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- Basic Structures and Dynamics
- General Circulation in the
Troposphere
- General Circulation in the
Stratosphere
- Jetstreams
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- Weight = mass x gravity
- Density = mass / volume
- Pressure = force / area
- = weight / area
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- Pascal (Pa): a SI (Systeme
Internationale) unit for air pressure.
- 1 Pa = a force of 1 newton
acting on a surface of one square
- meter
- 1 hectopascal (hPa) = 1
millibar (mb) [hecto = one
hundred =100]
- Bar: a more popular unit for air pressure.
- 1 bar = a force of 100,000
newtons acting on a surface of one
- square meter
- = 100,000 Pa
- = 1,000 hPa
- = 1,000 mb
- One atmospheric pressure = standard value of atmospheric pressure at lea
level = 1013.25 mb = 1013.25 hPa.
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- Atmospheric pressure tells you how much atmospheric mass is above a
particular altitude.
- Atmospheric pressure decreases by
about 10mb for every 100 meters increase in elevation.
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- Since P= rRT (the
ideal gas law), the hydrostatic equation becomes:
- dP = -P/RT x gdz
- č dP/P = -g/RT x dz
- P = Ps exp(-gz/RT)
- P = Ps exp(-z/H)
- The atmospheric pressure decreases exponentially with height
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- PG = (pressure difference) /
distance
- Pressure gradient force goes from high pressure to low pressure.
- Closely spaced isobars on a
weather map indicate steep pressure gradient.
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- The Coriolis force also causes the east-west wind to deflect to the
right of its intent path in the Northern Hemisphere and to the left in
the Southern Hemisphere.
- The deflections are caused by the centrifugal force associated with the
east-west motion, and , therefore, related to rotation of the Earth, and
are also considered as a kind of Coriolis force.
- Although the description of the deflection effect for north-south and
east-west motions are very different, their mathematical expressions are
the same.
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- Coriolis force causes the wind to
deflect to the right of its intent path in the Northern Hemisphere and
to the left in the Southern Hemisphere.
- The magnitude of Coriolis force depends on (1) the rotation of the
Earth, (2) the speed of the moving object, and (3) its latitudinal location.
- The larger the speed (such as wind speed), the stronger the Coriolis
force.
- The higher the latitude, the stronger the Coriolis force.
- The Corioils force is zero at the equator.
- Coriolis force is one major factor that determine weather pattern.
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- Friction Force = c * V
- c = friction coefficient
- V = wind speed
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- Thermally Direct Cells (Hadley and Polar Cells)
- Both cells have their rising
branches over warm temperature zones and sinking braches over the cold
temperature zone. Both cells directly convert thermal energy to kinetic
energy.
- Thermally Indirect Cell (Ferrel Cell)
- This cell rises over cold
temperature zone and sinks over warm temperature zone. The cell is not
driven by thermal forcing but driven by eddy (weather systems) forcing.
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- Yes and No!
- (Due to sea-land contrast and
topography)
- Yes: the three-cell model
explains reasonably well the surface wind distribution in the
atmosphere.
- No: the three-cell model can
not explain the circulation pattern in the upper troposphere. (planetary
wave motions are important here.)
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- The Aleutian, Icelandic, and
Tibetan lows
- The oceanic (continental) lows achieve maximum strength during winter
(summer) months
- The summertime Tibetan low is important to the east-Asia monsoon
- Siberian, Hawaiian, and
Bermuda-Azores highs
- The oceanic (continental) highs achieve maximum strength during summer
(winter) months
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- ¶U/¶z µ - ¶T/¶y
- The vertical shear of zonal wind
is related to the latitudinal gradient of temperature.
- Jet streams usually are formed
above baroclinic zone (such as the polar front).
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- Temperature differences between the equator and poles
- The rate of rotation of the Earth.
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- Carl Rossby mathematically expressed relationships between mid-latitude
cyclones and the upper air during WWII.
- Mid-latitude cyclones are a large-scale waves (now called Rossby waves)
that grow from the “baroclinic” instabiloity associated with the
north-south temperature differences in middle latitudes.
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- Bjerknes, the founder of the
Bergen school of meteorology, developed polar front theory during WWI to
describe the formation, growth, and dissipation of mid-latitude
cyclones.
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- Cyclogenesis
- Mature Cyclone
- Occlusion
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- The hurricane is characterized by a strong thermally direct circulation
with the rising of warm air near the center of the storm and the sinking
of cooler air outside.
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- Hurricanes: extreme tropical storms over Atlantic and eastern Pacific
Oceans.
- Typhoons: extreme tropical storms over western Pacific Ocean.
- Cyclones: extreme tropical storms over Indian Ocean and Australia.
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- The Vostok ice record shows a series of cyclic variations in methane
concentration, ranging between 350 to 700 ppb (part per billion).
- Each CH4 cycle takes about 23,000 years.
- This cycle length points to a likely connection with changes in orbital
procession.
- The orbital procession dominates insolation changes at lower latitudes.
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- Air moves freely through snow and ice in the upper 15 m of an ice sheet.
- Flow is increasingly restricted below this level.
- Bubbles of old air are eventually sealed off completely in ice 50 to 100
m below the surface.
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- On the 23,000-year cycle, methane variations closely resemble the
variations of monsoon strength.
- The peak values of methane match the expected peaks in monsoon intensity
not only in timing but also in amplitude.
- This match suggests a close connection between CH4 concentrations and
the monsoon on the 23,000-year climate cycle.
- By why?
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- First, Earth spins around on its axis once every day č The Tilt.
- Second, Earth revolves around the
Sun once a year č
The shape of the Orbit.
- Both the tilt and the shape of the orbit have changed over time and
produce three types of orbital variations:
- (1) obliquity variations
- (2) eccentricity variations
- (3) precession of the spin
axis.
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- There are two kinds of precession: (1) the precession of the spin axis
and (2) the precession of the ellipse.
- Earth’s wobbling motion is called the axial precession. It is caused by the gravitational pull
of the Sun and Moon.
- Axial precession is a slow turning of Earth;s axis of rotation through a
circular path, with a full turn every 25,700 years.
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- The precession of the ellipse is known as the elliptical shape of
Earth’s orbit rotates itself at a slower rate than the wobbling motion
of the axial precession.
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- The combined effects of these two precessions cause the solstices and
equinoxes to move around Earth’s orbit, completing one full 360° orbit
around the Sun every 23,000 years.
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- The 23,000-year cycle of orbital procession increases (decreases) summer
insolation and at the same time decreases (increases) winter insolation
at low and middle latitudes.
- Departures from the modern seasonal cycle of solar radiation have driven
stronger monsoon circulation in the past.
- Greater summer radiation intensified the wet summer monsoon.
- Decreased winter insolation intensified the dry winter monsoon.
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- Orbital procession affects solar radiation at low latitudes
- č solar radiation affects the strength of
low-latitude monsoons
- č monsoon fluctuations changes the precipitation
amounts in Southeast Asia
- č heavy rainfalls increase the amount of standing
water in bogs
- č decaying vegetation used up any oxygen in the
water and creates the oxygen-free conditions needed to generate methane
- č the extent of these boggy area must have
expanded during wet monsoon maximum and shrunk during dry monsoon
minimum.
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- The greatest production of ozone occurs in the tropics, where the solar
UV flux is the highest.
- However, the general circulation in the stratosphere transport
ozone-rich air from the tropical upper stratosphere to mid-to-high
latitudes.
- Ozone column depths are highest during springtime at mid-to-high
latitudes.
- Ozone column depths are the lowest over the equator.
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- Quasi-Biennial Oscillation (QBO)
- Sudden Warming: in Northern Pole
- Ozone Hole: in Southern Pole
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- Every other year or so the normal
winter pattern of a cold polar stratosphere with a westerly vortex is
interrupted in the middle winter.
- The polar vortex can completely
disappear for a period of a few weeks.
- During the sudden warming period,
the stratospheric temperatures can rise as much as 40°K in a few days!
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- Planetary-scale waves propagating
from the troposphere (produced by big mountains) into the stratosphere.
- Those waves interact with the
polar vortex to break down the polar vortex.
- There are no big mountains in the
Southern Hemisphere to produce planetary-scale waves.
- Less (?) sudden warming in the
southern polar vortex.
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- The decrease in ozone near the South Pole is most striking near the
spring time (October).
- During the rest of the year, ozone levels have remained close to normal
in the region.
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- In winter the polar stratosphere is so cold (-80°C or below) that
certain trace atmospheric constituents can condense.
- These clouds are called “polar stratospheric clouds” (PSCs).
- The particles that form typically consist of a mixture of water and
nitric acid (HNO3).
- The PSCs alter the chemistry of the lower stratosphere in two ways:
- (1) by coupling between the
odd nitrogen and chlorine cycles
- (2) by providing surfaces on
which heterogeneous reactions can occur.
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- Long Antarctic winter (May through September)
- The stratosphere is cold enough to form PSCs
- PSCs deplete odd nitrogen (NO)
- Help convert unreactive forms of chlorine (ClONO2 and HCl) into more
reactive forms (such as Cl2).
- The reactive chlorine remains bound to the surface of clouds particles.
- Sunlight returns in springtime (September)
- The sunlight releases reactive chlorine from the particle surface.
- The chlorine destroy ozone in October.
- Ozone hole appears.
- At the end of winter, the polar vortex breaks down.
- Allow fresh ozone and odd nitrogen to be brought in from low latitudes.
- The ozone hole recovers (disappears) until next October.
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