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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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Http www aos wisc edu aalopez aos101 wk12 html

http://www.aos.wisc.edu/~aalopez/aos101/wk12.html

Thickness and Thermal Wind


Pressure

  • 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


Thickness

Thickness


A thought experiment

A Thought Experiment:

Start with a column of air.


A thought experiment1

A Thought Experiment:

The base of this column is at the surface, so lets say its pressure is about 1000mb.

1000mb


A thought experiment2

A Thought Experiment:

The top of this column is quite high—let’s say that its pressure is 500mb.

500mb

1000mb


A thought experiment3

A Thought Experiment:

This column has some thickness: it is some distance between 1000mb and 500mb.

500mb

1000mb


A thought experiment4

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


A thought experiment5

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


A thought experiment6

A Thought Experiment:

In fact, temperature is the ONLY factor in the atmosphere that determines the thickness of a layer!


A thought experiment7

A Thought Experiment:

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


Thickness and thermal wind

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


Thickness and thermal wind

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

700mb


Thickness and thermal wind

  • …whereas it is quite low to the ground near the poles.

700mb


Thickness and thermal wind

These layers are much less “thick”.

See how “thick” these layers are.


General circulation review

General Circulation Review


Conceptual circulation

Conceptual Circulation


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

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


Thickness and thermal wind

This is a cross section of the atmosphere.

North

COLD

South

WARM


Thickness and thermal wind

Cold air is coming from the north. This air comes from the polar high near the North Pole.

North

COLD

South

WARM


Thickness and thermal wind

Warm air is coming from the south. This air comes from the subtropical high near 30°N.

North

COLD

South

WARM


Thickness and thermal wind

These winds meet at the polar front.

POLAR FRONT

North

COLD

South

WARM


Thickness and thermal wind

Now, think about what we just learned about how temperature controls the THICKNESS of the atmosphere.

POLAR FRONT

North

COLD

South

WARM


Thickness and thermal wind

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


Thickness and thermal wind

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


Thickness and thermal wind

Above the front, the thickness of the atmosphere changes rapidly.

400mb

500mb

400mb

500mb

POLAR FRONT

North

COLD

South

WARM


Thickness and thermal wind

Now, let’s think about the pressure gradient force above the front.

400mb

500mb

400mb

500mb

POLAR FRONT

North

COLD

South

WARM


Thickness and thermal wind

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


Thickness and thermal wind

What is the pressure at point A?

400mb

A

500mb

B

400mb

500mb

POLAR FRONT

North

COLD

South

WARM


Thickness and thermal wind

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


Thickness and thermal wind

What is the pressure at point B?

400mb

A

500mb

300mb

B

400mb

500mb

POLAR FRONT

North

COLD

South

WARM


Thickness and thermal wind

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


Thickness and thermal wind

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


Thickness and thermal wind

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


Key points

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).


Polar front and the jet

Polar Front and The Jet

  • So, how does this all cause the midlatitude jet stream?


Polar front and the polar jet

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.


Polar front and the jet1

Polar Front and The Jet

  • All along the front, there is a strong pressure gradient force pushing northward.


Polar front and the jet2

Polar Front and The Jet

  • Winds aloft are in geostrophic balance…


Polar front and the jet3

Polar Front and The Jet

  • …so the true wind will be a WEST wind, directly above the polar front.


Another view

Another View:

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


In perspective

In Perspective:

Here is the polar front at the surface.


In perspective1

In Perspective:

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


In perspective2

In Perspective:

The midlatitude jet stream is found directly above the polar front.


Conclusions

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.


Thermal wind

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


Backing and veering of wind

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


Backing or veering

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


Cold or warm advection

Cold or Warm Advection?

  • Thermal wind always travels with COLDER AIR ON ITS LEFT !

Recall that

Cold advection brings

Cold air into warm region


Definition of the thermal wind

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


Thickness and thermal wind

No thermal advection:

Thermal wind is parallel to low level wind, so geostrophic wind at lower and upper levels are parallel

Cold Air Advection:

Thermal wind is to the left of the low level wind, so geostrophic wind must back with height => CAA

Warm Air Advection:

Thermal wind is to the right of the low level wind, so geostrophic wind must veer with height => WAA


Thermal wind equation

Thermal Wind Equation

  • Thickness is proportional to the mean temperature in the layer. Lines of equal z (isobars of thickness) are equivalent to the isotherms of mean temperature in the layer.


Thermal equation cont

In the presence of a horizontal temperature gradient, the tilt of pressure surfaces increases with height.

p=p2

ug

p=p1

cold

warm

Thermal Equation (Cont)


Thermal wind relationship

Thermal wind relationship

The change in the Geostrophic Wind is directly proportional to the horizontal temperature gradientThis is the Thermal (temperature) Wind relationship (refer to fig 3.8 in the book)


Thermal wind1

Thermal Wind

  • A horizontal thermal gradient creates a PGF at upper levels. As you increase in altitude, the pressure gradient between the warm column and the cool column increase. Last week we saw that wind in geostrophic balance, balances the PGF and Coriolis force. As the PGF increases the magnitude of the wind will increase and so will the Coriolis force. In this figure, the size of the green circles represent the magnitude of the geostrophic wind and the x in the circle represents the tail end of the directional arrow, so we are looking at an arrow pointing into away from us.


Thickness and thermal wind

  • The vertical change in geostrophic wind is called the geostrophic vertical shear. Since the geostrophic vertical shear is directly proportional to the horizontal temperature gradient, it is also called the Thermal WindSo the Thermal wind is not an actual wind, but the difference between two winds at different levels.


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