1. Cyclone and Frontal Structure and Evolution
Professor Cliff Mass
Department of Atmospheric Sciences
University of Washington

2. For much of the 20th century the dominant paradigm for cyclone/frontal evolution has been the Norwegian Cyclone Model (Bergen School)
Bjernkes, 1919

3. Concept of Air Flows in Cyclones

4. Concept of Evolution of Cyclones Bjerknes and Solberg 1922

5. Stationary Polar Front
Wave Forming on Polar Front

6. Wave Amplifies
Occlusion as Cold Front Catches Up to Warm Front

7. Occlusion Lengthens and System Weakens

8. Warm and Cold Occlusions

9. Norwegian Cyclone Model (NCM)
It was an important and revolutionary advance at the time.
First to connect three dimensional trajectories with clouds and precipitation.
Still found in many textbooks today
Over flat land away from water and terrain, reality often approximates gross characteristics of the NCM.
However, there are some major problems with the Norwegian Cyclone model that have been revealed by modern observations and modeling.

10. Some Problems With The Norwegian Cyclone Model
Different structures and evolutions of fronts and cyclones often observed over water and over/downstream of mountain barriers.
Does not properly consider the role of the middle to upper troposphere.
No upper levels fronts.
Major deficiencies regarding the occlusion process.
Does not properly consider that cyclogenesis and frontogenesis occur simultaneously.

11. Consider one problem area: the occlusion process

12. Classic Idea: Occlusion Type Determined By Temperature Contrast Behind Cold Front and in Front of Warm Front (“the temperature rule”)

13. But reality is very different
From Stoelinga et al 2002, BAMS

14. Literature Review
Schultz and Mass (1993) examined all published cross sections of occluded fronts. Found no relationship between the relative temperatures on either side of the occluded front and the resulting structure. Of 25 cross sections, only three were cold-type occlusions.
Of these three, one was a schematic without any actual data, one had a weak warm front, and one could be reanalyzed as a warm-type occlusion
Cold-type occlusions appear rare.

15. An Improved View: The Static Stability Rule of Occluded Front Slope
An occluded front slopes over the statically more stable air, not the colder air.
A cold occlusion results when the statically more stable air is behind the cold front.
When the statically more stable air lies ahead of the warm front, a warm occlusion is formed.

16. An Example

17. Another Example

18. According to the Norwegian Cyclone Model Cyclones Begin to Weaken When They Start to Occlude
In reality, observations often show that cyclones continue to deepen for many hours after the formation of the occluded front, reaching central pressures many hPa deeper than at the time often occluded-front formation.
Example: 29 of the 91 northeast United States cyclones for which surface analyses appear in Volume 2 of Kocin and Uccellini (2004) deepen 8–24 mb during the 12–24 h after formation of the occluded front

19. Intensification after Occluded Frontogeneis
This makes sense since cyclogenesis depends on three-dimensional dynamics and dynamics.
Such mechanisms for cyclogenesis can be undertood from quasigeostrophic, Petterssen–Sutcliffe development theory, baroclinic instability ideas, or potential-vorticity.

20. Is Frontal Catch-Up the Essential Characteristic of Occluded Front Development?
Not all occluded fronts develop from the cold fronts overtaking warming fronts.
Far more fundamental is the distortion of warm and cold air by vortex circulations.

21. Even in a nondivergent barotropic model where “isotherms” are passively advected by the flow, occluded-like warm-air and cold-air tongues can develop

22. Occlusion
This gradient in tangential wind speed takes the initially straight isotherms and differentially rotates them.
The differential rotation of the isotherms increases the gradient (i.e., frontogenesis)
The lengthening and spiraling of the isotherms brings the cold- and warm-air tongues closer

23. Oceanic Cyclone Structure

24. Shapiro-Keyser Model of Oceanic Cyclones

25. Major Elements of S-K Model
Weak cold front
Northern part of cold front is very weak (“fractured”)
Not much evidence of classic occlusion (well defined tongue of warm air projected to low center).
“T-Bone” structure: cold front intersects the warms front at approximately a right angle
Strong back bent (or bent back) warm front.
Warm air seclusion near the low center.

26. Simulation of the QE-II Storm

27. Neiman
and
Shapiro
1993

29. Air-Sea Interactions Warm the Cold Air, Weakening the Cold Front

31. Cross Section Across Cold Front

32. Cross Section Across Warm Front

34. Warm Seclusion Stage

35. Cross Section Across Warm Front and Associated Low-level Jet

36. Cross Section Across Warm-Air Seclusion: Circulation Weakens With Height

37. Strongest Winds With Back-Bent Warm Front

41. The Norwegian Cyclone Model Was Developed over the Eastern Atlantic and Europe, Might Development be Different In Other Midlatitude Locations Where the Large Scale Flow is Different?

42. Confluent
Diffluent

43. Confluent
Diffluent

44. There is considerable literature demonstrating different cyclone-frontal evolutions in differing synoptic environments.

45. Confluent versus diffluent synoptic flow
The Norwegian Cyclone model was developed in a region of generally diffluent flow (eastern Atlantic and Europe).
How does confluent and diffluent flow influence evolution?

46. Add a vortex to various synoptic flows and simulate the thermal evolution

47. Just Vortex

48. Confluence-Like Western Side of Oceans

49. Looks Like Shapiro-Keyser Model of Oceanic Cyclones
S-K developed over western oceans during the Erica field experiment.
Fractured cold front, strong bent-back warm/occluded front.

50. Summary

51. Diffluent Flow

52. Confluent Flow Strong cold front and weaker warm front
Resembles Norwegian Cyclone Model (NCM)
NCM devised over a region of confluent flow.

53. Summary

54. Major Mountain Barriers and Land/Water Configurations Can Have a Large Impact on Cyclone and Frontal Structures

55. How Does Different Drag Between Ocean and Land Change Cyclone and Frontal Structures?

56. Adiabatic, Primitive Equation Model

57. Ocean Drag
Land Drag

58. The Impact of Mountains Barriers on Cyclone Structure
Major topographic barriers can have a profound influence on cyclone and frontal structure.
Barriers destroy low level front structures, weaken cyclone circulations, create new structures (e.g., lee troughs and windward ridges), and restricts the motions of cold and warm air.

59. U.S. Terrain Impacts
When flow is relatively zonal synoptic structures are greatly changed over and downstream of the Rockies.
Takes roughly 1000 km for structures to appear more “classical”
Classic reference: Palmen and Newton (1969)

61. Steenburgh and Mass (Mon. Wea. Rev., 1994)
Detailed modeling study of the cyclone/frontal development east of the Rockies.

65. Conceptual Model

72. Cold Fronts Aloft And Forward Tilting Frontal Zones

78. Dry Lines or

79. Dry Lines
Associated with large horizontal gradients in moisture, but not necessarily temperature.
Results from the interaction of cyclones and fronts with large-scale terrain.
Found over the U.S. Midwest, northern India, China, central West Africa and other locations.
Acts as a focus for convection, and particularly severe convection.
Most prevalent during spring/early summer in U.S.

80. Dry Line Surface boundary between warm, moist air and hot, dry air.
Surface dry line
Well-mixed warm air
Inversion or cap

81. Typical Dryline
Temperatures in
degrees Celsius
©1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic
Meteorology in Midlatitudes, Volume II

82. Southern Plains Dry Line
©1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic
Meteorology in Midlatitudes, Volume II
Temperatures in
degrees Celsius

83. Trajectories
Fundamentally the dry line represents a trajectory discontinuity between moist southerly flow and flow descending from higher elevations.
Can only happen relatively close to the upstream barrier (no more than 1000 km) since otherwise air would swing southward behind the low system and thus would be cool and somewhat moist.

84. DRY LINE L
Warm, Moist

85. NO
DRY LINE—
Get Cold
Front L

86. Indian Dryline

88. Dew Point Gradients Associated with Indian Dry Line

89. Dry Line: Tends to Move Eastward During the Day and Westward At Night
After sunrise, the sun will warm the surface which will warm the air near the ground.
This air will mix with the air above the ground.
Since the air above the moist layer is dry (and is much larger than the moist layer), the mixed air will dry out.
The dry line boundary will progress toward the deeper moisture.

90. Dry Line Boundary after mixing Top of moist layer before mixing
Hot, Dry Air—Usually Well Mixed
Warm, Moist Air
Initial Position
of the Dry Line
Position of the
Dry Line after
mixing

91. Dry Line
After sunset, a nocturnal inversion forms and the winds in the moist air respond to surface pressure features.
The dry line may progress back toward the west .

92. West East
Note weak inversion or “cap” over low-level moist layer east of the surface dry line

93. Sounding West of the Dryline
NCAR
Sounding West of the Dryline
Albuquerque, NM
12Z June 1998
West Winds
Very Dry

94. Sounding East of the Dryline
NCAR
Sounding East of the Dryline
Oklahoma City, OK
12Z June 1998
South Winds
Moist

95. Aircraft Study of the Dry Line

96. Convection Tends to Focus On the Dryline

97. Simulation of a Thunderstorm Initiation Along Dryline in TX Panhandle
Note converging winds and rising
motion

98. Storm Initiation Along a Dry Line

99. Why is a dry line conducive for strong convection?
Low level confluence and convergence produce upward motion.
The cap allows the build-up of large values of Convective Available Potential Energy (CAPE)
East of the surface dry line, the existence of a layer of dry air over moist air enhances convective/potential instability.

100. Greatest Potential for Convective Development Exists at the Intersection between the Dry Line and Approaching Cold Front