atmospheric and oceanic circulations (continued) chapter 6
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Atmospheric and Oceanic Circulations (continued) Chapter 6. Lecture 14 4 February 2005. Figure Credit: “Earth’s Climate” by W. Ruddiman. Figure Credit: “Earth’s Climate” by W. Ruddiman. Credit: www.physicalgeography.net. Wind. - PowerPoint PPT PresentationTRANSCRIPT
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Lecture 144 February 2005
Atmospheric and Oceanic Circulations(continued)
Chapter 6
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Figure Credit: “Earth’s Climate” by W.
Ruddiman
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Figure Credit: “Earth’s Climate” by W. Ruddiman
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Credit: www.physicalgeography.net
Wind simply put, wind is the horizontal flow of air
in response to differences in air pressurethese pressure differences are usually
due to uneven solar heating at the surface
wind flows because of
pressure gradient
‘heat rises’
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Wind
winds are designated as direction fromnot direction to
(oceanographers do it the opposite)
wind compass
so, a westerly wind would be coming from what angular
direction?
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Four forces that determine winds
1. Gravity - pulls gas molecules close to Earth density & pressure decrease with height
2. Pressure gradient force - the difference in air pressure between
areas3. Coriolis force - deflects wind from a
straight line to the right or left depending on hemisphere
4. Friction force - the drag on air flow from the Earth’s surface
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Pressure vs. Pressure Gradient
•The value of pressure itself is NOT
important
•The CHANGE in pressure over DISTANCE
is
•Change over distance is a GRADIENT
•The GRADIENT in pressure gives winds &
ocean currents their “push”
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Pressure Gradient Force (PGF)
isobar - a line of equal pressure (analogous to
isotherm)
gradient is 16 mb(note the closer isobars)
the PGF acts at right (90º) angles to the isobars
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Pressure Gradient Force
note the 1008 mb isobar
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Wind speed = Const * Pressure Gradient
Here, a 4x increase in PGF corresponds to a 4x increase in wind speed
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Pressure Gradient Force and Isobars
if there were no other forces acting on wind, it would flow in straight lines (perpendicular to isobars) from high to low pressure zones
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Coriolis Force (just the facts)
• Rotation of Earth acts to deflect any motion
from a straight line
• Deflection is to right (NH) to the left (SH)
• Coriolis “force” act on a right angle to the
motion
• Coriolis Force is NOT a real “force” but is
caused by viewing motion on a rotating
planet
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Coriolis Force
• Show the merry-go-round video
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the amount of rotation about a vertical axis
(’spinning’) is maximum at the poles and minimum at
the equator
Figure Credit: “Earth’s Climate” by W.
Ruddiman
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Earth’s Rotationevery point on earth rotates around a central
axis at 15 degrees/hour
LatitudeSpeed of
rotation (mph)0˚ 1041
30˚ 902
50˚ 670
60˚ 521
90˚ 0
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Coriolis Forcean object with an initial east-west velocity will maintain that velocity,
even as it passes over
surfaces with different velocities
as a result, it appears to be deflected over that surface (right in NH, left in SH)
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Coriolis Force and Deflection of Flight Path
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Coriolis Force and Deflection of Flight Path
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Coriolis Force and Flight Paths II.
Airplane animation
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Figure Credit: “Earth’s Climate” by W.
Ruddiman
The Coriolis Force affects air flow in response to pressure gradients in the
atmosphere
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geostrophic winds - PGF and Coriolis forces are opposite and balanced
Credit: www.physicalgeography.net
in the northern hemisphere (upper troposphere), the CF deflects the wind to
the right until wind flows parallel to isobars
~7km
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Geostrophic Winds
Balance between Pressure Gradient & Coriolis
Forces
Flow along isobars not across
Works for upper atmosphere winds & ocean
currents
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500 mb Pressure Map
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PGF, CF & isobars in upper troposphereisobars
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Friction Force
surface friction reduces wind speed and reduces the Coriolis force (remember CF
increases with wind speed)because of this, it causes winds to
move across isobars at an angle
the friction force operates only in the bottom 0.5-1 km of the atmosphere,
and it acts opposite to the direction of motion
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Figure Credit: “Earth’s Climate” by W.
Ruddiman
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PGF + Coriolis + Friction Forces
isobars
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The inter-tropical convergence zone (ITCZ)solar heating in the tropics expands air and
decreases its density - leading to increased buoyancy
How would this change the average molecular weight of air?
average molecular weight of air is ~29
g/mol
average density of air is 1.3 kg/m^3
what happens to air density if
you add water vapor?
It also gets more humid (adding water vapor)
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Convection on your Stove
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Convection on Earth
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as this air rises, it cools and water condenses out, leading to intense
precipitation
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A satellite (GOES) view of the ITCZ over the eastern Pacific
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the position of the ITCZ tracks the sun (it is found in the summer hemisphere) - the
location of the ITCZ determines the rainy season in many tropical countries, especially
those in Africa
the horizontal winds within the ITCZ are calm - the doldrums
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The C in ITCZ
the intense uplift of air creates horizontal pressure gradients at the surface
Credit: NASA JPL
as a result, winds converge towards the equator from both hemispheres
what about the complete cycle - where does the uplifted air go?
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Equator-to-pole cross section of circulation
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Hadley cell circulation
this circulation refers to the complete circulation of rising air in the tropics,
descending air over 30 °N and °S, and trade winds converging at the equator
the descending branch of the Hadley circulation brings hot, dry air to the surface -
leading to high pressure areas and suppressed precipitation
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Subtropical high-pressure cellsthese cells occur where the tropical air
descends in either hemisphere
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Figure
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Figure
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Figure Credit: physicalgeography.net
Monsoon Circulation
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Asian monsoo
nintense, dry winds flow from the Asian interior in response
to the gradient between the
continental high pressure and the equatorial (ITCZ)
low pressure
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Asian monsoo
nin summer, the subsolar point
and the ITCZ shift northward,
reversing the pressure gradient
- as the winds flow over the
Indian ocean they gain moisture
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Daytime land-sea breeze
results from differential heating of land and sea - not from radiation differences - but
from the different specific heats of land and water
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Nighttime land-sea breeze
at night, the land cools more rapidly than the sea and thus overlying air becomes more
dense and has a higher pressure