Transcript Horizontal Motion (Con`t)

19-1
Atmospheric Motions & Climate
Horizontal Atmospheric Motion
Geostrophic Wind Approximation
Science Concepts
Newton’s Laws of Motion
Horizontal Forces
Pressure Gradient
Force
Coriolis Force
Friction Force
High and Low Pressure System Winds
Boundary Layer
Convergence-Divergence
The Earth System (Kump, Kastin & Crane)
•
Chap. 4 (pp. 57-63, Fig. 4-13)
Climate and Global Change Notes
19-2
Atmospheric Motions
Of what is this a picture?
http://www-misr.jpl.nasa.gov/gallery/galhistory/2000_dec_06.html
Climate and Global Change Notes
19-3
Atmospheric Motions
Note similar patterns between Bermuda grass
colorations and clouds in the lower right-hand corner?
Climate and Global Change Notes
19-4
Atmospheric Motions
Forces
•
Forces are a vector quantity - they have a direction as well as a magnitude.
Newton's Laws
•
First Law
-
•
An object's velocity (direction or speed) will remain unchanged unless
acted upon by a force.
Second Law
-
Object's acceleration =
-
Acceleration is defined as the change in velocity with time and it also is
a vector
a
>
=
net force
object's mass
(Change in velocity)
(Change in time)
=
( V2 - V1 )
( t 2 - t1 )
Change in velocity can be change in direction or speed or both
Climate and Global Change Notes
19-5
Atmospheric Motions
Horizontal Motion
•
Forces and accelerations
-
Pressure gradient force (PGF)
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Coriolis force (CF)
-
Friction force (FF)
-
Thus,
Acceleration on a parcel = ( PGF + CF + FF ) / Mass of parcel
•
Pressure gradient force (PGF)
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Gradient is defined as the change of a quantity with change of location,
i.e., the pressure gradient is the change in pressure with change in
position.
Climate and Global Change Notes
19-6
Atmospheric Motions
Horizontal Motion (Con’t)
•
Pressure gradient force (PGF) (Con’t)
PG =
(Change in pressure) =
(Change in location)
1020 mb
High
Pressure
Location 1
L1
( p2 - p1 )
( L2 - L1 )
Pressure
Gradient
Force
1016 mb
L2
Low
Pressure
Location 2
Isobars - Lines of equal pressure
Pressure gradient force is the force exerted on an air parcel by the pressure
gradient, i.e., the force which causes air to accelerate from an area of high
pressure (too much mass) toward an area of low pressure (too little mass) in
an attempt to reduce the pressure gradient.
Climate and Global Change Notes
19-7
Atmospheric Motions
Horizontal Motion (Con’t)
•
Pressure gradient force (PGF) (Con’t)
-
Example
<
Note PGF is larger where isobars are closely packed and smaller
where isobars are spaced further apart
Pressure
Gradient
Force
1016
Isobars
1020
1024
H
Note: The Pressure
Gradient Force is largest
where the Pressure
Gradient is largest.
Climate and Global Change Notes
19-8
Atmospheric Motions
Horizontal Motion (Con’t)
•
Coriolis Force
-
Example: Merry-Go-Round and Ball
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml
Climate and Global Change Notes
19-9
Atmospheric Motions
Horizontal Motion (Con’t)
•
Coriolis Force
-
Example: Merry-Go-Round and Ball
QuickTime™ and a
Video decompressor
are needed to see this picture.
Climate and Global Change Notes
19-10
Atmospheric Motions
Horizontal Motion (Con’t)
•
Coriolis Force
-
Example: Fire missile with speed of 1 mile per second from the North
Pole toward New York City. Flight would take 55 minutes.
Climate and Global Change Notes
19-11
Atmospheric Motions
Horizontal Motion (Con’t)
•
Coriolis Force Example (Con’t)
-
During that 55 minutes the Earth would have turned about 15°.
NP
CHI NY
60
30
150
120
90
Climate and Global Change Notes
19-12
Atmospheric Motions
Horizontal Motion (Con’t)
•
Coriolis Force (Con’t)
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How fast is a spot on the Equator moving?
Speed = Distance / Time
NP
= Circumference of Earth
at the Equator / Time
= ( 2 • π • 6,378.4 km ) / 24 h
= ( 2 • 3.14159 • 6,378.4 km ) / 24 h
= 1,670 km / h
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How fast is a spot with half the radius (60° Latitude) moving?
Speed = ( 2 • π • 3,189.2 km ) / 24 h
= 835 km / h
Climate and Global Change Notes
19-13
Atmospheric Motions
Horizontal Motion (Con’t)
•
Coriolis Force (Con’t)
-
How fast is a spot with a 1 km radius
moving?
Speed = ( 2 • π • 1 km ) / 24 h
NP
= 0.26 km / h
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Object moving from the Equator toward the
Pole is moving faster than the surface and
thus,appears to turn eastward
-
Object moving from the Poles toward the
Equator is moving slower than the surface and thus, appears to turn
westward
Climate and Global Change Notes
19-14
Atmospheric Motions
Horizontal Motion (Con’t)
•
•
Coriolis Force (Con’t)
-
Apparent force resulting from the Earth’s rotation
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Causes objects to deflect to the right of their direction of motion in the
Northern Hemisphere
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Zero for objects at rest, increasing as an object’s velocity increases
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Zero for objects located at the equator, increasing as the object moves
toward either pole
Geostrophic Wind Approximation
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Geostrophic - Earth turning
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Balance of the PGF and the Coriolis Force
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Approximation to the “real” wind - have neglected friction
-
Approximation improves as one proceeds upward out of the boundary
layer where friction is most important
Climate and Global Change Notes
19-15
Atmospheric Motions
Horizontal Motion (Con’t)
•
Geostrophic Wind Approximation (Con’t)
PGF
PGF
996 mb
PGF
1000 mb
PGF
V
V
PGF
V
CF
V
CF
1004 mb
CF
CF
1008 mb
Balance of the Pressure Gradient Force (PGF) and the Coriolis Force (CF)
Climate and Global Change Notes
19-16
Atmospheric Motions
Horizontal Motion (Con’t)
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Geostrophic Wind Circulations
Anticyclone
-
Cyclone
Implications
> In the Northern Hemisphere and at levels where friction is not a
dominate force, wind blows clockwise (in the anticyclonic direction)
with lower pressure to the left, around High pressure areas
>
In the Northern Hemisphere and at levels where friction is not a
dominate force, wind blows counterclockwise (in the cyclonic
direction) with lower pressure to the left, around Low pressure
areas
Climate and Global Change Notes
19-17
Atmospheric Motions
Horizontal Motion (Con’t)
•
Friction force
-
Always acts in the opposite direction to the velocity
-
Two types
-
>
Mechanical (similar to forced convection)
>
Thermal (similar to free convection)
Important within the boundary or well-mixed layer
Climate and Global Change Notes
19-18
Atmospheric Motions
Horizontal Motion (Con’t)
Friction force (Con’t)
-
Important within the boundary or well-mixed layer (Con’t)
>
Depth of the mixed layer is affected by
‡
‡
‡
Surface heating (more heating deeper layer)
Wind speed (higher wind speed deeper layer)
600
Height (m)
•
Strong
Mixing
Weak
Mixing
400
200
Surface roughness or terrain
(rougher surface - deeper layer)
0
5
10
Wind Speed (kt)
Climate and Global Change Notes
19-19
Atmospheric Motions
Horizontal Motion (Con’t)
•
Wind approximation with friction
PGF
PGF
996 mb
1000 mb
V
FF
1004 mb
CF
1008 mb
Friction slows the wind speed which reduces the CF thus
allowing the PGF to pull harder toward Low pressure than
the CF pulls to the right.
Climate and Global Change Notes
19-20
Atmospheric Motions
Horizontal Motion (Con’t)
•
Wind Circulations with Friction
-
Implications
>
In the Northern Hemisphere near the surface, wind blows clockwise
and slightly out of High pressure areas
>
In the Northern Hemisphere near the surface, wind blows
counterclockwise and slightly into Low pressure areas
Climate and Global Change Notes
19-21
Atmospheric Motions
Horizontal Motion (Con’t)
•
Circulations in the Northern and Southern Hemispheres
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Coriolis force is to the left in the Southern Hemisphere
>
>
In the Southern Hemisphere near the surface, wind blows
counterclockwise and slightly out of High pressure areas
In the Southern Hemisphere near the surface, wind blows
clockwise and slightly into Low pressure areas
Northern Hemisphere Low
http://earthobservatory.nasa.gov/NaturalHazards/
natural_hazards_v2.php3?img_id=10657
Southern Hemisphere Low
http://earthobservatory.nasa.gov/NaturalHazards/
natural_hazards_v2.php3?img_id=2108
Climate and Global Change Notes
19-22
Atmospheric Motions
Horizontal Motion (Con’t)
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Buys-Ballot Rule
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•
http://earthobservatory.nasa.gov/
Study/NAO/NAO_2.html
When one has his or her back to the
wind, low pressure will be to the left
and slightly ahead.
Convergence - Divergence
Divergence out of the center of the
High causes downward motion that
warms the air and decreases its
Relative Humidity
H
Convergence into the center of
the Low causes upward motion
that cools the air and increases its
Relative Humidity
L
Climate and Global Change Notes