Elements of the Sun; Solar Radiation

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Chapter 6 General Circulation

• This chapter discusses
• Mean Sea Level Pressure and Winds
• Zonal winds
• Meridional Transport of Momentum, Heat and
  Moisture

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Thermal Circulations
Solar heating and radiational cooling of earth's
surface generates cold-core thermal highs and
warm-core thermal lows. Winds can circulate
between these two systems.
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Wind Profiles
Changes in air temperature causing warm air to
rise and cool air to sink can also generate
horizontal winds. Rising warm air creates a
surface low and upper level high. Sinking cool
air creates a surface high and upper level low.
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Forces Motion
Pressure forces are only one influence on the
movement of atmospheric air. Air responds
similarly as water to this force, moving from
higher pressure to lower pressure. Centripetal,
friction, and apparent Coriolis are other forces,
however, determining winds.
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Sea and Land Breezes
Land heats more quickly than water, creating
land-water temperature differences along a
coastline. During the day the land's warm-core
thermal low draws a sea breeze, while at night,
the warmer sea draws a land breeze.
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Variation in Height
Isobaric (constant pressure) surfaces rise and
fall in elevation with changes in air temperature
and density. A low 500 mb height indicates denser
air below, and less atmosphere and lower pressure
above. Contour lines indicate rates of pressure
change.
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Ridges Troughs
Upper level areas with high pressure are named
ridges, and areas with low pressure are named
troughs. These elongated changes in the pressure
map appear as undulating waves.
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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.
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Pressure Gradient Force
Change in pressure per change in distance
determines the magnitude of the pressure gradient
force (PGF). Greater pressure changes across
shorter distances creates a larger PGF to
initiate movement of winds.
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PGF vs. Cyclonic Winds
Pressure gradient force (PGF) winds acting alone
would head directly into low pressure. Surface
observations of winds, such as the cyclonic flow
around this low, reveal that PGF winds are
deflected by other forces.
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Apparent Coriolis Force
Earth's rotation transforms straight line motion
into curved motion for an outside viewer. The
Coriolis force explains this apparent curvature
of winds to the right due to rotation. Its
magnitude increases with wind velocity and
earth's latitude.
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Actual Observed Paths
Airplane travel paths have an apparent curvature,
just as Coriolis forces affect winds. Again, the
deflection between actual and observed paths is
greater near the poles.
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Geostrophic Wind
Winds have direction and magnitude, and can be
depicted by vectors. Observed wind vectors are
explained by balancing the pressure gradient
force and apparent Coriolis force. These upper
level geostrophic winds are parallel to pressure
contours.
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Wind Speed Pressure Contours
Just as a river speeds and slows when its banks
narrow and expand, geostrophic winds blowing
within pressure contours speed as contour
intervals narrow, and slow as contour intervals
widen.
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Isobars Wind Prediction
Upper level pressure maps, or isobars, enable
prediction of upper level wind direction and
speed.
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Northern Southern Hemisphere Flow
Winds blow counterclockwise around low pressure
systems in the Northern Hemisphere, but clockwise
around lows in the Southern Hemisphere.
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Friction Surface Winds
Surface objects create frictional resistance to
wind flow and slows the wind, diminishing the
Coriolis force and enhancing the effect of
pressure gradient forces. The result is surface
winds that cross isobars, blowing out from highs,
and in toward lows.
Figure 9.29A
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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.
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Three Cell Circulation Model
A rotating earth breaks the single cell into
three cells. The Hadley cell extends to the
subtropics, the reverse flow Ferrel cell extends
over the mid latitudes, and the Polar cell
extends over the poles. The Coriolis force
generates westerlies and NE trade winds, and the
polar front redistributes cold air.
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Observed Winds in January
Observed average global pressure and winds have
increased complexity due to continents and the
tilted earth. Differential ocean-land heating
creates areas of semi-permanent high and low
pressure that guide winds and redistribute heat.
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Observed Winds in June
Global pressure and wind dynamics shift as the
Northern Hemisphere tilts toward the sun,
bringing the inter-tropical convergence zone, the
Pacific high, and blocking highs in the southern
oceans northward.
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North American Winter Weather
Semi-permanent highs redirect North American
winds, such as cold interior northerly flow from
the Canadian high. The Polar front develops a
wave like pattern as air flows around lows.
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Global Precipitation Patterns
Global low pressure zones around the equator and
60 latitude generate convergence at the surface,
rising air and cloud formation. Zones of high
pressure at 30 and the Poles experience
convergence aloft with sinking, drying air.
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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.
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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.
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Mean Sea Level Pressure (January)
http//www.atmo.arizona.edu/students/courselinks/s
pring03/atmo421/sealevelpressure.html
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Mean Sea Level Pressure (July)
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Mean Sea Level Pressure and Winds
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Excerpt RIME OF THE ANCIENT MARINER Samuel
Taylor Coleridge The fair breeze blew, the white
foam flew, The furrow followed free We were
the first that ever burst Into that silent
sea Down dropt the breeze, the sails dropped
down, Twas sad as sad could be And we did speak
only to break The silence of the sea! All in
a hot and copper sky The bloody Sun, at
noon, Right up above the mast did stand, No
bigger than the moon. Day after day, day after
day, We stuck, nor breath nor motion As idle as
a painted ship Upon a painted ocean.
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Excerpt RIME OF THE ANCIENT MARINER Samuel
Taylor Coleridge The fair breeze blew, the white
foam flew, The furrow followed free (The
trades) We were the first that ever burst Into
that silent sea Down dropt the breeze, the sails
dropped down, Twas sad as sad could be And we
did speak only to break (The doldrums) The
silence of the sea! All in a hot and copper
sky The bloody Sun, at noon, (Vicinity of the
ITCZ) Right up above the mast did stand, No
bigger than the moon. Day after day, day after
day, We stuck, nor breath nor motion As idle as
a painted ship Upon a painted ocean.
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Meridional Zonal Flow
Wind direction and speed are indicated by lines,
barbs, and flags, and appear as an archer's
arrow. Upper level winds that travel a
north-south path are meridional, and those
traveling a west-east path are zonal.
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Jet Stream
High velocity Polar and subtropical jet stream
winds are located to break the tropopause, and
they oscillate along planetary ridges and troughs.
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Polar Jet Formation
Steep gradients of temperature change at the
Polar front trigger steep pressure gradients,
which then forces higher velocity geostrophic
winds. This is the trigger for jet stream flow.
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Winds Angular Momentum
Angular momentum is the product of mass,
velocity, and the radius of curvature and it must
be conserved. As northward-flowing air
experiences a smaller radius, it increases in
velocity and augments the jet stream flow.
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Zonal Mean Winds
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Convergence Divergence
Deepening of cyclones into explosive cyclogenesis
is prohibited when low pressure aloft is directly
above the surface low. In this scenario, the
convergence at the surface low builds up air
pressure and fills in the low. The same stacking
of high pressure, with divergence at the surface,
will weaken the anticyclone.
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Storm Vertical Structure
Divergence of air aloft occurs as isobars
intervals widen. Low pressure systems deepen and
intensify (e.g. cyclogenesis) when upper-level
divergence is stronger than the surface
convergence, which requires a vertical staggering
of surface and upper lows.
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Summary of Cyclone Weather
Upper and surface maps illustrate the role of
convergence and divergence aloft, and the pattern
of clouds, precipitation, and temperatures on the
ground.
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Meridional Momentum Transport (DJF 2001)
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Meridional Momentum Transport (JJA 2001)
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Meridional Heat Transport (DJF 2001)
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Meridional Heat Transport (JJA 2001)
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Meridional Moisture Transport (DJF 2001)
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Meridional Moisture Transport (JJA 2001)
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Seasonally Changing Winds The Monsoon
Seasonal reversal of monsoon winds in southern
Asia is explained by continental temperature
shifts. Summer monsoon depressions of low
pressure and rains are enhanced by insolation,
latent heat of condensation, and jet stream
patterns. El Nino Southern Oscillation is also
known to affect monsoon intensity.
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Asian Monsoon Circulation
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African Monsoon Circulation
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Seasonal Wind Changes
Monsoon like winds develop in July across the
southwestern U.S. region. As the continental
interior heats and rises, humid Gulf air sweeps
in creating instability and thunderstorms.
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North American Monsoon Circulation