• 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
• 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
• 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

• 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
• 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
. 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
.

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
• 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
• - 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

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 .

April 15
•
An example of a mid-latitude cyclone

April 16

April 17

April 18

Summary