Geoscience Reference
In-Depth Information
lower levels in the atmosphere to compensate for the
removal of air aloft. The significance of this fact will be
discussed in Chapter 9G. The interaction of horizontal
and vertical air motions is outlined in B.2 (this chapter).
5 Frictional forces and the planetary
boundary layer
The last force that has an important effect on air move-
ment is that due to friction from the earth's surface.
Towards the surface (i.e. below about 500 m for flat
terrain), friction begins to reduce the wind velocity
below its geostrophic value. The slowing of the wind
towards 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 fric-
tional drag due to the earth's surface averaging about
10 to 20° at the surface over the sea and 25 to 35° over
land.
In summary, the surface wind (neglecting any
curvature effects) represents a balance between the
pressure-gradient force and the Coriolis force perpen-
dicular to the air motion, and friction roughly parallel,
but opposite, to the air motion.
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 PLB structure. Its depth varies over
land from a few hundred metres at night, when the
air is stable as a result of nocturnal surface cooling,
to 1 to 2 km during afternoon convective conditions.
Exceptionally, over hot dry surfaces, convective mixing
may extend to 4 to 5 km. Over the oceans, it is more
consistently near 1 km 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
Figure 6.5
The Ekman spiral of wind with height, in the northern
hemisphere. The wind attains the geostrophic velocity at between
500 and 1000 m 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.
(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 7D1.a) by V. W. Ekman; both are referred
to as
Ekman spirals
. The inflow of air towards the low-
pressure centre generates upward motion at the top of
the PBL, known as
Ekman pumping
.
Wind velocity decreases exponentially close to the
earth's surface due to frictional effects. These consist
of 'form drag' over obstacles (buildings, forests and
hills), and the stress exerted by the air at the surface
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
Bare soil
0.005
Snow
0.001
Smooth sand
0.0003
Water
0.0001
Source
: After Troen and Petersen (1989).
