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ESS5Prof. Jin-Yi Yu
Chapter 10: Mid-latitude Cyclones
Life Cycle of Cyclone
Cyclone Structures
Steering of Cyclone
ESS5Prof. Jin-Yi Yu
Mid-Latitude Cyclones
Mid-latitude cyclones form along a
boundary separating polar air from warmerair to the south.
These cyclones are large-scale systems
that typically travels eastward over greartdistance and bring precipitations over wideareas.
Lasting a week or more.
ESS5Prof. Jin-Yi Yu
Polar Front Theory
Bjerknes,
the founder of the
Bergen school of meteorology,developed polar front theoryduring WWI to describe theformation, growth, anddissipation of mid-latitudecyclones.
Vilhelm Bjerknes (1862-1951)
ESS5Prof. Jin-Yi Yu
Life Cycle of Mid-Latitude Cyclone
Cyclogenesis
Mature Cyclone
Occlusion
(from
Weather & Climate
)
ESS5Prof. Jin-Yi Yu
Cyclogenesis
Cyclogenesis
typically begins along the
polar front but may initiate elsewhere, suchas in the lee of mountains.
Minor perturbations occur along theboundary separating colder polar easterliesfrom warmer westerlies.
A low pressure area forms and due to thecounterclockwise flow (N.H.) colder airmigrates equatorward behind a developingcold front.
Warmer air moves poleward along adeveloping warm front (east of the system).
Clouds and precipitation occur inassociation with converging winds of thelow pressure center and along thedeveloping fronts.
ESS5Prof. Jin-Yi Yu
Mature Cyclone
ESS5Prof. Jin-Yi Yu
Mature Cyclones
Well-developed fronts circulating about adeep low pressure center characterize amature mid-latitude cyclone.
Heavy precipitation stems from cumulusdevelopment in association with the coldfront.
Lighter precipitation is associated withstratus clouds of the warm front.
Isobars close the low and are typicallykinked in relation to the fronts due tosteep temperature gradients.
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Occlusion
When the cold front joins the warm front, closing offthe warm sector, surfacetemperature differences areminimized.
The system is in occlusion,the end of the system’s lifecycle.
ESS5Prof. Jin-Yi Yu
Parameters Determining
Mid-latitude Weather
Temperature differences between theequator and poles
The rate of rotation of the Earth.
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Vorticity
The rotation of a fluid (such as air and
water) is referred to as its vorticity.
Absolute Vorticity (viewed from space)
Earth (or Planetary) Vorticity
Relative Vorticity (relative to the Earth)
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Earth (Planetary) Vorticity
Earth vorticity is a functionsolely of latitude.
The higher the latitude, thegreater the vorticity.
Earth vorticity is zero at theequator.
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Relative Vorticity
Air which rotates in thedirection of Earth’s rotationis said to exhibit positivevorticity.
Air which spins oppositelyexhibits negative vorticity.
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Vorticity and Rossby Wave
Rossby waves are produced from the conservation of absolutevorticity.
As an air parcle moves northward or southward over different latitudes, itexperiences changes in Earth(planetary) vorticity.
In order to conserve the absolutevorticity, the air has to rotate toproduce relative vorticity.
The rotation due to the relative vorticity bring the air back to whereit was.
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Vortocity and Divergence
Decreasing vorticity in the zonebetween a trough and ridgeleads to upper air convergenceand sinking motions throughthe atmosphere, which supportssurface high pressure areas.
Increasing vorticity in the zonebetween a ridge and troughleads to upper air divergenceand rising motions through theatmosphere, which supportssurface low pressure areas.
ESS5Prof. Jin-Yi Yu
Trough and Cold Front
Upper air troughs develop behind surface cold fronts with the vertical pressure differences proportional to horizontaltemperature and pressure differences.
This is due to density considerations associated with thecold air.
Such interactions also relate to warm fronts and the upperatmosphere.
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An Example
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Typical Winter Mid-latitude Cyclone Paths
Alberta Clippers
are
associated with zonal flow andusually produce lightprecipitation.
Colorado Lows
are usually
stronger storms which producemore precipitation.
East Coast
storms typically
have strong uplift and highwater vapor content.
ESS5Prof. Jin-Yi Yu
Modern View of Mid-latitude Cyclones
