Transcript Circulation_Notes

Pressure and Winds
General Circulation of the Atmosphere
Geog 210 - Physical Geography
Geog 311 - Climatology
Global balance – local imbalances
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Is the earth system in radiative equilibrium? The diagram below shows the annual mean, averaged around
latitude circles, of the balance between the solar radiation absorbed at the ground (in blue) and the
outgoing infrared radiation from Earth into space (in red). The two curves generally balance over the
entire globe, but not at every single latitude. In the tropics, there is an excess of radiation (solar radiation
absorbed exceeds outgoing terrestrial radiation) in middle and high latitudes all the way to the poles,
there is a deficit (Earth is radiating into space more than it receives from the sun). The atmosphere and
ocean systems are forced to move about by this imbalance, and bring heat by convection and advection
from equator to the poles.
Source: Columbia University
The imbalance of radiative budget between the tropics and higher
latitudes is the main cause that drives weather and climate! The
ultimate energy source is, of course, the sun.
Global weather phenomena as observed by NOAA’s geostationary satellite
GOES-12 on Oct 7, 2003.
Whither the Wind?
Pressure Gradient
Coriolis Effect
Friction
•Pressure unevenness is the main cause of winds. This unevenness causes a pressure
gradient from areas of high pressure areas of low pressure. The pressure gradient force
causes winds to blow.
•In the absence of other forces, winds will blow from high pressure to low pressure.
•The unit of pressure is millibars (mb). The ‘mean’ sea-level pressure is 1013.25 mb, or
roughly 1000 mb.
Coriolis Force
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But there are other forces that may influence
winds. One of the most important is the
Coriolis force in honor of the French
physicist Coriolis.
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Coriolis force is not a “real” force but an
apparent force. It appears only because the
observer (i.e., people like you and I) is on a
rotating reference frame.
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Zero at equator; maximum at high latitudes
Coriolis Force
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The same phenomenon occurs for any
motion on earth due to the earth rotation.
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When we observe the movement of an air
parcel on earth, we find that the moment
the air parcel starts to move, it will be
subject to this force immediately. N
ET in
The deflection of the movement is such that
the parcel will be deflected to the right in
the Northern Hemisphere and to the left in
the Southern Hemisphere.
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Thus, the wind patterns are rotating in
opposite sense near high and low pressure
centers in Northern and Southern
Hemispheres, as shown in the figure to the
right.
Geostrophic Balance and Geostrophic Winds
Balance between the pressure gradient force and Coriolis force
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Will the wind direction forever turn to
right (left) in Northern (Southern)
Hemisphere? The answer is no. It will
keep turning until a balance between the
pressure gradient force and Coriolis
force is achieved.
This balance happens when the wind
direction becomes parallel to the isobars.
This is called the geostrophic balance.
The wind satisfying the geostrophic
balance condition is called the
geostrophic wind .
Geostrophic winds are very close to (but
not exactly the same as) the actually
observed winds.
Unisys Weather
http://weather.unisys.com/
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Datastreme Project
http://www.ametsoc.org/amsedu/dstreme/index.html
Northern Hemisphere
Friction
• Upper Atmosphere
Convergence and Divergence
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In the real world, there are more forces (such as friction and curvature effect)
involved in shaping wind direction and magnitude aside from the pressure gradient
and Coriolis forces. The final results are that winds tend to converge toward the
low pressure center and diverge outward from the high pressure center.
Scale - “swirls within swirls”
Sea Breeze
Land Breeze
Single-Cell Circulation Model
The basis for average
air flow around the
earth can be examined
using a non-rotating,
non-tilted, ocean
covered earth.
Heating is more
intense at the equator,
which triggers Hadley
cells to redistribute
rising heat from the
tropical low to the
polar highs.
3-Cell Model: simple!
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A simplified 3-cell model of
the general meridional
circulation of the
atmosphere.
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In general, descending air
has a drying effect while
ascending air causes
condensation.
The wind system on the earth surface
The earth is rotating, and rotation makes the wind system a bit more
complicated than implied by the 3-cell structure.
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In the northern hemisphere, the
tropical ( 0 – 30ºN) prevailing
winds are easterlies (NE winds)called trade winds. In the middle
latitudes (roughly 30 – 60ºN) the
prevailing winds are westerlies
(SW winds). The polar region’s
prevailing winds are easterlies.
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The wind system in the southern
hemisphere is like the mirror image
of the northern hemisphere.
Hadley Cell
Seasonal shifts
ITCZ Limits
Winter Season
Summer Season
Winter
Coastal Summer Weather
The semi-permanent Pacific high blocks moist maritime winds and rain
from the California coast, while the Bermuda high pushes moist tropical
air and humidity over the eastern states.
Coastal Winter Weather
During winter
months, the Pacific
high migrates
southward and
allows for maritime
winds with moisture
and rains to reach
California.
On the east coast,
precipitation is
rather even
throughout the year,
and moisture is
always from the Gulf
of Mexico and the
tropical Atlantic.
January Winds Aloft
Land-sea
temperature
differences trigger
ridges and troughs
in the isobaric
surface.
Whither the Westerlies?
Thermal wind is the most fundamental and significant dynamical balance controlling the
large-scale circulation of the atmosphere and ocean. It is a consequence of hydrostatic
and geostrophic balance, and relates horizontal buoyancy gradients to changes in the
horizontal wind with height.
“What goes up must come down,
spinning wheel got to go ‘round.”
Blood, Sweat and Tears
http://www.youtube.com/watch?v=8T97f2kBzOQ
Surface & 500 mb Maps
Surface maps chart
pressure contours, highs
and lows, and wind
direction.
Winds blow clockwise
around highs, called
anticyclones.
500 mb maps reveal
patterns that on average
are 5600 m above the
surface, where westerly
winds rise and fall across
ridges and troughs.
Climagraphs of U.S. Cities
http://drought.unl.edu/whatis/climographs.htm
World Climagraphs
http://cwx.prenhall.com/bookbind/pubbooks/lutgens3/m
edialib/abcontrol/pages/question.html
World Climates (Ritter)
Please go to the website below and look at the climagraphs for
locations all over the world. Study the controlling factors
http://southhill.vsb.bc.ca/Departments/Hum
anities/Geogpraphy/Kyle/Notes/2_Atmosph
ere/Climographs/WorldClimates.html
.