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Basic
Structures and Dynamics |
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General
Circulation in the Troposphere |
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General
Circulation in the Stratosphere |
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Jetstreams |
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Weight = mass x gravity |
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Density = mass / volume |
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Pressure = force / area |
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= weight / area |
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Pascal
(Pa): a SI (Systeme Internationale) unit for air pressure. |
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1 Pa
= a force of 1 newton acting on a surface of one square |
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meter |
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1
hectopascal (hPa) = 1 millibar (mb)
[hecto = one hundred =100] |
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Bar: a more popular unit for air pressure. |
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1
bar = a force of 100,000 newtons acting on a surface of one |
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square meter |
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= 100,000 Pa |
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= 1,000 hPa |
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= 1,000 mb |
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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. |
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Atmospheric pressure decreases by about 10mb for every 100
meters increase in elevation. |
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The
hydrostatic equation tells us how quickly air pressure drops wit height. |
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čThe rate at which air pressure decreases with
height (DP/ Dz) is equal to the air density (r) times the
acceleration of gravity (g) |
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An equation of state describes the relationship
among pressure, temperature, and density of any material. |
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All gases are found to follow approximately the
same equation of state, which is referred to as the “ideal gas law
(equation)”. |
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Atmospheric gases, whether considered
individually or as a mixture, obey the following ideal gas equation: |
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Since P= rRT (the ideal gas law), the hydrostatic
equation becomes: |
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dP = -P/RT x gdz |
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č
dP/P = -g/RT x dz |
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P
= Ps exp(-gz/RT) |
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P = Ps
exp(-z/H) |
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The atmospheric pressure decreases exponentially
with height |
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One way to measure how soon the air runs out in
the atmosphere is to calculate the scale height, which is about 10 km. |
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Over this vertical distance, air pressure and
density decrease by 37% of its surface values. |
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If pressure at the surface is 1 atmosphere, then
it is 0.37 atmospheres at a height of 10 km, 0.14 (0.37x0.37) at 20 km,
0.05 (0.37x0.37x0.37) at 30 km, and so on. |
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Different atmospheric gases have different
values of scale height. |
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PG =
(pressure difference) / distance |
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Pressure gradient force force goes from high
pressure to low pressure. |
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Closely
spaced isobars on a weather map indicate steep pressure gradient. |
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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. |
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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. |
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The stronger the speed (such as wind speed), the
stronger the Coriolis force. |
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The higher the latitude, the stronger the
Coriolis force. |
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The Corioils force is zero at the equator. |
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Coriolis force is one major factor that
determine weather pattern. |
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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. |
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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. |
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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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Friction
Force = c * V |
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c =
friction coefficient |
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V =
wind speed |
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The
three-way balance of horizontal pressure gradient, Coriolis force, and the
centrifugal force is call the gradient wind balance. |
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The
gradient wind is an excellent approximation to the actual wind observed
above the Earth’s surface, especially at
the middle latitudes. |
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Thermally Direct Cells (Hadley and Polar Cells) |
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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. |
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Thermally Indirect Cell (Ferrel Cell) |
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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! |
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(Due
to sea-land contrast and topography) |
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Yes: the
three-cell model explains reasonably well the surface wind distribution in
the atmosphere. |
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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 |
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The oceanic (continental) lows achieve maximum
strength during winter (summer) months |
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The summertime Tibetan low is important to the
east-Asia monsoon |
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Siberian,
Hawaiian, and Bermuda-Azores highs |
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The oceanic (continental) highs achieve maximum
strength during summer (winter) months |
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¶U/¶z µ ¶T/¶y |
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The
vertical shear of zonal wind is related to the latitudinal gradient of
temperature. |
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Jet
streams usually are formed above baroclinic zone (such as the polar front). |
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Temperature differences between the equator and
poles |
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The rate of rotation of the Earth. |
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Cyclogenesis |
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Mature
Cyclone |
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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. |
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Typhoons: extreme tropical storms over western
Pacific Ocean. |
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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). |
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Each CH4 cycle takes about 23,000 years. |
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This cycle length points to a likely connection
with changes in orbital procession. |
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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. |
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Flow is increasingly restricted below this
level. |
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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. |
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The peak values of methane match the expected
peaks in monsoon intensity not only in timing but also in amplitude. |
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This match suggests a close connection between
CH4 concentrations and the monsoon on the 23,000-year climate cycle. |
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By why? |
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First, Earth spins around on its axis once every
day č The Tilt. |
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Second,
Earth revolves around the Sun once a year č The
shape of the Orbit. |
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Both the tilt and the shape of the orbit have
changed over time and produce three types of orbital variations: |
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(1)
obliquity variations |
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(2)
eccentricity variations |
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(3)
precession of the spin axis. |
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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. |
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Departures from the modern seasonal cycle of
solar radiation have driven stronger monsoon circulation in the past. |
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Greater summer radiation intensified the wet
summer monsoon. |
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Decreased winter insolation intensified the dry
winter monsoon. |
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Orbital procession affects solar radiation at
low latitudes |
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č solar
radiation affects the strength of low-latitude monsoons |
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č monsoon
fluctuations changes the precipitation amounts in Southeast Asia |
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č heavy
rainfalls increase the amount of standing water in bogs |
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č decaying
vegetation used up any oxygen in the water and creates the oxygen-free
conditions needed to generate methane |
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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. |
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However, the general circulation in the
stratosphere transport ozone-rich air from the tropical upper stratosphere
to mid-to-high latitudes. |
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Ozone column depths are highest during
springtime at mid-to-high latitudes. |
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Ozone column depths are the lowest over the
equator. |
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Quasi-Biennial Oscillation (QBO) |
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Sudden
Warming: in Northern Pole |
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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. |
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The
polar vortex can completely disappear for a period of a few weeks. |
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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. |
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Those
waves interact with the polar vortex to break down the polar vortex. |
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There
are no big mountains in the Southern Hemisphere to produce planetary-scale
waves. |
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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). |
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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. |
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These clouds are called “polar stratospheric
clouds” (PSCs). |
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The particles that form typically consist of a
mixture of water and nitric acid (HNO3). |
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The PSCs alter the chemistry of the lower
stratosphere in two ways: |
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(1)
by coupling between the odd nitrogen and chlorine cycles |
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(2)
by providing surfaces on which heterogeneous reactions can occur. |
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Long Antarctic winter (May through September) |
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The stratosphere is cold enough to form PSCs |
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PCSs deplete odd nitrogen (NO) |
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Help convert unreactive forms of chlorine
(ClONO2 and HCl) into more reactive forms (such as Cl2). |
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The reactive chlorine remains bound to the
surface of clouds particles. |
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Sunlight returns in springtime (September) |
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The sunlight releases reactive chlorine from the
particle surface. |
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The chlorine destroy ozone in October. |
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Ozone hole appears. |
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At the end of winter, the polar vortex breaks
down. |
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Allow fresh ozone and odd nitrogen to be brought
in from low latitudes. |
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The ozone hole recovers (disappears) until next
October. |
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