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

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 cyclone 1. 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 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.

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 ZonalMeridional

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