Climatology Lecture 9

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Climatology Lecture 9
General Circulation Mid-latitudes

• Michael Palmer

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Course Outline

• Heat and the Earths Atmosphere
• Vertical Motion Stability
• Horizontal Motion Winds
• The General Circulation Midlatitudes
• The General Circulation Tropics
• Variability of the General Circulation

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Lecture Outline

• Westerlies
• Waves in the westerlies
• Role of waves in the westerlies cyclone
  formation and lifecycle frontal characteristics

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Poles
Subtropics
Pressure Gradient Force
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Lecture Outline

• Westerlies
• Waves in the westerlies
• Role of waves in the westerlies cyclone
  formation and lifecycle frontal characteristics

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Mountain lee waves

• Trough of low pressure in the lee of the mountain
  range

LOW
Conservation of Pot. Vort. i.e. c ?r / f
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Relative and absolute

• Vorticity of air relative to the Earth, ?r
• Earths vorticity, f
• Absolute vorticity, ?a ?r f
• For horizontal motion absolute vorticity is
  conserved
• Increase (decrease) in ?r is balanced by decrease
  (increase) in f (change in latitude)

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Conserving absolute vorticity

• Southward moving air
• f decreases?r increases
• cyclonic curvature
• Northward moving air
• reverse

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T2m
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Lecture Outline

• Westerlies
• Waves in the westerlies
• Role of waves in the westerlies cyclone
  formation and lifecycle frontal characteristics

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The Dynamical View

• Life of a wave cyclone
• Birth
• Energy source Deepening
• Maturity
• Changing shape Energy of winds Movement
  Vertical motion
• Death
• End of movement Filling pressure Winds die out

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Birth energy source

• Strong temperature difference between polar and
  tropical air

LOW
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Airflow through the upper level wave
LOW
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Airflow through the upper level wave
LOW
Cyclonic curvature gradient wind for cyclonic
flow Sub Geostrophic Wind slows down!
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Airflow through the upper level wave
LOW
Anticyclonic curvature gradient wind for
anticyclonic flow Super Geostrophic Wind
speeds up!
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Birth deepening

• Upper divergence and convergence develop in wave

LOW
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Convergence or divergence?
LOW

• Both above friction layer
• Non-parallel isobars
• convergence
• Trough of low pressure
• subgeostrophic wind at trough

HIGH
CON
DIV
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Birth deepening

• Upper divergence from weak wave produces reduced
  surface pressure

LOW
Increasing pressure
Reducing pressure
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Maturitychanging shape

• Surface trough and ridge grow due to convergence
  and divergence

LOW
LOW
Increasing pressure
Reducing pressure
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Maturity changing shape

• Upper trough deepens due to temperature advection

Low
LOW
Decreasing contour heights
Warm air advection
Cold air advection
LOW
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Maturity vertical motion

• Divergence and convergence produce upward and
  downward motion

LOW
Vortex stretching - increasing vorticity
LOW
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Maturity vertical motion

• Horizontal motion is larger than vertical and
  combines with it

LOW
Vorticity maximum
LOW
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Maturity energy of winds

• Wind speed increases Kinetic energy increase
  Potential energy decreases
• More cold air, less warm air at surface

LOW
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Maturity movement

• Upper trough moves with jet stream Deepening
  moves ahead of trough

LOW
Most rapid decrease
LOW
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Death end of movement

• Upper trough moves faster than surface system
  divergence now out of sync

LOW
LOW
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Death filling pressure

• Convergence aloft replaces divergence

LOW
Increasing pressure
LOW
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Death winds die out

• Cold air undercuts all warm air so source of
  energy ends friction slows winds

At birth
North
South
At death
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Life-cycle of wave cyclone

• Surface view

Day 1
Day 2
Day 3
Day 5
Day 4
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Lecture Outline

• Westerlies
• Waves in the westerlies
• Role of waves in the westerlies cyclone
  formation and lifecycle frontal characteristics

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Fronts - Review

• See Weather vol 55 no 4 april 2000 page 120 for
  good review on frontal theory

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Reading for todays lecture

• Barry and Chorley 1997 chapter 6 7
• Briggs et al. 1997 Fundamentals of the Physical
  Environment chp 9
• Henderson-Sellers and Robinson 1999 chp 7 9
• Linacre and Geerts 1997 Climates and Weather
  Explained p 258-264, 271-283
• McIlveen 1992 chp 11