Geoscience Reference
In-Depth Information
Component of
earth's vorticity
around its axis
(2
A
1
A
2
2
Ω
15
Anticyclonic
vorticity
φ
sin
)
Ω
φ
=
90°
decreases
to 0
Cyclonic
vorticity
15
Equator
ANTICYCLONE
ς
< 0
CYCLONE
ς
> 0
φ
=
0°
B
1
B
2
Cyclonic
vorticity
Anticyclonic
vorticity
Figure 6.8
Sketch of the relative vertical vorticity
(
) about a cyclone and an anticyclone in the
Northern Hemisphere. The component of the
earth's vorticity around its axis of rotation (or the
Coriolis parameter, f), is equal to twice the angular
velocity (
20
10
15
15
10
20
) times the sine of the latitude (f). At
the pole f = 2
C
D
, diminishing to 0 at the equator.
Cyclonic vorticity is in the same sense as the
earth's rotation about its own axis, viewed from
above, in the Northern Hemisphere: this cyclonic
vorticity is defined as positive ( >0).
Strong
anticyclonic
vorticity
20
10
10
Strong
cyclonic
vorticity
20
horizontal or vertical axis around which the
rotation occurs) and the sense of rotation.
Rotation in the same sense as the earth's rotation
- cyclonic in the Northern Hemisphere - is
defined as positive. Cyclonic vorticity may result
from cyclonic curvature of the streamlines, from
cyclonic shear (stronger winds on the right side of
the current, viewed downwind in the Northern
Hemisphere), or a combination of the two (
Figure
6.9
). Lateral shear (see
Figure 6.9B
) results from
changes in isobar spacing. Anticyclonic vorticity
occurs with the corresponding anticyclonic
situation. The component of vorticity around an
axis vertical to the earth's surface is referred to as
the vertical vorticity. This is generally the most
important, but near the ground surface frictional
shear causes vorticity around an axis parallel to the
surface and normal to the wind direction.
Vorticity is related not only to air motion
around a cyclone or anticyclone (
relative vorticity
),
but also to the location of that system on the
rotating earth. The vertical component of
absolute
vorticity
consists of the relative vorticity (
E
F
Indeterminate
20
10
10
20
Indeterminate
Figure 6.9
Streamline models illustrating in plan
view the flow patterns with cyclonic and anti-
cyclonic vorticity in the Northern Hemisphere. In C
and D, the effects of curvature (a
1
and a
2
) and
lateral shear (b
1
and b
2
) are additive, whereas in E
and F they more or less cancel out. Dashed lines
are schematic isopleths of wind speed.
Source: After Riehl et al. (1954).
local vertical is at right angles to the earth's axis,
so
f
= 0, but at the North Pole cyclonic relative
vorticity and the earth's rotation act in the same
sense (see
Figure 6.8
) and
f
= 2
Ω
.
C LOCAL WINDS
For a weather observer, local controls of air
movement may present more problems than the
effects of the major planetary forces just discussed.
Diurnal tendencies are superimposed upon both
) and
the latitudinal value of the Coriolis parameter,
f
= 2
ζ
Ω
sin
φ
(see Chapter 6A). At the equator, the
