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Air Masses, Fronts, and Middle-Latitude Cyclones-II

GEOL 1350: Introduction To Meteorology

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Cyclone – an area of low pressure around which the winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere

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How do middle-latitude cyclone (wave cyclone) form ? – Polar Front Theory (Norwegian model, or the wave cyclone model)

1.  Stationary Front 2.  Nascent Stage 3.  Mature Stage 4.  Partially Occluded Stage 5.  Occluded Stage 6.  Dissipated Stage

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Cyclone Development begins with a stationary front (usually a segment of the polar front) N

Before Birth

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Nascent stage of Cyclone or incipient cyclone Frontal wave developed N

Birth and adolescence

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Mature stage of Cyclone Development Fully developed open wave with cold and warm front

Adulthood

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Mature Wave Cyclone

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The Partially Occluded Stage begins when the cold front starts to overrun the warm front

Middle age

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Partially occluded wave cyclone

Cold-occluded front

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Relationship between occluded fronts and midlatitude cyclone

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The Occluded Stage is characterized by more warm air being pushed aloft and the size of the warm air wedge at the surface decreases significantly

Over the hill

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Relationship between occluded fronts and midlatitude cyclone

Partially occluded wave cyclone

Occluded wave cyclone

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The final decay stage of the cyclone. The warm air is isolated aloft with cold air beneath.

Death

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Stationary front Incipient cyclone Open wave

Mature stage occlusion dissipating

Summary of

Cyclone development stages

• Life time of a typical cyclone is several days to a week •  Moves 1000’s of km during lifecycle

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A low pressure system may develop into a cyclone if the low pressure system intensifies (pressure drops in the center of the low).

What cause surface low pressure to intensify?

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When upper air divergence is stronger than surface convergence (more air is taken out from the top than is brought in at the bottom), surface pressure drop, and the low intensifies, or deepens.

Air going into the surface low

Air going out of the column

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What initiates “cyclogenesis?” 500 mb height

Low

High

When upper-level divergence is stronger than lower-level convergence, more air is taken out at the top than is brought in at the bottom. Surface pressure drops, and the low intensifies, or “deepens.”

Convergence

behind trough

Divergence ahead of trough

Cyclogenesis region

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The relationship between upper-level ridge/trough and surface high and low

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Upper-level convergence/divergence and surface high/low

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Cyclone development: Strong north south gradient + passage of a shortwave trough can lead to rapid cyclogenesis via baroclinic instability (baroclinic means temperature varies on an isobaric surface)

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•  upper level shortwave passes •  upper level divergence -> surface low •  cold advection behind the trough and warm advection ahead of the trough intensifies the wave and deepens the trough • Increase the divergence over the surface low •  Eventually a upper-level low right above surface low, divergence aloft weakens, cyclone dissipate

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The role of jet stream in the development of midlatitude cyclone

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SUMMARY

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Cloud bands associated with mid-latitude cyclone look like a giant comma, called comma clouds

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Storm track – the position of the lows

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1.  Fronts are actually part of the mid-latitude cyclone.

2.  Mid-latitude cyclone goes through a series of stages from birth, to maturity, to death as an occluded storm.

3.  An important influence on the development of a mid-latitude cyclonic storm is the upper-air flow, including the jet stream.

4.  When an upper-level low lies to the west of the surface low, and the polar jet stream bends and then dips south the surface storm, an area of divergence above the surface low provides the necessary ingredients for the surface mid-latitude cyclone to develop into a deep low-pressure area.

SUMMARY