Geoscience Reference
In-Depth Information
Wind velocity decreases exponentially close to
the earth's surface due to frictional effects. These
consist of 'form drag' over obstacles (buildings,
forests, hills), and the frictional stress exerted by
the air at the surface interface. The mechanism of
form drag
involves the creation of locally higher
pressure on the windward side of an obstacle and
a lateral pressure gradient. Wind stress arises
from, first, the molecular resistance of the air to
the vertical wind shear (i.e., increased wind speed
with height above the surface); such molecular
viscosity operates in a laminar sublayer only
millimeters thick. Second, turbulent eddies, a few
meters to tens of meters across, brake the air
motion on a larger scale (eddy viscosity). The
aerodynamic roughness of terrain is described by
the
roughness length
(
z
0
), or height at which the
wind speed falls to zero based on extrapolation of
the neutral wind profile.
Table 6.1
lists typical
roughness lengths.
Turbulence in the atmosphere is generated by
the vertical change in wind velocity, (i.e., a vertical
wind shear), and is suppressed by an absence of
buoyancy. The dimensionless ratio of buoyant
suppression of turbulence to its generation by
shear, known as the Richardson number (
Ri
),
provides a measure of dynamic stability. Above a
critical threshold, turbulence is likely to occur.
accelerate (decelerate), leading to velocity diver-
gence (convergence). When streamlines (lines of
instantaneous air motion) spread out or squeeze
together, this is termed diffluence or confluence,
respectively. If the streamline pattern is
strengthened by that of the isotachs (lines of equal
wind speed), as shown in the third panel of
Figure
6.6A
, then there may be mass divergence or
convergence at a point (
Figure 6.6B
). In this case,
the compressibility of the air causes the density to
decrease or increase, respectively. Usually,
however, confluence is associated with an increase
(A)
10
20
10
20
20
10
DIVERGENCE
DIFFLUENCE
STRONG
DIVERGENCE
INDETERMINATE
20
10
10
20
20
10
CONVERGENCE
CONFLUENCE
STRONG
CONVERGENCE
INDETERMINATE
(B)
AIR MOTION
AIR MOTION
DIFFLUENCE
+
VERTICAL STRETCHING
CONFLUENCE
+
VERTICAL SHRINKING
(C)
B DIVERGENCE, VERTICAL
MOTION AND VORTICITY
These three terms are the key to proper under-
standing of wind and pressure systems on a
synoptic and global scale. Mass uplift or descent
of air occurs primarily in response to dynamic
factors related to horizontal airflow and is only
secondarily affected by air-mass stability. Hence
the significance of these factors for weather
processes.
AIR MOTION
AIR MOTION
HORIZONTAL CONVERGENCE
+
VERTICAL STRETCHING
HORIZONTAL DIVERGENCE
+
VERTICAL SHRINKING
Figure 6.6
Convergence and divergence. A:
Plan view of horizontal flow patterns producing
divergence and convergence - the broken lines
are schematic isopleths of wind speed (isotachs).
B: Schematic illustration of local mass divergence
and convergence, assuming density changes. C:
Typical convergence-stretching and divergence-
shrinking relationships in atmospheric flow.
1 Divergence
Different types of horizontal flow are shown in
Figure 6.6A
. The first panel shows that air may
