Chapter 13 Middle Latitude Cyclones

1. Chapter 13 Middle Latitude Cyclones • This chapter discusses: • The location, vertical structure, and developmental stages of middle latitude cyclones • How upper level convergent winds, abrupt topographic features, and planetary longwaves may enhance cyclonic development at the surface

2. Polar Front Theory (Norwegian) Figure 13.1C-Open Wave Figure 13.1A-Cylconic Shear Figure 13.1D- Mature Cyclone Figure 13.1B-Frontal Wave

3. Mid-latitude Cyclone Life Cycle • Begins with a stationary front with warm and cold winds in opposite directions (e.g. wind shear). • A wavelike kink, such as a low pressure system, then creates a frontal wave, or incipient cyclone. • As the storm develops into an open wave, a broad band of precipitation forms ahead of the warm sector. • As cyclone matures, the cold air “catches up” with the warm sector, and low becomes occluded • The mature cyclone is now at it’s strongest, and begins to slowly weaken • Eventually, the lowbecomes cut-off and dissapates

4. Triple Point Figure 13.1E-Occlusion Figure 13.1F-Dissapation

6. Wave Cyclone Development A series, or family, of cyclones, at various stages of development, may extend across North America. Energy originates from kinetic sources such as rising warm air, sinking cold air, and converging air, as well as latent heat of condensation. Figure 13.2

7. Cyclone & Anticyclone Paths Generally NE Lee Side lows Generally SE Figure 13.3A Figure 13.3B • Many well-known paths for low and high pressure systems extend across North America • Cyclone development most common in a few specific areas • Cyclogenesis describes the strengthening of these storms into huge unstable waves.

8. Convergence & Divergence Deepening of cyclones through explosive cyclogenesis is prohibited when low pressure aloft is directly above the surface low. In this scenario, convergence at the surface low builds up mass and fills in the low (converging air has nowhere to go). The same stacking of high pressure, with divergence at the surface, will weaken the anticyclone. Figure 13.4

9. Vertical Storm Structure Divergence of air aloft occurs as contour interval widen (the inverse happens when height contours become more packed together) Low pressure systems deepen and intensify (cyclogenesis) when upper-level divergence is stronger than the surface convergence This which requires a vertical offset of the surface low and low pressure above it. Figure 13.5

10. Upper Level Waves Earth's poles are encircled by 3 to 6 longwaves, or Rosby waves, directing upper level winds around lows at the 500 mb surface. Small disturbances (shortwaves) in these larger waves can trigger storms Cyclones move in the direction of the 500mb flow, at about ½ the speed Figure 13.6

11. Shortwave Disturbance Shortwave “kinks” within the Rossby waves move faster, and propagate downwind into the Rossby troughs and cause them to deepen. Barotropic conditions, where isobars and isotherms are parallel, then degenerate into a baroclinic state where the lines cross and cold or warm air is advected downwind.

12. Cyclone Development: Upper Winds Figure 13.8A-Barotropic Figure 13.8C-Barotropic Differential temperature advection intensifies the wave. + Existing surface frontal boundary upper level shortwave moving over the boundary--cyclogenesis occurs Figure 13.8B-Barclinic Instability

13. Atmospheric conditions at the surface and aloft affect cyclogenesis. An upper level shortwave can trigger baroclinic instability. -Convergent flow aloft helps strengthen surface high pressure, and supports cold air advection -Downwind of the upper level trof, divergent flow aloft deepens the surface low, and warm advection aids rising air flow

15. Finding a Low Center Using Wind Observations

16. Jet Streak Divergence aloft is enhanced by the polar jet stream, where the jet maximum, or jet streak, forms in the tightly packed pressure gradients. Strong C Strong D Weak D Weak C Figure 13.9

17. Jet Convergence & Divergence Figure 13.10A Figure 13.10B -The polar jet forces air convergence aloft upstream of the deepening open wave cyclone, and then divergence downstream. -When these winds are gone, the cyclone degrades.

18. Summary of Cyclone Weather Upper and surface maps summarize the role of convergence and divergence aloft; and the pattern of clouds, precipitation, and temperatures on the ground. Figure 13.11

19. Conveyor Belt Model This model describes rising and sinking air along three conveyor belts, warm conveyor belt rises with water vapor above the cold conveyor belt which also rises and turns. Finally the dry conveyor belt descends brining clearer weather behind the storm. (Upper Level) Figure 13.12

20. Comma Cloud Pattern

21. Comma Clouds Rising and turning moist air, illustrated in the conveyor belt model, condenses into a large comma-shaped cloud typical of the open wave cyclone. This March 1993 storm wreaked havoc along the East Coast. Figure 13.13

22. Mar 1993 “Storm of the Century” During the “storm of the century”, the low pressure center reached 980 mb, and the storm extended across several southeastern states. Figure 13.14

23. 500mb Temperature Advection Figure 13.15 Upper level winds flowed along a deep trough with steep baroclinic cold and warm air advection.

24. Storm of Century Path Low pressure values and location are charted with time to illustrate the storm track and intensity, moving from Texas to Maine in 2 days. Figure 13.16

25. Size of Polar Lows Cyclones that develop north of the polar front, called polar lows, are smaller in size than mid-latitude cyclones. They form during the winter in arctic air, have (relatively) warm central cores, strong winds, and can generate snow. Figure 13.17

26. Vorticity & Cyclonic Spin Figure 13.18 Figure 13.19 - Vorticity describes the spin of an air parcel, which is positive in counterclockwise cyclonic flow. - Due to the conservation of angular momentum, vorticity increases with a decrease in parcel radius (e.g. stretching due to divergence aloft) and increase in earth's latitude.

27. Sources of Vorticity Figure 13.21 Figure 13.20 - Curvature of upper level isobars and wind, as well as strong changes in wind speed (shear), generate the spin of relative vorticity. - Additional earth vorticity is generated by the earth's spin, and together they comprise absolute vorticity.

28. Trough to Ridge Vorticity Change Anticyclonic spin around a ridge reduces absolute vorticity, but the convergence and cyclonic spin in the trough enhance the relative and hence absolute vorticity, as illustrated in this upper level diagram. Figure 13.22

29. Vorticity & Vertical Motion The 500 mb map vorticity maximum is a signal that to its east, air is diverging aloft. If there is also convergence below, then an open wave cyclone will likely deepen. Hence, 500 mb charts are useful to find the “vort max” and predict potential storms. Figure 13.23

30. 500mb Vorticity Chart

31. Imaging Shortwave Centers GOES West satellite infrared imagery of water vapor are useful in identifying swirling vorticities, seen off Pacific Northwest coast. Figure 13.24

32. Vertical Storm Profile Surface, 500mb, and 200mb charts are used to illustrate the structure of the February 1983 open wave cyclone exploding over North Carolina. The 500mb chart shows a shortwave dashed line moving into the longwave trough and baroclinic cold air advection. Figure 13.25

33. February 1983 Vorticity Lines of equal vorticity are plotted on the 500 mb chart for the February 1983 open wave cyclone that buried the east coast in snowfall. Note that the vort max is west of the storm center, strengthening the cyclogenesis. Figure 13.26

34. Key Facts and Concepts • The polar front theory is a model of how an ideal storm progresses through the stages of birth, maturity, and dissipation • For a surface mid-latitude cyclone to develop or intensify, the upper-level low must be located to the west (or behind) the surface low • Convergence is the piling up of air, divergence is the spreading out of air • Upper-level divergence above the developing surface cyclone must be greater than surface convergence • Upper-level convergence above developing surface anticyclone must be greater than surface divergence

35. As the jet stream develops into a looping wave pattern, it produces areas of convergence and divergence • The jet stream tends to direct anticyclones to the southeast and cyclones to the NE • When upper-level flow is disturbed by a shortwave, horizontal and vertical air motions begin to enhance the formation of a surface storm • Rising warm air and sinking cold air provide energy for storm formation • As air flows over a mountain range, wind flow causes storms to form or intensify • Polar lows are storms that develop over water in polar regions behind the main polar front (on the arctic front) • Vorticity is a measure of the spin in a column of air • An area of positive relative vorticity at 500mb can initiate surface mid latitude cyclogenesis