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Chapter 10
Mid-latitude Cyclones
The Polar Front Theory was postulated in the early
part of the twentieth century to describe the formation,
development, and dissipation of mid-latitude cyclones.
Mid-latitude cyclones are large systems that travel
great distances and often bring precipitation
and sometimes severe weather to wide areas.
Lasting a week or more and covering large
portions of a continent, they are familiar as
the systems that bring abrupt changes
in wind, temperature, and sky conditions.
Cyclogenesis is the formation
of a mid-latitude cyclone.
Initially, the polar front
separates the cold easterlies
and the warmer westerlies.
As cyclogenesis begins, a “kink”
develops along the boundary.
The cold air north of the front
begins to push southward behind
the cold front, and air behind the
warm front advances northward,
creating a counterclockwise
rotation around a weak
low-pressure system.
With further intensification, the
low pressure deepens even
further and distinct warm and
cold fronts emerge from the
original polar front. Convergence
associated with the low pressure
can lead to uplift and cloud
formation, while linear bands of
deeper cloud cover develop
along the frontal boundaries.
Occlusion represents the end
of the cyclone’s life cycle
and takes place as the center
of the low pressure pulls back
from the warm and cold fronts.
The figure depicts the typical
structure of a mature cyclone
and the processes causing uplift.
Shaded areas represent the
presence of cloud cover.
The numbers represent
an approximation of the
precipitation probability.
Rossby waves are waves in the
mid-latitude westerlies having
wavelengths on the order of
thousands of kilometers. In the
figure moving from points 1 to 3,
the air rotates counterclockwise.
Between points 3 and 5, it rotates
clockwise. The rotation of a fluid
is referred to as its vorticity.
The figure shows vorticity
changes in the moving air,
relative to the surface.
The overall rotation of air, or its absolute vorticity, has two components:
relative vorticity, or the vorticity relative to Earth’s surface, and
Earth vorticity, which is due to Earth’s daily rotation about its axis.
Counterclockwise rotation in the Northern Hemisphere is said to have
positive vorticity while air rotating clockwise possesses negative vorticity.
The figure represents change in vorticity along a Rossby wave trough.
As the air flows from positions 1 to 3, it undergoes little change in
direction and thus has no relative vorticity. From positions 4–6, it
turns counterclockwise and thus has positive relative vorticity. The air
flows in a constant direction from positions 7–9. Thus, the trough has
three segments based on vorticity, separated by two transition zones.
Divergence in the upper atmosphere, caused by decreasing vorticity,
draws air upward from the surface and provides a lifting mechanism for
the intervening column of air. This can initiate and maintain low-pressure
systems at the surface. Conversely, increasing upper-level vorticity leads
to convergence and the sinking of air, which creates high pressure at the
surface. Surface low-pressure systems resulting from upper-tropospheric
motions are called dynamic lows and are distinct from thermal lows
caused by localized heating of air from below.
Temperature distributions in the lower atmosphere lead to variations
in upper-level pressure. Above A the entire column of air in the lower
atmosphere is warm, so the pressure drops relatively slowly with height.
At B cold air occupies the lowest 500 m, with warmer air aloft. This leads
to slightly lower pressure at the 1 km level. At C cold air occupying the
lowest 1000 km causes a greater rate of pressure decrease with altitude,
and lower pressure at the 1 km level. Thus, the existence of sloping
frontal boundaries establishes horizontal pressure gradients
in the upper and middle atmosphere.
The effect of differing vertical
pressure gradients on either
side of warm and cold fronts
leads to upper-tropospheric
troughs and ridges.
Speed divergence and convergence
occur when air moving in a constant
direction either speeds up or slows
down. Speed divergence occurs
where contour lines come closer
together in the downwind direction.
In the top figure, the wind speed,
indicated by the length of the arrows,
increases in the direction of flow and
causes speed divergence. Speed
convergence occurs when fastermoving air approaches the slowermoving air ahead (bottom).
Diffluence and confluence occur
when air stretches out or converges
horizontally due to variations in wind
direction. In the top figure, a certain
amount of air is contained in the
shaded area between points 1 and 3.
As it passes to the region between
points 2 and 4, the same amount of
air occupies a greater horizontal area.
This is diffluence, a pattern that
commonly appears wherever vorticity
changes cause divergence to occur.
Confluence is shown in the bottom.
In the figure, the height contours exhibit a zonal pattern with a
minimum of north–south displacement. Because they have
no pronounced vorticity changes, zonal patterns hamper the
development of intense cyclones and anticyclones. They are
therefore more often associated with a large-scale pattern of light
winds, calm conditions, and no areas of widespread precipitation.
The pattern above shows a strong meridional component, which
can lead to the formation of major cyclones and anticyclones.
Some areas will experience cloudy and wet conditions
while others are calm and dry.
The figure shows a center of low surface pressure gradually moving
northeastward relative to a Rossby wave. In doing so, it moves away
from the region of maximum divergence aloft and evolves as it travels.
It goes through the various stages of its life cycle, typically occluding
and dissipating as it approaches the upper-level ridge.
In the conveyor belt model, the warm conveyor belt originates near
the surface in the warm sector and flows toward and over the wedge
of the warm front. The cold conveyor belt lies ahead of the warm front.
It enters the storm at low levels as an easterly belt flowing westward
toward the surface cyclone. The dry conveyor belt originates in the
upper troposphere as part of the generally westerly flow.
(a)
A barotropic atmosphere (a) exists
where the isotherms (the dashed lines
showing the temperature distribution)
and height contours (solid lines) are
aligned in the same direction.
No temperature advection occurs
when the atmosphere is barotropic.
A baroclinic atmosphere occurs
where the isotherms intersect the
height contours. Cold air advection
(the horizontal movement of cold air)
is occurring in (b), while warm air
advection is occurring in (c).
(b)
(c)
The next chapter examines
lightning, thunder, and tornadoes.