Post on 18-Jan-2018
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Chapter 9: Mid-Latitude Cyclones
IntroductionIntroduction• 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
• 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
The Life Cycle of a Mid-Latitude Cyclone
• 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
Mature Cyclones
• 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
• 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
Occlusion
• 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
Evolution and Migration
• 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
Process of the Middle and Upper Troposphere
• 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)
• 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
WHAT’S THE POINT OF VORTICITY????
Necessary ingredients for a developing wave cyclone1. Upper-air support
- 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.
filling
Values of absolute vorticity on a hypothetical 500 mb map
Changes in vorticity through a Rossby wave
Necessary ingredients for a developing wave cyclone1. 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 cyclone2. 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.
• 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
The Effect of Fronts on Upper-Level Patterns
• Upper air troughs develop behind surface cold fronts
Cold Fronts and the Formation of Upper-Level Troughs
• 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
Interaction of Surface and Upper-Level Patterns
• 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)
Flow Patterns and Large-Scale Weather
Zonal Meridional
• 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.
Steering of Mid-latitude Cyclones
• An example of a mid-latitude cyclone
April 15
April 16
April 17
April 18
Summary