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