Thickness and Thermal Wind /aos101/wk12.html /aos101/wk12.html

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

Thickness

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

A Thought Experiment: The base of this column is at the surface, so lets say its pressure is about 1000mb. 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. The top of this column is quite high—let’s say that its pressure is 500mb. 1000mb 500mb

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

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

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

A Thought Experiment: In fact, temperature is the ONLY factor in the atmosphere that determines the thickness of a layer! 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…. It wouldn’t have mattered which pressure we had chosen. They are all higher above the ground when it is warmer….

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

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

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

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

General Circulation Review

Conceptual Circulation

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

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

Cold air is coming from the north. This air comes from the polar high near the North Pole. 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. 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. These winds meet at the polar front. North COLD South WARM POLAR FRONT

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

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

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

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

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

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

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

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. 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. North COLD South WARM POLAR FRONT 500mb 400mb 500mb 400mb A B 300mb

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

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. 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. North COLD South WARM POLAR FRONT 500mb 400mb 500mb 400mb A B 300mb 600mb

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

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

Key Points: This strong pressure gradient force happens: This strong pressure gradient force happens: Aloft (above the surface) Aloft (above the surface) Directly above the Polar Front Directly above the Polar Front Also, this force pushes toward the north (in the Northern Hemisphere). 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? 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. 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. All along the front, there is a strong pressure gradient force pushing northward.

Polar Front and The Jet Winds aloft are in geostrophic balance… 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. …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. 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. 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 is because of the changes in THICKNESS associated with the polar front. This process is known as the THERMAL WIND RELATIONSHIP. This process is known as the THERMAL WIND RELATIONSHIP.

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

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

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

Cold or Warm Advection? Thermal wind always travels with COLDER AIR ON ITS LEFT ! 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 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

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 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. coldwarm ugug p=p 1 p=p 2 Thermal Equation (Cont)

Thermal wind relationship The change in the Geostrophic Wind is directly proportional to the horizontal temperature gradient This 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.geostrophic balance

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 Wind So the Thermal wind is not an actual wind, but the difference between two winds at different levels.