General Circulation

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General Circulation Thermal Wind

• April 14, 2009
• AOS 101-302

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General Circulation

• What is the global picture?
• The average flow on the globe...

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General Circulation Hadley Cell

• Thermally-driven convection
• Warm air rises and cold air sinks, creating
  circulation

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General Circulation 3 Cells

• Hadley Thermally driven circulation confined to
  tropics
• Ferrell Mid-latitude circulation cell
  (subtropics to polar front)
• Polar Sinking air at the poles

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General Circulation Winds

• Trade Winds Surface easterly winds diverging
  from subtropical Highs and converging near the
  Equator
• Westerlies Diverge from subtropical Highs
  converge toward polar front
• Polar Easterlies Converge along the polar front

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General Circulation Sea Level Pressure

• Low Pressure (converging air!)
• ITCZ (Intertropical convergence zone), near the
  equator
• Subpolar Lows along the polar front, near 60
• High Pressure (diverging air!)
• Subtropical Highs near 30 (warm dry)
• Polar High at the pole (cold dry)

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General Circulation Climate

• Deserts at subtropical highs (High sinking
  air!)
• Rainforests near ITCZ (Low rising air
  clouds!)
• Polar regions are deserts and receive very little
  precipitation each year (High sinking air!)

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General Circulation Jet Streams
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Pressure

• Pressure is the weight of air molecules ABOVE you
• Pressure decreases with altitude because there
  are less air molecules above you as your rise
• As a result of pressure changes, Temperature,
  Density, and Volume change too as you rise

http//www.srh.noaa.gov/jetstream//atmos/images/mb
_heights.jpg
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Thickness...
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• Start with a column of air.

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

1000 mb
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• The top of this column is quite highlets say
  that its pressure is 500 mb

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

500 mb
1000 mb
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500 mb

• 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

1000 mb
Warmer
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• 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

500 mb
1000 mb
Colder
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Thickness

• In fact, temperature is the ONLY factor in the
  atmosphere that determines the thickness of a
  layer
• It wouldnt have mattered which pressure we had
  chosen. They are all higher above the ground
  when it is warmer.

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Thickness

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

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Thickness

• At the poles, 700 mb is quite low to the ground
• These layers are not very thick
• In the tropics, 700mb is much higher above the
  ground
• See how thick these layers are

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General Circulation!
Lets think about what thickness means near a
polar front, where cold air and warm air are
meeting
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This is a cross section of the atmosphere
North COLD
South WARM
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Cold air is coming from the north. This air
comes from the polar vortex near the North Pole
North COLD
South WARM
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Warm air is coming from the south. This air
comes from the subtropical high near 30N
North COLD
South WARM
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These winds meet at the polar front (a strong
temperature gradient)
POLAR FRONT
North COLD
South WARM
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Now, think about what we just learned about how
temperature controls the THICKNESS of the
atmosphere
POLAR FRONT
North COLD
South WARM
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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
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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
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Above the front, thickness of atmosphere changes
rapidly
400mb
500mb
400mb
500mb
POLAR FRONT
North COLD
South WARM
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Now, what about the PGF above the front?
400mb
500mb
400mb
500mb
POLAR FRONT
North COLD
South WARM
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Lets draw a line between the cold side of the
front and the warm side
400mb
500mb
A
B
400mb
500mb
POLAR FRONT
North COLD
South WARM
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What is the pressure at point A?
400mb
500mb
A
B
400mb
500mb
POLAR FRONT
North COLD
South WARM
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The pressure at point A is less than 400mb, since
it is higher than the 400mb isobar on this plot.
Lets estimate the pressure as 300mb
400mb
500mb
A
B
300mb
400mb
500mb
POLAR FRONT
North COLD
South WARM
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What is the pressure at point B?
400mb
500mb
A
B
300mb
400mb
500mb
POLAR FRONT
North COLD
South WARM
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The pressure at point B is more than 500mb, since
it is lower than the 500mb isobar on this plot.
Lets estimate the pressure as 600mb
400mb
500mb
A
B
300mb
600mb
400mb
500mb
POLAR FRONT
North COLD
South WARM
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The pressure gradient force between point B A
is HUGE Therefore, all along the polar front,
there will be a strong pressure gradient force
aloft, pushing northward
400mb
PGF
500mb
A
B
300mb
600mb
400mb
500mb
POLAR FRONT
North COLD
South WARM
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• Strong PGF is
• Aloft (above the surface)
• Above the Polar Front (strong temperature
  gradient!)
• PGF pushes to the north (in the Northern
  Hemisphere)
• How does this cause the midlatitude jet stream?

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Midlatitude Jet Stream

• Suppose we have a polar front at the surface
• This purple line is the polar front at the
  surface
• As well learn, this is NOT how fronts are
  correctly drawn, but it will work for now

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Midlatitude Jet Stream

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

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Midlatitude Jet Stream

• Winds aloft are in geostrophic balance

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Midlatitude Jet Stream

• So the wind will be accelerated North by the PGF,
  then turned to the East by the Coriolis effect
• The true wind will be a WESTERLY wind, directly
  above the polar front

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Midlatitude Jet Stream
The same diagram from a different angle

• Here is the polar front at the surface

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Midlatitude Jet Stream

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

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Midlatitude Jet Stream

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

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Midlatitude Jet Stream

• The (Northern Hemisphere) 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 same relationship exists near ANY front
  (temperature gradient) known as the THERMAL WIND
  RELATIONSHIP

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Large temperature gradients at the surface
correspond to strong winds aloft!
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Large temperature gradients at the surface
correspond to strong winds aloft!
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Thermal Wind

• Upper-level winds will be much stronger than
  low-level winds (i.e. thermal wind will be very
  close to upper-level wind)
• Equal to the SHEAR of the geostrophic wind (i.e.
  change of geostrophic wind with height)
• Not an actual wind
• Stronger temperature gradients imply stronger
  thermal wind
• Blows along thickness contours with (low
  thickness) air to the left

Thermal Wind
Lower Level Geostrophic Wind
Upper level geostrophic wind
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Thermal Wind
VT
Upper level geostrophic wind
Lower Level Geostrophic Wind
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Thermal Wind
COLD
5540 m
VT
5600 m
Upper level geostrophic wind
5660 m
Lower Level Geostrophic Wind
WARM
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Backing Veering
If winds rotate clockwise from lower level to
upper-level ? veering!
Thermal Wind
Lower level Geostrophic winds
Upper Level Geostrophic wind
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Backing Veering
If winds rotate clockwise from lower level to
upper-level ? veering!
If winds rotate counter-clockwise with height ?
backing!
Upper Level Geostrophic Wind
Thermal Wind
Lower level Geostrophic winds
Lower Level Geostrophic Wind
Upper Level Geostrophic wind
Thermal Wind
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Backing Veering
If winds rotate clockwise from lower level to
upper-level ? veering!
If winds rotate counter-clockwise with height ?
backing!
Upper Level Geostrophic Wind
Thermal Wind
Lower level Geostrophic winds
Lower Level Geostrophic Wind
Upper Level Geostrophic wind
Thermal Wind
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Backing Veering
If winds rotate clockwise from lower level to
upper-level ? veering!
If winds rotate counter-clockwise with height ?
backing!
Upper Level Geostrophic Wind
Thermal Wind
Lower level Geostrophic winds
Lower Level Geostrophic Wind
Upper Level Geostrophic wind
Thermal Wind
Warm Air Advection!
Cold Air Advection!
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Some terminology.
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Fronts Symbols(point in direction of front
movement)
COLD
WARM
OCCLUDED
STATIONARY
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Warm Front
WARM
COOL

• Gradual Slope
• Stratiform rain
• Long lasting light rain
• Occurs on cool side of front
• Temperature increases prior to frontal passage
• Wind becomes southerly after passage

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Cold Front
WARM
COOL

• Much Steeper Slope
• More intense (convective) rain
• Thunderstorms for a shorter period
• occurs on warm side of front
• Temperature decreases after frontal passage
• Wind becomes northerly after passage

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Frontal-Cyclone Structure
Cool Air
L
Lighter Rain
Cold Air
Warm Air
Heavier Rain
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Finding a Front

• Temperature (Dewpoint) Gradient
• Change in wind direction
• Converging winds at the front
• Kink or trough in isobars (lower pressure)
• Banded precipitation

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Upper-level terminology

• Trough area of lower heights
• Ridge area of higher heights

L
H
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Geopotential Height
The height of a pressure surface above ground is
analogous to the pressure. As an example, a low
height of the 500 mb surface is analogous to
lower pressure. This will be very important when
we analyze upper tropospheric data.
Figure A 3-dimensional representation of the
height of the 500 mb surface (in meters)
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Hydrostatic Balance

• Pressure decreases with height
• Thus we have a vertical pressure gradient force
  pointing...Upward!
• This is counteracted by the gravitational force,
  which points downward
• This balance is called Hydrostatic Balance
• Often we take this to be the case in our
  atmosphere