Transcript Thickness and the Thermal Wind

General Circulation & Thermal Wind
AOS 101
Lecture 11
General Circulation
• What is the global picture?
• The average flow on the globe...
General Circulation: Hadley Cell
• Thermally-driven convection:
– Warm air rises and cold air sinks, creating circulation
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
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
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)
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!)
General Circulation: Jet Streams
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
Upper Tropospheric Pressure Surfaces
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)
Thickness...
• Start with a column of air.
• The base of this column is at
the surface, so lets say its
pressure is about 1000 mb
1000 mb
• The top of this column is
quite high—let’s say that
its pressure is 500 mb
500 mb
1000 mb
• This column has some
thickness: it is some distance
between 1000 mb and 500 mb
500 mb
1000 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
500 mb
1000 mb
Warmer
• 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
Thickness
• In fact, temperature is
the ONLY factor in the
atmosphere that
determines the thickness
of a layer
• It wouldn’t have
mattered which pressure
we had chosen. They
are all higher above the
ground when it is
warmer….
Thickness
• In fact, temperature is
the ONLY factor in the
atmosphere that
determines the thickness
of a layer
• 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
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
General 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
North
COLD
South
WARM
Cold air is coming from the north. This air comes from
the polar vortex 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
(a strong temperature gradient)
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
Above the front, thickness of atmosphere changes rapidly
400mb
500mb
400mb
500mb
POLAR
FRONT
North
COLD
South
WARM
Now, what about the PGF above the front?
400mb
500mb
400mb
500mb
POLAR
FRONT
North
COLD
South
WARM
Let’s 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
What is the pressure at point A?
400mb
500mb
A
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
500mb
A
300mb
B
400mb
500mb
POLAR
FRONT
North
COLD
South
WARM
What is the pressure at point B?
400mb
500mb
A
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
500mb
A
300mb
600mb
B
400mb
500mb
POLAR
FRONT
North
COLD
South
WARM
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
A
300mb
600mb
500mb
B
400mb
500mb
POLAR
FRONT
North
COLD
South
WARM
• 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?
Midlatitude Jet Stream
• 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
Midlatitude Jet Stream
• All along the
front, there is a
strong pressure
gradient force
pushing
northward
Midlatitude Jet Stream
• Winds aloft are in
geostrophic
balance…
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”
Midlatitude Jet Stream
The same diagram from a different angle
•Here is the polar
front at the surface
Midlatitude Jet Stream
•Remember, it’s a polar
front because it is where
warm air from the south
meets cold air from the
north.
Midlatitude Jet Stream
•The midlatitude jet
stream is found
directly above the
polar front.
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
Large temperature gradients at the
surface correspond to strong winds aloft!
Large temperature gradients at the
surface correspond to strong winds aloft!
Thermal Wind



VT  VUPPER  VLOWER
• 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)
Thermal Wind
• Not an actual wind
• Stronger temperature gradients imply
Lower Level
stronger thermal wind
Geostrophic Wind
• “Blows” along thickness contours with
Upper level
geostrophic wind
(low thickness) air to the left
Thermal Wind
VT
Upper level
geostrophic wind
Lower Level
Geostrophic Wind
Thermal Wind
COLD
VT
5600 m
Upper level
geostrophic wind
Lower Level
Geostrophic Wind
5540 m
WARM
5660 m
Backing & Veering
If winds rotate clockwise from lower
level to upper-level  veering!
Lower level
Geostrophic
winds
Thermal Wind
Upper Level
Geostrophic
wind
Backing & Veering
If winds rotate clockwise from lower If winds rotate counter-clockwise
level to upper-level  veering!
with height  backing!
Lower level
Geostrophic
winds
Thermal Wind
Upper Level
Geostrophic
wind
Lower Level
Geostrophic
Wind
Upper Level
Geostrophic
Wind
Thermal Wind
Backing & Veering
If winds rotate clockwise from lower If winds rotate counter-clockwise
level to upper-level  veering!
with height  backing!
Lower level
Geostrophic
winds
Thermal Wind
Upper Level
Geostrophic
wind
Lower Level
Geostrophic
Wind
Upper Level
Geostrophic
Wind
Thermal Wind
Backing & Veering
If winds rotate clockwise from lower If winds rotate counter-clockwise
level to upper-level  veering!
with height  backing!
Lower level
Geostrophic
winds
Thermal Wind
Upper Level
Geostrophic
wind
Warm Air Advection!
Lower Level
Geostrophic
Wind
Upper Level
Geostrophic
Wind
Thermal Wind
Cold Air Advection!