VORTICITY
LEARNING OBJECTIVES: Recognize the two
components of relative vorticity. Define the
term absolute vorticity. Determine vorticity
impacts on weather processes.
Vorticity measures the rotation of very small air
parcels. A parcel has vorticity when it spins on its axis
as it moves along its path. A parcel that does not spin
on its axis is said to have zero vorticity. The axis of
spinning or rotation can extend in any direction, but for
our purposes, we are mainly concerned with the
rotational motion about an axis that is perpendicular to
the surface of the Earth. For example, we could drop a
chip of wood into a creek and watch its progress. The
chip will move downstream with the flow of water, but
it may or may not spin as it moves downstream. If it
does spin, the chip has vorticity. When we try to isolate
the cause of the spin, we find that two properties of the
flow of water cause the chip to spin: (1) If the flow of
water is moving faster on one side of the chip than the
other, this is shear of the current; (2) if the creek bed
curves, the path has curvature. Vorticity always applies
to extremely small air parcels; thus, a point on one of
our upper air charts may represent such a parcel. We
can examine this point and say that the parcel dots or
does not have vorticity. However, for this discussion,
larger parcels will have to be used to more easily
visualize the effects.
Actually, a parcel in the
atmosphere has three rotational motions at the same
time: (1) rotation of the parcel about its own axis
(shear), (2) rotation of the parcel about the axis of a
pressure system (curvature), and (3) rotation of the
parcel due to the atmospheric rotation. The sum of the
first two components is known as relative vorticity, and
the sum total of all three is known as absolute vorticity.
RELATIVE VORTICITY
Relative vorticity is the sum of the rotation of the
parcel about the axis of the pressure system (curvature)
and the rotation of the parcel about its own axis (shear).
Figure 1-4.-Illustration of vorticity due to the shear effect.
The vorticity of a horizontal current can be broken down
into two components, one due to curvature of the
streamlines and the other due to shear in the current.
Shear
First, let us examine the shear effect by looking at
small air parcels in an upper air pattern of straight
contours. Here the wind shear results in each of the
three parcels having different rotations (fig. 1-4).
Refer to figure 1-4. Parcel No. 1 has stronger wind
speeds to its right. As the parcel moves along, it will be
rotated in a counterclockwise direction. Parcel No. 2
has the stronger wind speeds to its left; therefore, it will
rotate in a clockwise direction as it moves along. Parcel
No. 3 has equal wind speeds to the right and left. It will
move, but it will not rotate. It is said to have zero
vorticity.
Therefore, to briefly review the effect of shear-a
parcel of the atmosphere has vorticity (rotation) when
the wind speed is stronger on one side of the parcel than
on the other.
Now let’s define positive and negative vorticity in
terms of clockwise and counterclockwise rotation of a
parcel. The vorticity is positive when the parcel has a
counterclockwise rotation (cyclonic, Northern
Hemisphere) and the vorticity is negative when the
parcel has clockwise rotation (anticyclonic, Northern
Hemisphere).
Thus, in figure 1-4, parcel No. 1 has positive
vorticity, and parcel No. 2 has negative vorticity.
Curvature
Vorticity can also result due to curvature of the
airflow or path. In the case of the wood chip flowing
with the stream, the chip will spin or rotate as it moves
along if the creek curves.
To demonstrate the effect of curvature, let us
consider a pattern of contours having curvature but no
shear (fig. 1-5).
Figure 1-5.-Illustration of vorticity due to curvature effect.
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