Chapter 10Chapter 10

Extratropical Cyclones and Anticyclones

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 upper-level 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.