Transcript Understanding Weather and Climate Ch 10
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