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  air and warmer southerly air masses separating polar • 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  wind direction leads to shifts in • 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

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  - Earth vorticity  motion of air relative to Earth’s surface 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  • referred to as

dynamic lows

(v. thermal lows) • dynamic lows (surface) exist downwind of trough axis surface LP • 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: Colorado Lows:

zonal flow, light ppt.

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