Geoscience Reference
In-Depth Information
over orography begins to reduce the wind velocity
below its geostrophic value. This slowing of the
wind near the surface modifies the deflective force,
which is dependent on velocity, causing it also to
decrease. Initially, the frictional force is opposite
to the wind velocity, but in a balanced state -
when the velocity and therefore the Coriolis
deflection decrease ( the vector sum of the Coriolis
and friction components balances the pressure
gradient force (
Figure 6.3B
). The friction force
now acts to the right of the surface wind vector.
Thus, at low levels, due to frictional effects, the
wind blows obliquely across the isobars in the
direction of the pressure-gradient. The angle of
obliqueness increases with the growing effect of
frictional drag due to the earth's surface averaging
about 10-20
The layer of frictional influence is known as
the
planetary boundary layer
(PBL). Atmospheric
profilers (lidar and radar) can routinely measure
the temporal variability of PBL structure. Its depth
varies over land from a few hundred meters at
night, when the air is stable as a result of nocturnal
surface cooling, to 1-2km during afternoon
convective conditions. Exceptionally, over hot,
dry surfaces, convective mixing may extend to
4-5km. Over the oceans it is more consistently
near 1km deep and in the tropics especially is
often capped by an inversion due to sinking air.
The boundary layer is typically either stable or
unstable. Yet, for theoretical convenience, it is
often treated as being neutrally stable (i.e., the
lapse rate is that of the DALR, or the potential
temperature is constant with height; see
Figure
5.1
). For this ideal state, the wind turns clockwise
(veers) with increased height above the surface,
setting up a wind spiral (
Figure 6.5
). This spiral
profile was first demonstrated in the turning of
ocean currents with depth (see Chapter 7D.1a) by
V. W. Ekman; both are referred to as
Ekman
spirals
. The inflow of air towards the low pressure
center generates upward motion at the top of the
PBL, known as
Ekman pumping
.
°
at the surface over the sea and
25-35
over land.
In summary, the surface wind (neglecting any
curvature effects) represents a balance between
the pressure-gradient force and the Coriolis force
perpendicular to the air motion, and friction
roughly parallel, but opposite, to the air motion.
Where the Coriolis force is small, friction may
balance the pressure gradient force and the wind
(known as antitriptic) flows down the pressure
gradient.
°
Geostrophic wind
(500-1000m)
Table 6.1
Typical roughness lengths (m) associated
with terrain surface characteristics
Terrain surface characteristics
Roughness length (m)
Groups of high buildings
1-10
Temperate forest
0.8
Groups of medium buildings
0.7
Suburbs
0.5
Trees and bushes
0.2
Farmland
0.05-0.1
Grass
0.008
Figure 6.5
The Ekman spiral of wind with height
in the Northern Hemisphere. The wind attains
the geostrophic velocity at between 500 and
1000m in the middle and higher latitudes as
frictional drag effects become negligible. This is a
theoretical profile of wind velocity under conditions
of mechanical turbulence.
Bare soil
0.005
Snow
0.001
Smooth sand
0.0003
Water
0.0001
Source:After Troen and Petersen (1989).
