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Chapter 10
Extratropical Cyclones
and Anticyclones
Figure CO: Chapter 10, Extratropical Cyclones and Anticyclones
Courtesy of NASA
Figure UN01: A vase, or two faces
Figure UN02: Jacob Bjerknes
Courtesy of Geophysical Institute, University of Bergen
The Norwegian Cyclone Model Life
Cycle
• The cyclone starts as a frontal wave
– A stationary front separates cold, dry cP air from
warm, moist mT air
– Called a wave because warm sector (mT) air
resembles a gradually steepening ocean wave
• The open wave in adolescence
– Moves east or northeast
– Develops warm and cold fronts
– Precipitation falls ahead of the warm front and in the
vicinity of the cold front
The Norwegian Cyclone Model Life
Cycle (continued)
• The occluded cyclone in full maturity sprouts an
occluded front as warm air mass rises
– Usually lowest barometric pressure in this stage
– Winds usually strongest in this stage
– Cloudiness associated with the fronts wraps poleward
and around the back side of the cyclone
• The cut-off cyclone
– Final stage
– Clouds and precipitation around the low’s center
dissipate
Figure T01a: The Life Cycle of the Extratropical Cyclone, Based on the
Bergen School Model
Modified from Bjerknes, J. and Solberg, H., 1922: 'Life cycle of cyclones and the
polar front theory of atmospheric circulation'. Geofys. Publ., 12, pp 1-61.
Courtesy of Norwegian Geophysical Society. Courtesy of
CIMSS/SSEC/University of Wisconsin-Madison
Figure T01b: The Life Cycle of the Extratropical Cyclone, Based on the
Bergen School Model
Modified from Bjerknes, J. and Solberg, H., 1922: 'Life cycle of cyclones and the
polar front theory of atmospheric circulation'. Geofys. Publ., 12, pp 1-61.
Courtesy of Norwegian Geophysical Society. Courtesy of
CIMSS/SSEC/University of Wisconsin-Madison
The story of an extratropical cyclone
• Day 1, Sat., Nov. 8,1975: Low (Panhandle
Hook) forms just NE of Amarillo
• Day 2, Sun., Nov. 9: Ship sails at 2 p.m.; Storm
has matured to young adult stage
• Day 3, Mon., Nov. 10: The storm is occluding.
Ship sinks at 7:20 p.m.
• Day 4, Tues., Nov. 11: The storm is dying
• Subtract 7 from date to get Day #
Figure 02: Edmund Fitzgerald
Courtesy of Ruth Hudson
Figure 03: Photo of two Fitzgerald sailors.
Courtesy of Ruth Hudson
Figure 01: Map of Great Lakes
Day 1: Cyclogenesis
• Cyclogenesis: cycle of cyclone birth and growth
• Key ingredients for cyclogenesis
– Surface temperature gradients, a front
– A strong jet stream, helps the low deepen and the
fronts intensify
– Presence of mountains or other surface boundaries
like a coastline near a warm ocean current
– Winds blowing across temperature gradients
– Baroclinic instability, the process by which cyclones
get their energy—related to horizontal temperature
gradients and vertical wind shear
Figure 04A: Weather maps (surface and 500 mb) and satellite picture for
Nov. 8, 1975.
Source: NOAA
Figure 04B: Weather maps (surface and 500 mb) and satellite picture for
Nov. 8, 1975.
Source: NOAA
Figure 04C: The satellite picture is centered over extreme western Texas;
clouds are visible along the west coast of Baja California at the left of the image.
Courtesy of National Snow and Ice Data Center
Typical cyclone paths
• Depend on topography
• Depend on position of the polar front
• Depend on upper-level winds
– Extratropical cyclones move approximately with
the 500mb wind and at about half the speed
Figure 05: Regions of cyclogenesis
Source: Courtesy of Pam Naber Knox, former Wisconsin State Climatologist.
Figure B01_1: Conservation of angular momentum
Figure B01_2: Cyclone going over Rockies
Figure 06: Regions of cyclogenesis across North America.
Adapted from Zishka, K. M., and P. J. Smith, Monthly Weather Review,
April 1980: 391-392
Day 2: Cyclone as a Young Adult
• Comma cloud is characteristic of mature
extratropical cyclones
– Quite different from the circular tropical cyclone
• Cloudless region between the comma head
and tail is the dry slot
– A feature of mature extratropical cyclones
– Not seen in hurricanes
Figure 07A: Weather maps and satellite image for Nov. 9, 1975.
Source: NOAA
Figure 07B: Weather maps and satellite image for Nov. 9, 1975.
Source: NOAA
Figure 07C: Weather maps and satellite image for Nov. 9, 1975.
Courtesy of National Snow and Ice Data Center
Figure 08: Zoom-in of satellite image in previous figure.
Courtesy of National Snow and Ice Data Center
Back to the story, Day 2
• The cyclone moves very rapidly, steered by the
upper-level winds at 500mb.
• On Day 2 (Nov. 9) gale warnings were issued in
the mid-afternoon for the next day (Monday, Day
3, Nov. 10)
• Gale warnings are for winds up to 38 knots (44
mph), not typical, not too unusual
• Ships take the northern, longer route to protect
ships from high seas caused by north and
northeast winds.
Figure 09: Madison, Wisconsin, weather during the passage of the
Edmund Fitzgerald cyclone
Data from NWS
Figure 10A: The approximate positions of the Fitzgerald cyclone and its fronts
Data from NWS
Figure 10B: Cross-sections of weather on Nov. 9, 1975.
Data from NWS
Figure 10C: Cross-sections of weather on Nov. 9, 1975.
Day 3: The Strengthening Storm
• At midnight the cyclone is strengthening
rapidly and aiming northward over Lake
Superior
• The strongest winds will soon be coming out
of the west, not the northeast
• This will leave the freighter exposed to
hurricane-force west winds and high seas on
the 10th (Day 3)
Cyclone—Jet-Stream Relationships
• Surface pressure drops when there is divergence
of the wind in the column of air above the low
• Upper-level divergence can occur in two different
ways
– Straight-line acceleration is called speed divergence
– Spreading out is called diffluence
• Divergence commonly occurs east of a trough
• The surface pressure falls along and ahead of the
low
Figure 11: The relationship between the two types of divergence (speed
divergence and diffluence)
Figure 12: Cyclone stages and upper-level winds and temperatures.
Adapted from Carlson, T. N. Mid-Latitude Weather Systems. American
Meteorological Society, 1998.
Figure 13: Jet stream on Nov. 9, 1975.
Data from NWS
Figure B02_1: The conveyor belts of an extratropical cyclone
Modified from Wilson, E.E., “Great Lakes Navigation Season’ Mariners
Weather Log 20 (1975): 139-149
Figure B02_2: Surface ozone levels in and near Chicago on
November 9–11, 1975
Day 3: The Mature Cyclone
• To the west of the low, blizzard conditions, as
much as 14 inches of snow in northern Wisconsin
• From Iowa to Tennessee 15 tornadoes
• Occluded front joins the low and the cold and
warm fronts
– Occlusion: Warm air ascends over the warm front,
which removes warm air from the surface
• The narrowing region of warm air lifts completely above the
surface near the low, leaving the boundary between two
cold air masses called the occluded front
Figure 14A: Weather maps and satellite image for Nov. 10, 1975.
Source: NOAA
Figure 14B: Weather maps and satellite image for Nov. 10, 1975.
Figure 14C: Weather maps and satellite image for Nov. 10, 1975.
Courtesy of National Snow and Ice Data Center
Figure 15: Pressure changes at four different weather stations.
Data from NWS
Figure 16: Winds at Sault Ste. Marie and Marquette, MI.
Data from NWS
Figure 17: The Wreck of the Edmund Fitzgerald
Courtesy of Frederick Stonehouse
Results of the Occlusion Process
• The surface low gradually retreats to the cold
air to the north and west
• The surface low ends up beneath the upperlevel low
• Eventually the cyclone is isolated from its fuel
source, the mT air
• An occluding cyclone can develop further if it
gets energy from latent heating
• This intensification was fatal for the Fitzgerald
Figure 18A: Weather maps for Nov. 11, 1975
Source: NOAA
Figure 18B: Weather maps for Nov. 11, 1975
Source: NOAA
Figure 18C: Weather maps for
Nov. 11, 1975
Source: NOAA
Figure B04_1: Paths of 1975 and 1998 storms
Source: Don Rolfson, National Weather Service Marquette/NOAA
Figure B04_3: Wave and lighthouse.
© Kalamazoo Gazette, Taya Kashuba/AP Photos
Figure 19: Capsized Fitzgerald lifeboat
Courtesy of Le Sault de Sainte Marie Historical Sites, Inc.
Figure 20: The Wreck Site II
Courtesy Great Lakes Shipwreck Historical Society
The Extratropical Anticyclone
•
•
•
•
•
An anticyclone is a high-pressure system
Highs are air masses, nearly homogeneous
Highs can linger as long as weeks in summer
Air in a high diverges at the surface
Weak gradients of temperature and humidity
mean no fronts in a high
• Highs are stable and often cloudless on account
of sinking
• Highs have weak PGF, weak winds
Figure 21: Weather map of 1979 blizzard and high pressure.
Adapted from Kocin, P. and Uccellini, L. Snowstorms Along the Northeastern Coast
of the United States: 1955 to 1985. American Meteorological Society, 1990.
The Extratropical Anticyclone
(continued)
• Anticyclogenesis, the development of
anticyclones, occurs away from jet streams
• Summertime highs
– Can thrive and intensify if cut off or blocked from
the main jet-stream winds and surface
temperature gradients
• Can be responsible for deadly heat waves
Figure 22: Typical regions of anticyclogenesis (shaded) and anticyclone paths
Adapted from Zishka, K. M., and P. J. Smith, Monthly Weather Review,
April 1980: 394-395.
Figure B07: The Groundhog Day Blizzard of 2011
© Kiichiro Sato/AP Photos
Figure 23: 500-mb height anomalies over Europe.
From G. A. Meehl et al., Science 305, 994 -997 (2004). Reprinted with
permission from AAAS.
Figure 24: Temperature anomalies over Europe.
Image by Reto Stöckli, Robert Simmon and David Herring, NASA Earth
Observatory, based on data from the MODIS land team.
Figure 25: Average summer temperatures in Switzerland for the years
1864-2003.
Adapted from C. Schär et al., Nature 427, 332-336.
Figure 26: Daily mortality rate in the state of Baden-Württemberg,
Germany, from January 2002 through August 2003.
Adapted from Koppe, C. and Jendritzky, G. Gesundheitliche Auswirkungen der
Hitzewelle. Sozialministerium Baden-Württemberg, Stuttgart, 2004.