Understanding Weather and Climate Ch 10

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Chapter 9 Mid-Latitude
Cyclones
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Introduction

• mid-latitude cyclones ? produce winds as strong
  as some hurricanes but different mechanisms
• contain well defined fronts separating two
  contrasting air masses
• form along a front in mid- and high-latitudes ?
  separating polar air and warmer southerly air
  masses
• polar front theory Bjerknes (Norwegian
  Geophysical Institute Bergen)
• Surface and Upper Atmosphere processes

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The Life Cycle of a Mid-Latitude Cyclone

• cyclogenesis formation of mid-latitude
  cyclones along the polar front
• boundary separating polar easterlies from
  westerlies
• low pressure area forms ? counterclockwise flow
  (N.H.)
• cold air migrates equatorward
• Warmer air moves poleward

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Mature Cyclones

• Well-developed fronts circulating about a deep
  low pressure center characterize a mature
  mid-latitude cyclone.
• Deep low pressure center
• Chance of precipitation increases toward the
  storm center
• cold front heavy ppt. (cumulus clouds)
• warm front lighter ppt. (stratus clouds)
• warm sector unstable conditions

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• pressure pattern interrupted at frontal
  boundaries ? leads to shifts in
• wind direction
• idealized pattern V shape ? can take many
  forms BUT warm front
• located ahead of cold front

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Two examples of mid-latitude cyclones
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Occlusion

• difficult to define exactly ? when the cold front
  joins the warm front, closing off the warm
  sector, surface temperature differences are
  minimized
• effectively the warm air is cut-off from the
  surface
• The system is in occlusion, the end of the
  systems life cycle
• evolution ? eastward migration

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Evolution and Migration

• passage of system and associated effects
• increase in cloud cover (cirrus)
• deepening clouds and light ppt. (altostratus,
  nimbostratus)
• southwest winds lasting 1-2 days
• cold front approach fast-moving, thick heavy
  ppt. bearing clouds

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Process of the Middle and Upper Troposphere

• Rossby waves ? long waves in the upper
  atmosphere (mid-latitudes)
• Ridges/ troughs waves of air flow, defined by
  wavelength and amplitude
• seasonal change fewer, more well-developed
  waves in winter, with stronger winds
• instrumental in meridional transport of energy
  and storm development
• C. G. Rossby ? linkage btw upper and middle
  troposphere winds and cyclogenesis

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• Vorticity describes the tendency of a fluid to
  rotate.
• clockwise rotation gt negative vorticity
• counterclockwise rotation gt positive
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  vorticity
• voticity is an attribute of rotation. Any
  rotation generates vorticity.

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• The vorticity generated by the earth rotation is
  called planetary vorticity. Any object in a
  place between the equator and poles has
  vorticity.
• Planetary vorticity f (Coriolis force).
• The other rotations rather than the earth
  rotation also generate vorticity, called
• relative vorticity.

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• Vorticity measures the intensity of rotation.
  more intense rotation ltgt larger
• vorticity

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Rossby Waves and Vorticity

• vorticity ? rotation of a fluid (air)
• Absolute vorticity
• - relative vorticity ? motion of air relative to
  Earths surface
• - Earth vorticity ? rotation of Earth around
  axis
• Air rotating in same direction as Earth
  rotation ? counterclockwise ? ive vorticity
• Air rotating in opposite direction as Earth
  rotation ? clockwise ? -ive vorticity
• maximum and minimum vorticity associated with
  troughs and ridges, respectively

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• two segments of no relative vorticity (1,3)
• one of maximum relative vorticity (2)
• Vorticity increases across zone A, decreases
  across zone B
• (beginning to turn more in A, starting to
  straighten in B)

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WHATS THE POINT OF VORTICITY????

• changes in vorticity in upper troposphere leads
  to surface pressure changes
• Increase in absolute vorticity ? convergence
• decrease in absolute vorticity ? divergence
• decrease vorticity ? divergence ? draws air
  upward from surface ? surface LP
• referred to as dynamic lows (v. thermal lows)
• dynamic lows (surface) exist downwind of trough
  axis
• increase vorticity ? convergence ? air piles
  up, sinks downward ? surface High

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Necessary ingredients for a developing wave
cyclone
1. Upper-air support
filling
- When upper-level divergence is stronger than
surface convergence, surface pressure drops and
low intensifies (deepens) - When upper-level
convergence exceeds low-level divergence, surface
pressure rise, and the anticyclone builds.
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Values of absolute vorticity on a hypothetical
500 mb map
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Changes in vorticity through a Rossby wave
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Necessary ingredients for a developing wave
cyclone
1. Upper-air support

• A shortwave moves through this region,
  disturbing the flow.
• - Diverging air aloft causes the sfc pressure to
  decreases beneath
• position 2 ? rising air motion.
• Cold air sinks and warm air rises potential
  energy is transformed into
• kinetic energy
• - Cut-off low

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Necessary ingredients for a developing wave
cyclone
2. Role of the jet stream upper-level divergence
above the surface low
The polar jet stream removing air above the
surface cyclone and supplying air to the surface
anticyclone.
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The Effect of Fronts on Upper-Level Patterns

• Upper-level divergence ? maintains/intensifies
  surface Low (mid-latitude cyclones)
• Upper-level conditions influence surface
  conditions
• Surface conditions ? influence upper-level via
  cold/warm fronts
• steeper pressure gradient in cold column ? at any
  given elevation, pressure will be lower over cold
  air than warm air
• therefore across a cold front temperature
  gradient leads to upper level pressure differences

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Cold Fronts and the Formation of Upper-Level
Troughs

• Upper air troughs develop behind surface cold
  fronts

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Interaction of Surface and Upper-Level Patterns

• upper atmosphere and surface conditions are
  inherently connected and linked
• Divergence/ convergence ? surface pressure
  differences in cyclones and anticyclones,
  respectively
• Surface temperatures influence VPG and upper
  atmospheric winds
• Upper level flow patterns explain why
  mid-latitude cyclones exist
• E.g. typical position of mid-latitude cyclones
  downwind of trough axes in the area of decreasing
  vorticity and upper-level divergence

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Flow Patterns and Large-Scale Weather

• meridional v. zonal flow patterns
• Zonal limited vorticity ? hampers
  cyclone/anti-cyclone development
• - light winds, calm conditions, limited ppt.
• Meridional vorticity changes between troughs
  and ridges ? supports cyclone
• development
• - cyclonic storm activity results
• Droughts (zonal) v. intense ppt. (meridional)

Zonal
Meridional
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Steering of Mid-latitude Cyclones

• movement of surface systems can be predicted by
  the 500 mb pattern
• movement in same direction as the 500 mb flow,
  at about 1/2 the speed
• Winter mid-latitude cyclones ? grouped by paths
  across North America
• Alberta Clippers zonal flow, light ppt.
• Colorado Lows stronger storms, heavier ppt.
• East Coast strong uplift, high vapor content,
  v. heavy ppt.

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• An example of a mid-latitude cyclone

April 15
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April 16
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April 17
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April 18
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Summary