Mid-latitude Cyclones

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Chapter 10 Mid-latitude Cyclones
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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.
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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.
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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 cyclones
life cycle and takes place as the center of the
low pressure pulls back from the warm and cold
fronts.
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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.
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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.
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The overall rotation of air, or its absolute
vorticity, has two components relative
vorticity, or the vorticity relative to Earths
surface, and Earth vorticity, which is due to
Earths daily rotation about its
axis. Counterclockwise rotation in the Northern
Hemisphere is said to have positive vorticity
while air rotating clockwise possesses negative
vorticity.
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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 46, it turns counterclockwise and
thus has positive relative vorticity. The air
flows in a constant direction from positions 79.
Thus, the trough has three segments based on
vorticity, separated by two transition zones.
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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.
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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.
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The effect of differing vertical pressure
gradients on either side of warm and cold fronts
leads to upper-tropospheric troughs and ridges.
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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
faster-moving air approaches the slower-moving
air ahead (bottom).
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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.
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In the figure, the height contours exhibit a
zonal pattern with a minimum of northsouth
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.
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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.
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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.
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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.
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(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)
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The next chapter examines lightning, thunder, and
tornadoes.