Transcript Atmospheric Forces

Atmospheric Forces
AOS 101 Discussion Sections 302 and 303
Why Does the Wind Blow?
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What makes the wind blow?
We need to think about Newton's Laws
1st Law
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An object at rest will remain at rest; an object in motion will
remain in motion as long as no force is exerted on the object.
2nd Law
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The total force exerted on an object is equal to the
acceleration of the object times its mass
Pressure Gradient Force
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Compels fluids to move from high pressure to lower
pressure
PGF and Wind
Isobars and PGF
PGF
Coriolis Force
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An apparent force
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Northern Hemisphere
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Results from the constant rotation
of the Earth
Acts at a 90°angle to the right of
the object in motion (such as the
wind)
This means that a wind from the
south would have a CF acting
toward the east
Imagine Dallas, TX fires a missile at Winnipeg, Manitoba…
Missile starts at Dallas, which is at a latitude of 37.28 N, rotates
with the Earth at a speed of 465.11 m/s.
Missile travels toward Winnipeg which, at a latitude of 52.00 N,
rotates with the Earth at a speed of 286.35 m/s
Geostrophic Balance
L
996 mb
X
1000 mb
1004 mb
H
Geostrophic Balance
L
996 mb
1000 mb
1004 mb
H
Pressure
Gradient Force
Geostrophic Balance
L
Pressure
Gradient Force
996 mb
1000 mb
Coriolis Force
1004 mb
H
Geostrophic Balance
L
Pressure
Gradient Force
996 mb
Geostrophic
Wind
1000 mb
Coriolis Force
1004 mb
H
Upper Level Flow
PGF
CF
Geopotential Height
Geopotential Height
PGF/ CF/ Centripetal
Friction Force
Friction
Wind
Friction Force
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This throws the wind out of geostrophic balance
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There is now a net force acting on the wind in the direction
opposite its motion
PGF
FR
Wind
CF
Friction Force
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Upper Level Wind
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Lower Level Wind
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Balance: PGF/ CF
Balance: PGF/ CF/ Friction
Friction causes wind to cross
isobars at ~30°angle at surface
Front Collapse Experiment
Front Collapse Experiment
Rotating Tank Experiment
Atmospheric Fronts
AOS 101 Discussion Sections 302 and 303
Warm Front
Warm Front
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Drawn as a red line with red semi-circles pointing in the
direction of the front’s movement
Cold Front
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Drawn as a blue line with blue triangles pointing in the
direction of the front’s movement
Stationary Front
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Stalled
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No movement of the
temperature gradient
Convergence of wind
Drawn as alternating
segments of red semicircles
(warm front) and blue
triangles (cold front) in
opposite directions
Occluded Front
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A region where a faster
moving cold front has
caught up to a slower
moving warm front.
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Generally occurs near the
end of the life of a cyclone
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Drawn with a purple line
with alternating semicircles
and triangles
Cold Occlusion
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The type most associated
with mid-latitude cyclones
Cold front "lifts" the warm
front up and over the very
cold air
Associated weather is similar
to a warm front as the
occluded front approaches
Once the front has passed,
the associated weather is
similar to a cold front
Vertical structure is often
difficult to observe
Warm Occlusion
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Cold air behind cold
front is not dense
enough to lift cold air
ahead of warm front
Cold front rides up
and over the warm
front
Upper-level cold front
reached station
before surface warm
occlusion
Fronts
Identifying Fronts
We know that we need to look for low pressure
and a boundary of cold and warm air.
To pinpoint the parts of our cyclone, look for
specifics in the observation maps
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Find the center of cyclonic rotation
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Find the large temperature gradients
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Identify regions of wind shifts
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Identify the type of temperature advection
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Look for kinks in the isobars