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Thickness and Thermal Wind

http://www.aos.wisc.edu/~aalopez/aos101/wk12.html. Thickness and Thermal Wind. Pressure is the weight of molecules ABOVE you Fewer molecules above you as you go up causes pressure to decrease with altitude

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Thickness and Thermal Wind

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  1. http://www.aos.wisc.edu/~aalopez/aos101/wk12.html Thickness and Thermal Wind

  2. Pressure is the weight of molecules ABOVE you • Fewer molecules above you as you go up causes pressure to decrease with altitude • Temp, density, volume change because of pressure change – do not cause the pressure change as a parcel rises Pressure

  3. Thickness

  4. A Thought Experiment: Start with a column of air.

  5. A Thought Experiment: The base of this column is at the surface, so lets say its pressure is about 1000mb. 1000mb

  6. A Thought Experiment: The top of this column is quite high—let’s say that its pressure is 500mb. 500mb 1000mb

  7. A Thought Experiment: This column has some thickness: it is some distance between 1000mb and 500mb. 500mb 1000mb

  8. A Thought Experiment: If we heat the column of air, it will expand, warm air is less dense. The thickness of the column will increase. 500mb is now farther from the ground. 500mb 1000mb Warmer

  9. A Thought Experiment: If we cool the column of air, it will shrink, cool air is more dense. The thickness of the column will decrease. 500mb is now closer to the ground. 500mb 1000mb Colder

  10. A Thought Experiment: In fact, temperature is the ONLY factor in the atmosphere that determines the thickness of a layer!

  11. A Thought Experiment: It wouldn’t have mattered which pressure we had chosen. They are all higher above the ground when it is warmer….

  12. …which is what this figure is trying to show.

  13. In the tropics, 700mb is quite high above the ground… 700mb

  14. …whereas it is quite low to the ground near the poles. 700mb

  15. These layers are much less “thick”. See how “thick” these layers are.

  16. General Circulation Review

  17. Conceptual Circulation

  18. Let’s think about what thickness means near a polar front, where cold air and warm air are meeting.

  19. This is a cross section of the atmosphere. North COLD South WARM

  20. Cold air is coming from the north. This air comes from the polar high near the North Pole. North COLD South WARM

  21. Warm air is coming from the south. This air comes from the subtropical high near 30°N. North COLD South WARM

  22. These winds meet at the polar front. POLAR FRONT North COLD South WARM

  23. Now, think about what we just learned about how temperature controls the THICKNESS of the atmosphere. POLAR FRONT North COLD South WARM

  24. On the warm side of the front, pressure levels like 500mb and 400mb are going to be very high above the ground. 400mb 500mb POLAR FRONT North COLD South WARM

  25. On the cold side of the front, pressure levels like 500mb and 400mb are going to be very low to the ground. 400mb 500mb 400mb 500mb POLAR FRONT North COLD South WARM

  26. Above the front, the thickness of the atmosphere changes rapidly. 400mb 500mb 400mb 500mb POLAR FRONT North COLD South WARM

  27. Now, let’s think about the pressure gradient force above the front. 400mb 500mb 400mb 500mb POLAR FRONT North COLD South WARM

  28. Let’s draw a line from the cold side of the front to the warm side. 400mb A 500mb B 400mb 500mb POLAR FRONT North COLD South WARM

  29. What is the pressure at point A? 400mb A 500mb B 400mb 500mb POLAR FRONT North COLD South WARM

  30. The pressure at point A is less than 400mb, since it is higher than the 400mb isobar on this plot. Let’s estimate the pressure as 300mb. 400mb A 500mb 300mb B 400mb 500mb POLAR FRONT North COLD South WARM

  31. What is the pressure at point B? 400mb A 500mb 300mb B 400mb 500mb POLAR FRONT North COLD South WARM

  32. The pressure at point B is more than 500mb, since it is lower than the 500mb isobar on this plot. Let’s estimate the pressure as 600mb. 400mb A 500mb 300mb B 400mb 600mb 500mb POLAR FRONT North COLD South WARM

  33. The pressure gradient force between point B and point A is huge! 400mb A 500mb 300mb B 400mb 600mb 500mb POLAR FRONT North COLD South WARM

  34. Therefore, all along the polar front, there will be a strong pressure gradient force aloft, pushing northward. 400mb A 500mb 300mb B 400mb 600mb 500mb POLAR FRONT North COLD South WARM

  35. Key Points: This strong pressure gradient force happens: Aloft (above the surface) Directly above the Polar Front Also, this force pushes toward the north (in the Northern Hemisphere).

  36. Polar Front and The Jet • So, how does this all cause the midlatitude jet stream?

  37. Polar Front and The Polar Jet • Suppose we have a polar front at the surface. This purple line is the polar front at the surface. As we’ll learn, this is NOT how fronts are correctly drawn, but it will work for now.

  38. Polar Front and The Jet • All along the front, there is a strong pressure gradient force pushing northward.

  39. Polar Front and The Jet • Winds aloft are in geostrophic balance…

  40. Polar Front and The Jet • …so the true wind will be a WEST wind, directly above the polar front.

  41. Another View: Here’s the same diagram, shown from a slightly different angle, which might make this all more clear.

  42. In Perspective: Here is the polar front at the surface.

  43. In Perspective: Remember, it’s a polar front because it is where warm air from the south meets cold air from the north.

  44. In Perspective: The midlatitude jet stream is found directly above the polar front.

  45. Conclusions: The Midlatitude Jet Stream is found directly above the polar front, with cold air to the LEFT of the flow. This is because of the changes in THICKNESS associated with the polar front. This process is known as the THERMAL WIND RELATIONSHIP.

  46. Thermal Wind • The strength and direction of the wind changes with altitude above the front Thermal Wind Lower Level Geostrophic wind Upper level geostrophic wind

  47. Backing and Veering of Wind If winds rotate counter- Clockwise with height  Backing ! If winds rotate clockwise From lower level to upper Level  veering ! Upper Level Geo Wind Lower Level Geo Wind Thermal Wind Lower level Geo winds Upper Level Geo wind Thermal Wind

  48. Backing or Veering? • Find the lower level geostrophic winds • Track angle (shortest) FROM lower level wind to upper level wind • Did you go clockwise? • Did you go counterclockwise? • CLOCKWISE  VEERING • COUNTERCLOCKWISE  BACKING

  49. Cold or Warm Advection? • Thermal wind always travels with COLDER AIR ON ITS LEFT ! Recall that Cold advection brings Cold air into warm region

  50. Definition of the thermal wind • The thermal wind (VT) is not a wind at all, but a vector difference between the geostrophic wind at one level and the geostrophic wind at another level, i.e., it is a wind shear : VT = Vupper level - Vlower level

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