Thickness and Thermal Wind

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# Thickness and Thermal Wind - PowerPoint PPT Presentation

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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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
A Thought Experiment:

A Thought Experiment:

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

1000mb

A Thought Experiment:

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

500mb

1000mb

A Thought Experiment:

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

500mb

1000mb

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 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 Experiment:

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

A Thought Experiment:

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

These layers are much less “thick”.

See how “thick” these layers are.

Let’s think about what thickness means near a polar front, where cold air and warm air are meeting.
Cold air is coming from the north. This air comes from the polar high near the North Pole.

North

COLD

South

WARM

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

North

COLD

South

WARM

These winds meet at the polar front.

POLAR FRONT

North

COLD

South

WARM

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

POLAR FRONT

North

COLD

South

WARM

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

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

400mb

500mb

400mb

500mb

POLAR FRONT

North

COLD

South

WARM

400mb

500mb

400mb

500mb

POLAR FRONT

North

COLD

South

WARM

400mb

A

500mb

B

400mb

500mb

POLAR FRONT

North

COLD

South

WARM

What is the pressure at point A?

400mb

A

500mb

B

400mb

500mb

POLAR FRONT

North

COLD

South

WARM

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

What is the pressure at point B?

400mb

A

500mb

300mb

B

400mb

500mb

POLAR FRONT

North

COLD

South

WARM

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

400mb

A

500mb

300mb

B

400mb

600mb

500mb

POLAR FRONT

North

COLD

South

WARM

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:

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
• So, how does this all cause the midlatitude jet stream?
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 Jet
• All along the front, there is a strong pressure gradient force pushing northward.
Polar Front and The Jet
• Winds aloft are in geostrophic balance…
Polar Front and The Jet
• …so the true wind will be a WEST wind, directly above the polar front.
Another View:

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

In Perspective:

Here is the polar front at the surface.

In Perspective:

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

In Perspective:

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

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
• 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

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

Recall that

Cold air into warm region

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

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

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

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

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.

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

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 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.
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.