1. Midlatitude Weather Systems ATMS 301

2. The Large Scale Picture

3. A jet stream (or jet) is a narrow current of strong winds.
Can exist at several levels, but most often applied to the high velocity winds in the vicinity of the midlatitude tropopause

4. Midlatitude Jet Stream Facts
Can reach 250 mph (or more)
Centered on the upper troposphere…around 250 hPa
Stronger in winter. Generally, weakens and moves northward during summer.
Closely associated with a tropospheric temperature gradient (thermal wind!)
Not uniform zonally

5. Wasaburo Oishi, Japanese Discoverer (1920s)

7. Fu-Go Balloon Weapon During WWII

9. Strahlstromung (German) for “jet stream” first used in 1939
Big effects on bomber flights during WWII

10. B-29s flying westward to Japan

11. Jet Stream winds are not uniform

12. 250 mb isotach

13. Shaded 70—110 knot

14. Jet Streams are NOT uniform

16. Main Upper Tropospheric Jets Closely Associated with the Tropopause

17. Tropopause

18. There are TWO types of upper tropospheric jet streams
Polar front jet: associated with main midlatitude frontal/baroclinic zone. Typically 30-45N, ~ hPa
Subtropical jet: associated with the northern portion of the tropical Hadley circulation. Typically around 30N, ~200 hPa

19. Subtropical Jet Stream

21. Actually Two Jet Streams
10-16km
7-12km

22. Subtropical Jet Stream Often Associated with a cloud band

23. http://itg1. meteor. wisc

24. Jet Stream’s Relationship to Temperature Is Expressed Through Thermal Wind Arguments

25. Thermal Wind Equation

26. 250 mb isotach

27. 1000-500 mb thickness-large thickness gradient and thus thermal wind near jet core

28. Or alternatively…

29. The Jet Stream Recall the horizontal temperature effects
on the pressure:
500 mb
700 mb
850 mb
Psurface
Warm
Cold

30. The Jet Stream Consider the balance of forces at each level: Co PGF
500 mb
700 mb
850 mb Ps
Warm
Cold

31. Air Masses

34. Maritime Tropical (mT) Source Region

35. Continental Polar (cP) Source Region

36. Continental Tropical (cT) Source Region

38. Maritime Polar (mP) Source Region

39. Bering Sea Cloud Streets (cP to mP)

43. An Important Midlatitude Weather Feature
Fronts
An Important Midlatitude Weather Feature

44. Definition You Should NEVER Forget
A front is a boundary between relatively uniform warm air and a zone in which temperatures cools rapidly

46. Four Main Types of Fronts

47. Warm Front

48. Stationary Fronts

49. Occluded Front (a hybrid)

50. As a front passes there are changes in:
Temperature
Dew point
Wind direction
Pressure
cloudiness

51. Fronts and Pressure Fronts are associated with troughs of low pressure

52. Fronts are associated with bands of clouds

53. Vertical Structure of Fronts

54. Cold Front
Slope ~1:50, moves fast (20-30 mph), convection on leading edge

55. Warm Front
Smaller slope (~1:200), slower (1—15 knots), more stratiform clouds

56. Stationary Front similar structure to warm front, but without movement

57. There is a typical progression of clouds as cold and warm fronts approach and pass by
Cirrus
Cirrostratus
Altostratus
Nimbostratus
Cumulus after cold front

64. There is another type of front: the occluded front
But to understand this this front, you need to learn about the life cycle of fronts and cyclones.

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

66. Concept of Evolution of Cyclones Bjerknes and Solberg 1922

67. Stationary Polar Front
Wave Forming on Polar Front

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

69. Occlusion Lengthens and System Weakens

71. Warm and Cold Occlusions

73. In the real world, only the warm occlusion is observed

74. During the 1930s-1950s we learned the relationship between cyclones and fronts and upper level flow
Upper troughs associated with surface lows. Usually lagging to the west.
Upper ridges asociated with surface highs. Usually lagging to the west.

76. In front of Ridges associated with sinking, in front of troughs with rising motions

77. https://atmos.washington.edu/~ovens/wxloop.cgi?h500_slp+/-168//

78. What is the energy source of midlatitude cyclones?

79. The answer: warm air rising and cold air sinking

80. Warm (less dense) air rising and cold (more dense) air sinking lowers the center of gravity of the atmosphere
Like dropping a weight.
Potential energy (energy inherent in being aloft) is converted to kinetic energy (energy of moving air)

81. The conversion to kinetic energy is enhanced by having large differences of temperatures (large horizonal temperature gradients)

82. No accident that cyclones grow in regions of large temperature gradients