Climate and Global Change Notes 19-1 Atmospheric Motions & Climate Horizontal Atmospheric Motion...

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)


19-2

Atmospheric Motions

Of what is this a picture?

http://www-misr.jpl.nasa.gov/gallery/galhistory/2000_dec_06.html


19-3

Atmospheric Motions

Note similar patterns between Bermuda grass colorations and clouds in the lower right-hand corner?


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


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


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


19-7

Atmospheric Motions


• 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


19-8

Atmospheric Motions


• Coriolis Force

- Example: Merry-Go-Round and Ball

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml


19-9

Atmospheric Motions


• Coriolis Force

- Example: Merry-Go-Round and Ball

QuickTime™ and aVideo decompressor

are needed to see this picture.


19-10

Atmospheric Motions


• 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


19-11


• 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


19-12

Atmospheric Motions


• 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


19-13

Atmospheric Motions



- 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


19-14

Atmospheric Motions



- 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


19-15

Atmospheric Motions


• 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


19-16

Atmospheric Motions


• 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


19-17

Atmospheric Motions


• 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


19-18

Atmospheric Motions


• 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)


19-19

Atmospheric Motions


• 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.


19-20

Atmospheric Motions


• 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


19-21

Atmospheric Motions


• 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


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


• 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

Climate and Global Change Notes 19-1 Atmospheric Motions & Climate Horizontal Atmospheric Motion...

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