Atmospheric circulation 1

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Atmospheric circulation 1

• ITCZ
• Hadley Circulation
• Non-rotating Wind Patterns

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Day after day, day after day,We stuck, nor
breath nor motion As idle as a painted ship,
Upon a painted ocean.Water, water, every where,
And all the boards did shrink Water, water,
every where, Nor any drop to drink. Rime of
the Ancient Mariner by Samuel Taylor Coleridge
Or, put another way, they were stuck in the horse
latitudes
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Global radiation imbalance There is a distinct
decrease in received solar radiation with
increasing latitude (N or S). Recall that net
radiation is energy received minus energy
emitted. Figure 10 illustrates the approximate
distribution of received solar and net emitted
infrared versus latitude. This assumes a spatial
average over an entire year for each latitude
band.
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Radiation imbalance

• In the tropics, the solar radiation input exceeds
  the terrestrial infrared radiation losses. Put
  another way, there is a surplus of energy or
  positive net radiation.
• Recall that net radiation may be used for latent
  heat or sensible heat
• The middle and high latitudes lose more infrared
  than they receive from solar. This deficit
  results in a net loss of radiation.

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Radiation imbalance

• The cross over points lie at about 38 N and 38 S.
  
• So a majority of the Earths surface suffers a
  net loss of radiation, while the tropics are the
  continued beneficiary of a surplus.
• If no other processes were available to mediate
  this situation, the tropics would grow incredibly
  hot while the rest of the planet would experience
  extreme cold.
• Of course, this does not occur. Such a gradient
  in temperature cannot exist, as heat will flow in
  response. There must exist mechanisms to
  transport heat poleward from tropical regions.

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Idealized earth

• The Earth is known to rotate
• That said it will serve our purposes here to
  consider the Earth as a non-rotating body
• This will enable us to conceptualize the basic
  motions of the atmospheric circulation

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Radiation surplus/deficit

• What does it mean to have a surplus or deficit of
  radiation?
• A surplus indicates that there is excess energy
  available with which to evaporate water and/or to
  warm water and land
• In the tropics, which are mostly water, the vast
  majority of the radiation surplus is used to
  evaporate water
• The energy used to evaporate the water is not
  lost, but stored in a potential state as latent
  heat. When it cools, the latent heat of
  condensation will release heat

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Persistent tropical low pressure

• Recall that PVnRT
• Rearranged P(nRT)/V
• In the case of the tropics, the volume of an air
  parcel increases, reducing pressure
• Evaporation of water to water vapor, which is
  lighter than N2 and O2,also acts to reduce the
  pressure in the tropics
• Consequently, equatorial latitudes generally
  experience low pressure conditions

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Horizontal wind movement

• Surface winds will move from areas of high
  pressure to areas of low pressure
• Winds thus tend to converge at the equator
• The converging winds then rise forcefully,
  cooling and condensing.
• Condensation produces more heat and more rising
• Eventually the air dries out and/or hits the
  Tropopause
• Here, they cease rising and begin to diverge

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The rising air is replaced by air from the north
or south. The zone of convergence is near the
equator but varies seasonally. Summer ITCZ north
of equator. Winter ITCZ south of equator. The
zones of divergence are at about 30N and 30S and
coincide with major deserts.
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Causes of subsidence

• Subsidence at 30N and 30S at the outer reaches of
  the Hadley cells occurs for three reasons
• The same mass of air is being forced into a
  smaller volume. This increases density
• Radiative cooling cools the air, increasing
  density
• Drying of the air decreases the amount of lighter
  water vapor, increasing density

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Polar forcing

• On the other hand, the cold air at the poles,
  especially in winter, is extremely dense and is
  characterized by high pressure
• Naturally, as in all cases of pressure
  differentials, the flow is from areas of high
  pressure to areas of low pressure
• This means there must be southerly winds from the
  north pole and northerly winds from the south pole

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Wind does in fact flow from the poles. Globally,
this means that there are three cells Hadley
cells from the equator to 30NS Ferrel Cells
from 30-60 and the polar cell from 60-90. These
are extremely generalized values and are subject
to great variability.
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Hadley circulation and subsidence at 30N and 30S
explains the presence of major global subsidence
deserts.