PART IV: Disturbances
CHAPTER 10. Mid-latitude Cyclones
Chapter Overview:
The chapter highlights aspects of the mid-latitude cyclone including associated fronts, life cycle,
and upper air relationships. Particular attention is paid to the critical aspects of upper atmospheric
divergence and convergence and relationships to mid-latitude cyclones. Further, a real world
mid-latitude cyclone example is used to augment aspects of cyclogenesis, maturity, and occlusion.
Chapter at a Glance:
Bjerknes, founder of the Bergen school of meteorology, developed polar front theory to describe
interactions between unlike air masses and related aspects of the mid-latitude cyclone.
• The Life Cycle of a Mid-latitude Cyclone - Cyclogenesis typically begins along the polar front
but may initiate elsewhere, such as in the lee of mountains, as well. <Web>
A. Cyclogenesis - Minor perturbations occur along the boundary separating colder
polar easterlies from warmer westerlies. A low pressure area forms and due to the
counterclockwise flow (N.H.) colder air migrates equatorward behind a developing
cold front (west of the system) while warmer air moves poleward along a
developing warm front (east of the system). Clouds and precipitation occur in
association with converging winds of the low pressure center and along the
developing fronts. The original Norwegian scientists observed that cyclogenesis
occurred along steep thermal gradients and in lee of mountains, but they could not
explain why this occurred. <Web>
B. Mature Cyclones - Well developed fronts circulating about a deep low pressure
center characterize a mature mid-latitude cyclone. Chance of precipitation
increases toward the storm center, with heavy precipitation stemming from
cumulus development in association with the cold front and lighter precipitation
associated with stratus clouds of the warm front. Also, highly unstable conditions
are associated with the warm sector, ahead of the cold front. This area may produce
heavy precipitation and severe thunderstorms associated with prefrontal waves
(squall lines). The system is also capable of creating snow, sleet, freezing rain,
and/or hail. Isobars close the low and are typically kinked in relation to the fronts
due to steep temperature gradients. Winds, spiraling counterclockwise toward the
low, change accordingly as the system, and its associated fronts, moves over
particular regions. <ME10.1;10.2>
C. Occlusion - When the cold front joins the warm front, closing off the warm
sector, surface temperature differences are minimized. The system is in occlusion,
which marks the end of the system life cycle.
D. Evolution and Movement of Cyclones - A hypothetical mid-latitude cyclone
may develop as a weak disturbance east of Japan and travel eastward, guided by
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upper air trajectories. The system may then bring rain to western North America
and snow to high elevations. On the lee side of the Rocky Mountains, the system
may undergo strengthening causing blizzard conditions in the central and
northeastern US. Occlusion typically occurs in the western North Atlantic. For
particular locations, weather conditions may progress from clear conditions to ones
in which cloud cover thickens and lowers. Eventually, light precipitation may
begin with warm front advancement. Winds, originally easterly, shift to
southeasterly and eventually southwesterly with warm front passage as temperature
and humidity conditions increase. Heavy clouds and precipitation advance with
the cold front approach. As the front passes, temperature and humidity plummets
while winds shift to northwesterly.
E. Processes of the Middle and Upper Troposphere - A major breakthrough in
polar front theory occurred during WWII with increasing information of the upper
atmosphere. Carl Rossby mathematically expressed relationships between
mid-latitude cyclones and the upper air.
F. Rossby Waves and Vorticity - The rotation of air, or vorticity, may be viewed
as either being absolute, the overall rotation, or relative to the Earth’s surface.
<CD7.5> Air that rotates in the direction of Earth rotation is said to exhibit positive
vorticity while that which spins oppositely exhibits negative vorticity. In relation
to the upper atmosphere, maximum and minimum vorticity occurs in relation to
troughs and ridges, respectively. Vorticity changes in the upper atmosphere lead to
surface pressure changes. Decreasing vorticity, that which occurs in the transition
zone between a trough and a ridge, leads to upper air divergence. Oppositely,
increasing vorticity, that which occurs in the transition zone between a ridge and a
trough leads to upper air convergence. Divergence creates a vacuum in the upper
atmosphere which pulls air from the surface. <CD8.4> This maintains low pressure
at the surface which supports mid-latitude cyclones. These systems are also known
as dynamic, or cold core lows. Convergence areas lead to subsidence through the
atmosphere which supports surface anticyclones. <Web>
• The Effect of Fronts on Upper-Level Patterns - Interactions between the upper air and
the surface and vise versa helps establish and develop mid-latitude cyclones. Thermal
differences across cold and warm fronts lead to upper atmospheric pressure differences due
to density differences which equate to air temperature as expressed through the hydrostatic
equation.
A. Cold Fonts and the Formation of Upper-Level Troughs - Upper atmospheric
troughs develop behind surface cold fronts with the vertical pressure differences
proportional to horizontal temperature and pressure differences. This is due to
density considerations associated with the cold air. Such interactions also relate to
warm fronts and the upper atmosphere.
B. Interaction of Surface and Upper-Level Patterns - The upper atmosphere and
the surface are inherently connected and linked. Divergence and convergence
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relate to surface pressure differences in cyclones and anticyclones, respectively,
while surface temperatures influence vertical pressures and upper atmospheric
winds. Upper level flow patterns explain why mid-latitude cyclones exist in
addition to explaining aspects of their life cycles. An example is the typical position
of mid-latitude cyclones downwind of trough axes in the area of decreasing
vorticity and upper-level divergence.
• An Example of a Mid-Latitude Cyclone - The subsequent 1994 example follows a
typical mid-latitude cyclone through its life cycle.
A. April 15 - A mid-latitude cyclone centered over the upper mid-western US is
depicted. Heavy rains, high winds and overcast skies dominate the regions near the
central low pressure center. Recordings of wind trajectories at stations throughout
the central US depict the counterclockwise rotation of the system. The 500 mb
chart shows the storm upstream of the trough axis, in the region of decreasing
vorticity and upper level divergence.
B. April 16 - The northeasterly movement of the storm system is seen through a
comparison of weather maps over a 24 hour period. Occlusion occurs as the low
moves over the northern Great Lakes. In the upper air, the trough has increased in
amplitude and strength and become oriented northwest to southeast. In addition,
isobars have closed about the low initiating a cutoff low.
C. April 17 - Continual movement towards the northeast is apparent although
system movement has lessened. The occlusion is now sweeping northeastward of
the low bringing snowfall to regions east of the low. In the upper air, continued
deepening is occurring in association with the more robust cutoff low.
D. April 18 - The system has moved over the northwestern Atlantic Ocean but
evidence persists on the continent in the form of widespread precipitation. The
upper atmosphere also shows evidence of the system with an elongated trough
pattern.
• Flow Patterns and Large-Scale Weather - Zonal height patterns obstruct development
of surface pressure systems as vorticity remains constant. Zonal conditions are, therefore,
indicative of rather benign atmospheric conditions at the surface, although small scale
disturbances may occur. Meridional conditions actively support the development of
surface cyclones and anticyclones as vorticity changes appreciably between troughs and
ridges. Large-scale flow conditions in the upper atmosphere may persevere for long
periods, locking in particular weather situations to affected regions. <Web>
• The Modern View-Mid-latitude Cyclones and Conveyor Belts - The conveyor belt
cyclone model helps describe conditions associated with mid-latitude cyclones through the
entire profile of the atmosphere. <CD8.5> The model is divided into three distinct
movements. The first is the warm conveyor belt which originates in the lower atmosphere
of the warm sector. Air flowing toward the storm center is displaced aloft until it overrides
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the warm front. Once over the warm front, the air turns to the right (N.H.), as it becomes
part of the westerly flow aloft. The cold conveyor belt lies north of the warm front, in the
general path of the system. This air streams westward near the surface toward the surface
low. It also ascends and eventually turns clockwise (N.H.) to become part of the westerly
upper air flow. The third motion involves the dry conveyor belt which originates in the
upper troposphere as part of the normal westerly flow. This air sinks into the trough only to
rise over the region of the surface low. This flow then continues along its eastward path
through the upper atmosphere. The dry belt is integral to maintaining separate cloud
bands, which give the entire system the characteristic comma shape. <ME10.3>
• Migration of Surface Cyclones Relative to Rossby Waves - When divergence aloft
exceeds surface convergence, a mid-latitude cyclone will intensify. <CD8.4> In opposite
conditions, the low will fill and dissipate. Mid-latitude cyclones move approximately one
half the speed of the 700 mb steering currents and in the same direction as these winds. In
doing, they move away from the area of maximum divergence aloft, leading to weakening
and occlusion near the upstream ridge axis.
• Anticyclones - Anticyclones are also driven by upper atmospheric conditions. These dry
and often cold air masses may remain over particular regions for long periods as dictated
by upper atmospheric conditions. <CD8.4>
Chapter Boxes:
10-1 Physical Principles: Vorticity and Rossby Waves - Earth vorticity, associated with
planetary spin, is a function of latitude. This vorticity increases with latitude. Relative
vorticity relates to spinning objects in correlation to the surface and is a function of shear
and curvature. Shear is caused by variations in speed across the direction of flow.
Curvature, the most important function of relative vorticity, occurs as a result of forces
which mandate curved flow. Absolute vorticity, or overall rotation, is conserved such that
it remains constant unless other forces dictate otherwise. When absolute vorticity is
conserved, relative vorticity must increase. The interplay of Earth and relative vorticity
dictate that air moving poleward turn clockwise while air moving equatorward turn
counterclockwise.
10-2 Special Interest: A Closer Look at Divergence and Convergence - Although
divergence and convergence aloft relate to the horizontal area occupied by air, such
motions are influential to vertical atmospheric motions. Divergence (convergence) occurs
when air spreads (contracts) horizontally. Divergence and convergence are governed by
wind speed and stretching, or pinching of air, respectively. A speed increase leads to
divergence while a speed decrease leads to convergence. Diffluence occurs in relation to
air stretching while confluence occurs in pinching situations. On isobaric charts, such is
noticeable where isobars spread or contract, respectively.
10-3 Special Interest: Short Waves in the Upper Atmosphere and Their Effect on
Surface Conditions - Small eddies of air superimposed on larger Rossby waves may
affect convergence and divergence aloft. These short waves depend on temperature
advection, which relates to the transportation of warm or cold air by wind. Warm (cold) air
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advection occurs when air is transported from warm (cold) regions toward colder (warmer)
ones. Isotherms on upper level charts may indicate such advection. When isotherms and
height contours are in alignment, the atmosphere is said to be barotropic. When they
intersect, the atmosphere is baroclinic. Baroclinic instability arises when warm and cold
air advection causes vertical motions.
Related Web Sites:
Cyclogenesis: http://tgsv5.nws.noaa.gov/er/rah/research/wntrwx2.html
Mid-latitude Cyclone: www.arts.ouc.bc.ca/geog/G121/1j.html
Vorticity Charts: http://cirrus.mit.edu/~gempak/forecast_maps.html
Zonal and Meridional Flow:
http://pollux.geog.ucsb.edu/~joel/g110_w99/lect19/oh99_19_3.htm
Media Enrichment:
ME10.1 - Movie depicting a mid-latitude cyclone as it traverses the southeastern US.
ME10.2 - A violent mid-latitude cyclone. The storm spawned tornadoes over GA and NC.
ME10.3 - Movie of the January 1999 blizzard.
Key Terms:
polar front theory
cyclogenesis
mature cyclone
occlusion
warm air advection
baroclinic instability
absolute vorticity
relative vorticity
Earth vorticity
Rossby wave
cold air advection
speed convergence
diffluence
confluence
short waves
barotropic
cutoff low
conveyor belt model
speed divergence
temperature advection
baroclinic
Review Questions:
1. Define cyclogenesis. Where does it most commonly occur according to the original polar front
theory?
The original polar front theory began with the cyclogenesis phase. The polar front
separated the cold polar easterlies from the warm westerlies. A kink developed in the
stationary front between the two air masses which allowed colder polar air to displace
warmer air southward behind the kink while the opposite occurred in advance of the kink.
With counterclockwise flow being initiated, a weak low pressure cell developed. A cold
front developed behind the low while a warm front developed in advance of it. As the
system gained strength and intensity, the mature phase was initiated. Now the cold front
trailed the low pressure core to the south-southwest while the warm front occupied roughly
its original position relative to the low. The placement of the fronts defined a well
developed warm sector south of the core. Heavy precipitation characterized the cold front
region while light precipitation delineated the warm front. Eventually the faster moving
cold front would catch the slower moving warm front. This initiated the occlusion phase
which marked the end of the mid-latitude cyclone’s life cycle.
2. Describe the isobar and wind patterns associated with mature mid-latitude cyclones.
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Isobars close the low and are typically kinked in relation to the fronts due to steep
temperature gradients. Winds, spiraling counterclockwise toward the low, change
accordingly as the system, and its associated fronts, moves over particular regions.
3. Where is precipitation most likely to be found within mid-latitude cyclones?
The chance of precipitation increases toward the storm center, with heavy precipitation
stemming from cumulus development in association with the cold front and lighter
precipitation associated with stratus clouds of the warm front.
4. Where within a mid-latitude cyclone are overrunning, convergence, and instability likely to
serve as precipitation inducing processes?
Overrunning occurs in relation to the warm front as warm air overrides cold. Convergence
occurs at the storm center as surface winds converge into the low pressure center and rise.
Instability is mainly associated with the warm sector of the storm, between the cold and
warm fronts. This is where warm, moist air resides.
5. What is Earth, relative, and absolute vorticity?
Earth vorticity is simply the rotation of Earth about its axis. Absolute vorticity is simply
the overall rotation of any object. Relative vorticity is vorticity relative to the Earth’s
surface. Air that rotates in the direction of Earth rotation is said to exhibit positive vorticity
while that which spins oppositely exhibits negative vorticity.
6. Why is counterclockwise rotation in the Northern Hemisphere said to have positive vorticity?
Because the spin is in the same direction as Earth spin.
7. Where are the zones of positive, negative, and zero vorticity in a typical ridge and trough
pattern?
Maximum and minimum vorticity (rotation of air) occurs in relation to troughs and ridges,
respectively. Decreasing vorticity, which occurs in the transition zone between a trough
and a ridge, leads to upper air divergence which supports low pressure (cyclones) near the
surface. Vorticity increases from the ridge to the next trough. This supports upper level
convergence which supports high pressure (anticyclones) near the surface.
8. In what part of a ridge and trough system do you find the areas of decreasing and increasing
vorticity? Why is their existence important?
Maximum and minimum vorticity occurs in relation to troughs and ridges, respectively.
So, air traveling from a ridge to a trough will experience an increase in vorticity while air
traveling from a trough to a ridge will experience a decrease in vorticity. Vorticity changes
in the upper atmosphere lead to surface pressure changes. Decreasing vorticity, leads to
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upper air divergence while increasing vorticity leads to upper air convergence. Both
induce respective changes in surface pressure.
9. How do dynamic and thermal lows differ from each other?
Dynamic lows are initiated by convergence of air or other physical mechanisms. Thermal
lows develop as air near the surface heats and rises.
10. Where are upper-level divergence and convergence most likely to occur?
Divergence occurs where vorticity decreases, in the region between a trough and an
upstream ridge. Convergence occurs where vorticity increases, in the region between a
ridge and an upstream trough.
11. What type of upper-level condition typically lies above and behind a cold front?
The atmosphere is an integrated whole. Therefore, situations present in the upper
atmosphere support near surface features and vise versa. Differences across cold and
warm fronts lead to upper air pressure differences due to density inequalities equated to air
temperature as expressed through the hydrostatic equation. Typically, an upper-level
trough exists above and behind a cold front as heights lower in association with colder,
denser, air.
12. Where are upper-level ridges generally located relative to mid-latitude cyclones?
Ahead of, or upstream, from the surface cyclone, or behind the upper-level trough which is
immediately behind the surface system.
13. What are diffluence and speed divergence? How do they differ from confluence and speed
convergence?
Diffluence occurs where upper-level heights spread outward, or said another way, where
air stretches out, induced by a vorticity decrease. This typically occurs in the region
between a trough and ridge. As the heights spread, air speeds up, this is termed speed
divergence. In regions between a ridge and trough heights come together, in regions where
vorticity increases, which leads to air slowing. This is termed speed convergence.
14. If the upper-level air flow over North America is zonal, what can you infer with regard to
widespread precipitation conditions?
Zonal height patterns obstruct development or surface pressure systems as vorticity
remains constant. Small scale disturbances do develop in zonal conditions but they, and the
associated precipitation is rather limited.
15. Which type of general upper-level air flow is more likely to occur if minimal temperature
variations exist across North America?
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Zonal flow conditions will dominate.
16. Describe the three conveyor belts of the conveyor belt model of mid-latitude cyclones. How
does the conveyor belt model differ from the description of air flow presented in the
original polar front model?
The original model did not account for upper atmospheric interactions. The conveyor belt
model relies upon observations in three dimensions to explain mid-latitude cyclone
development. The conveyor belt model describes the cyclone in terms of three major air
flows. The first is the warm conveyor belt which stems from surface air flow in the warm
sector of the storm. This air is displaced aloft as it flows toward the low pressure core.
This initiates clouds and precipitation associated with the cold front. Once near the core,
the air, which is now in the middle troposphere, turns and flows to the northeast as part of
the general westerly upper air flow. The cold conveyor belt originates ahead (north) of the
warm front. This northeasterly surface flow also ascends as it nears the cyclone core.
Once aloft it also turns and becomes part of the generalized westerly flow in the upper
atmosphere. The dry conveyor belt is comprised of upper air flow. This cold air sinks
slightly as it approaches the cold front from the west. It then rises and merges with the
general westerly upper-level flow near the core. It is important for maintaining a strong
temperature contrast across the cold front.
17. Explain how the movement of mid-latitude cyclones relative to the upper-level air flow
contributes to the demise of cyclones?
As a surface cyclone moves towards the region of an upper air ridge, vorticity decreases
which causes the low to “fill”. This occurs as surface convergence exceeds divergence
aloft. The system will typically occlude at this point and dissipate.
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