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Climate and Global Change Notes
19-1
Atmospheric Motions & Climate
Horizontal Atmospheric Motion
Geostrophic Wind Approximation
High and Low Pressure System Winds
Boundary LayerConvergence-Divergence
Science Concepts
Newton’s Laws of MotionHorizontal Forces
Pressure Gradient Force
Coriolis ForceFriction Force
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 = net force object's mass
- Acceleration is defined as the change in velocity with time and it also is
a vector
a = (Change in velocity) = ( V2 - V1 )
(Change in time) ( t2 - t1 )
> Change in velocity can be change in direction or speed or both
Climate and Global Change Notes
19-5
Horizontal Motion
• Forces and accelerations
- Pressure gradient force (PGF)
- Coriolis force (CF)
- Friction force (FF)
- Thus,
Acceleration on a parcel = ( PGF + CF + FF ) / Mass of parcel
• Pressure gradient force (PGF)
- 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.
Atmospheric Motions
Climate and Global Change Notes
19-6
Atmospheric Motions
Horizontal Motion (Con’t)
• Pressure gradient force (PGF) (Con’t)
PG = (Change in pressure) =( p2 - p1 )
(Change in location) ( L2 - L1 )
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.
1020 mb 1016 mb
Isobars - Lines of equal pressure
L1 L2High
Pressure Low
Pressure
Location 1 Location 2
PressureGradientForce
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
Note: The Pressure Gradient Force is largest
where the PressureGradient is largest.
H
1024
1020
1016 IsobarsPressureGradientForce
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 aVideo 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.9060300120150NPNY
9060300120150NPNYDesired Trajectory
Climate and Global Change Notes
19-11
Horizontal Motion (Con’t)
• Coriolis Force Example (Con’t)
- During that 55 minutes the Earth would have turned about 15°.
Atmospheric Motions
9060300120150NPNY NP906030120150NY
90
60 30
120
150
NP
CHI NY
Climate and Global Change Notes
19-12
Atmospheric Motions
Horizontal Motion (Con’t)
• Coriolis Force (Con’t)
- How fast is a spot on the Equator moving?
Speed = Distance / Time
= 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
- How fast is a spot with half the radius (60° Latitude) moving?
Speed = ( 2 • π • 3,189.2 km ) / 24 h
= 835 km / h
N P
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
= 0.26 km / h
- 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
N P
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
- Causes objects to deflect to the right of their direction of motion in the
Northern Hemisphere
- Zero for objects at rest, increasing as an object’s velocity increases
- Zero for objects located at the equator, increasing as the object moves
toward either pole
• Geostrophic Wind Approximation
- Geostrophic - Earth turning
- Balance of the PGF and the Coriolis Force
- 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)
Balance of the Pressure Gradient Force (PGF) and the Coriolis Force (CF)
996 mb
1000 mb
1004 mb
1008 mb
PGF
PGF
CF
V
PGF
V
CF
PGF
V
CF
PGF
V
CF
Climate and Global Change Notes
19-16
Atmospheric Motions
Horizontal Motion (Con’t)
• Geostrophic Wind Circulations
- 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
Anticyclone Cyclone
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)
‡ Surface roughness or terrain(rougher surface - deeper layer)
Wind Speed (kt)0 5 10
200
600
400StrongMixing
WeakMixing
Height (m)
Climate and Global Change Notes
19-19
Atmospheric Motions
Horizontal Motion (Con’t)
• Wind approximation with friction
996 mb
1000 mb
1004 mb
1008 mb
PGF PGF
V
CF
FF
Friction slows the wind speed which reduces the CF thusallowing 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
- 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 Southern Hemisphere Low
http://earthobservatory.nasa.gov/NaturalHazards/natural_hazards_v2.php3?img_id=2108
http://earthobservatory.nasa.gov/NaturalHazards/natural_hazards_v2.php3?img_id=10657
Climate and Global Change Notes
19-22
H
Divergence out of the center of the High causes downward motion that warms the air and decreases its
Relative Humidity
Atmospheric Motions
Horizontal Motion (Con’t)
• Buys-Ballot Rule
- When one has his or her back to the wind, low pressure will be to the left and slightly ahead.
• Convergence - Divergence
Convergence into the center of the Low causes upward motion that cools the air and increases its
Relative Humidity
L
http://earthobservatory.nasa.gov/Study/NAO/NAO_2.html