Transcript Understanding Weather and Climate Ch 10

Chapter 9: Mid-Latitude
Cyclones
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
The Life Cycle of a Mid-Latitude Cyclone
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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
Mature Cyclones
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Well-developed fronts circulating about a deep low pressure center
characterize a mature mid-latitude cyclone.
• Deep low pressure center;
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Chance of precipitation increases toward the storm center
– cold front: heavy ppt. (cumulus clouds)
– warm front: lighter ppt. (stratus clouds)
– warm sector: unstable conditions
• 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
Two examples of
mid-latitude cyclones
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 system’s life cycle
• evolution  eastward migration
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
Process of the Middle and Upper Troposphere
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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
• Vorticity: describes the tendency of a fluid to
rotate.
clockwise rotation => negative vorticity
counterclockwise rotation => positive
vorticity
voticity is an attribute of rotation. Any rotation
generates vorticity.
• 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.
• Vorticity measures the intensity of rotation.
more intense rotation <=> larger
vorticity
Rossby Waves and Vorticity
• vorticity  rotation of a fluid (air)
• Absolute vorticity:
- relative vorticity  motion of air relative to Earth’s 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
• 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)
WHAT’S 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
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.
Values of absolute vorticity on a hypothetical 500 mb map
Changes in vorticity through a Rossby wave
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
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.
The Effect of Fronts on Upper-Level Patterns
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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
Cold Fronts and the Formation of Upper-Level Troughs
• Upper air troughs develop behind surface cold fronts
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
Flow Patterns and Large-Scale Weather
• meridional v. zonal flow patterns
• Zonal: limited vorticity  hampers cyclone/anti-cyclone development
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- light winds, calm conditions, limited ppt.
• Meridional: vorticity changes between troughs and ridges  supports cyclone
development
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- cyclonic storm activity results
• Droughts (zonal) v. intense ppt. (meridional)
Zonal
Meridional
Steering of Mid-latitude Cyclones
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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.
• An example of a mid-latitude cyclone
April 15
April 16
April 17
April 18
Summary