Geoscience Reference
In-Depth Information
Figure 6.8
Sketch of the relative vertical vorticity (
ζ
) about
a cyclone and an anticyclone in the northern hemisphere. The
component of the earth's vorticity about its axis of rotation (or the
Coriolis parameter,
f
) is equal to twice the angular velocity (
Ω
)
times the sine of the latitude (f). At the pole
f
= 2
, 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
(
Ω
ζ
>0).
direction
(the horizontal or vertical axis about 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 about a vertical axis is referred
to as the vertical vorticity. This is generally the most
important, but near the ground surface frictional shear
causes vorticity about 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 (
Figure 6.9
Streamline models illustrating in plan view the flow
patterns with cyclonic and anticyclonic 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).
) and the latitudinal value of
the Coriolis parameter,
f
= 2
ζ
(see Chapter 6A).
At the equator, the 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).
Ω
sin
ϕ
