g
)
−
(12.20)
∂V
∂t
+
∂vw
∂z
=
g
θ
0
θ
β
y
,
f(U
g
−
U)
−
