General Circulation

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General Circulation

• Redistribution of trace components in the
  atmosphere
• Driving forces for all air circulation are
  temperature imbalances (pressure imbalances)
• Circulation is classified as either horizontal or
  vertical
• Vertical Circulation is dictated by
• Gravity
• Pressure Gradient
• Horizontal Circulation is dictated by
• Pressure Gradient
• Coriolis Forces
• Friction (below 1 km)

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Circulation around pressure centers

• High pressure
• denser, colder descending air
• clockwise circulation around center of high in N
  Hemisphere (b/c of Coriolis force)
• opposite circulation in S Hemisphere
• divergence at surface, convergence aloft
• Low pressure
• warmer, less dense, rising air
• counterclockwise circulation around center of low
  in N Hemisphere (b/c of Coriolis force)
• convergence at surface, divergence aloft

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Atmospheric circulation

• Hadley cells Large-scale convection cells from
  equator to subtropics
• Equatorial low pressure - warmer, moist, rising
  air, heavy precip (rain forests such as Amazon,
  Congo)
• Subtropical high pressure - air moving N and S
  from tropics cools becomes denser, descends,
  warms and dries forms subtropical deserts
  (Sahara) horse latitudes with lesser winds
• Surface flow toward equatorial low pressure from
  north, south deflected to west by Coriolis force
  - producing easterly trade winds (blow from east
  to west)
• Convergence at/near InterTropical Convergence
  Zone (ITCZ, near equator)

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Near the center of this high-pressure zone of
descending air, called the "Horse Latitudes," the
winds at the surface are weak and variable. The
name for this area is believed to have been given
by colonial sailors, who, becalmed sometimes at
these latitudes while crossing the oceans with
horses as cargo, were forced to throw a few
horses overboard to conserve water.
http//pubs.usgs.gov/gip/deserts/atmosphere/
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Atmospheric circulation

• Midlatitude eastward-moving winds (westerlies)
• Result from northward-moving air deflected to
  right in N Hemisphere
• Polar front jet at northern boundary of
  westerlies (30-70N)
• at boundary of cold polar air and warmer
  midlatitude airmasses
• acts as storm track for most of US
• height 25,000-35,000 feet, speed often 200 mph
• Rossby waves in jet stream
• may loop strongly north and south, or jet stream
  may be straighter
• Subtropical jet stream at southern extent of
  westerlies (20-50N)

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http//pubs.usgs.gov/gip/deserts/atmosphere/
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Atmospheric circulation

• Polar front - very cold descending air, high
  pressure, weaker easterly winds
• Polar vortex - Closed circulation around polar
  high pressure esp. in Antarctic winter

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http//pubs.usgs.gov/gip/deserts/atmosphere/
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Vertical Circulations

• Vertical distances much smaller than horizontal
  scales
• Vertical motions are less rapid, more random and
  occur over shorter periods of time.
• Vertical temperature gradient is due to pressure
  variation with altitude

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Vertical Circulations

• Amount of temperature change that takes place in
  a parcel of air as it is moved vertically. The
  motion is determined by the temperature of the
  parcel relative to the temperature of the
  surrounding air.
• (i) If the rate of decrease is greater than
  that of the surroundings, the air is stable and
  little or no convection occurs.
• (ii) If the rate of decrease is smaller than
  that of the surroundings, the air is unstable and
  a high degree of convective mixing occurs.

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Stable ELR less than adiabatic lapse rates (air
relatively warm at altitude) Rising air cools
more than surrounding air, becomes denser than
surrounding air, sinks back down
Environmental lapse rate ELR actual observed
change in temperature w/ altitude at a given
place and time
http//epswww.unm.edu/facstaff/gmeyer/envsc101/wk1
1atmcompcirc.htm
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Unstable ELR greater than adiabatic lapse rates
(air relatively cold at altitude) rising air
cools, but still stays warmer and less dense than
surrounding air, and keeps rising (often to
condensation and precipitation)
http//epswww.unm.edu/facstaff/gmeyer/envsc101/wk1
1atmcompcirc.htm
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http//epswww.unm.edu/facstaff/gmeyer/envsc101/wk1
1atmcompcirc.htm
Conditionally unstable air will become unstable
and continue rising if dew-point temperature is
reached and it becomes saturated (then moist
adiabatic rate will apply)
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Thermal Inversions
conditions are an extreme case of (i) ? no
mixing, high pollution levels.
http//www.pollution-china.com/item/2007/06/therma
l-inversion-and-pollution
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Mixing within the Stratosphere

• mixing is not restricted by cellular flows
• ? inter - hemispherical transport has no
  barriers.
• volcanic eruptions, nuclear explosions can push
  material directly through the tropopause into the
  stratosphere.
• other materials are transported from the
  troposphere into the stratosphere at breaks in
  the tropopause (areas of high circulation e.g.
  middle latitudes jet streams)
• ? long residence times and uniform (global)
  distribution

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Tropospheric ozone has two major sources, i.e.
intrusion from the stratosphere and production
from photochemical reactions. The tropospheric
ozone plays several key roles in the atmosphere
because although it oxidizes many chemical
substances in troposphere and controls
tropospheric chemistry, it is also a gaseous
pollutant harmful for human being and crops. It
oxidizes many chemical substances in troposphere,
controls tropospheric chemistry, and it is a
green house gas that contributes to the global
warming.
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Water

• Water comprises only 2 of the total volume of
  the atmosphere but it is one of the most
  important components.
• 25 of solar energy that reaches the Earth is
  used to evaporate water (extremely high heat of
  vaporization - Hydrogen-bonded network)
• The presence of water in air lowers the lapse
  rate dramatically
• Water is an efficient infrared absorber -
  greenhouse effect with positive feedback.

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Water cycle

• evaporation from the oceans is the main source of
  atmospheric H2O.
• precipitation is the main sink
• residence time is on the order of 10 days
  (uneven distribution in the atmosphere)
• Note that water is an excellent solvent!
• Atmospheric components which are water soluble
  will tend to have residence times of 10 days
  too!

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http//www.bom.gov.au/info/climate/change/gallery/
8.shtml
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• Water is not a permanent gas at atmospheric
  temperatures
• The maximum attainable pressures depend on the
  atmospheric temperatures (saturation pressure or
  equilibrium vapour pressure)
• Usually, the atmosphere is below the saturation
  point.
• Relative Humidity (R.H.)
• P(H2O) as a percentage of the saturation pressure

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Relative Humidity

• RH is NOT the concept of air holding water vapor
• likely a result of the use of the word saturation
  which is often misused in definitions of relative
  humidity.
• In the present context saturation refers to the
  saturation state of water vapor, not the
  solubility of one material in another.
• The thermophysical properties of water-air
  mixtures encountered at atmospheric conditions
  can be reasonably approximated by assuming that
  they behave like a mixture of ideal gases.
• This assumption implies that both components (air
  and water) behave independently of each other and
  therefore the physical properties of the mixture
  can be estimated by considering the physical
  properties of each component separately.
• This is reflected in the definition of RH - only
  the physical properties of water are considered
  when determining the RH of a mixture.

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where      is the relative humidity of the
mixture being considered         is the
partial pressure of water vapor in the mixture
        is the saturation vapor pressure of
water at the temperature of the mixture.
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• But note Relative Humidity is a function of
  temperature
• e.g. R.H. 80 at 0 ºC corresponds to P(H2O)
  0.0048 atm.
• R.H. 80 at 25 ºC corresponds to P(H2O)
  0.025 atm.
• Note also that solid and vapour phases of H2O can
  coexist.

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The relative humidity of a system is dependent
not only on the temperature but also on the
absolute pressure of the system of interest. A
change in relative humidity can be explained by a
change in system temperature, a change in the
absolute pressure of the system, or change in
both of these system properties.
http//en.wikipedia.org/wiki/Relative_humidity
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Relative Humidity

• Water vapor is a lighter gas than air at the same
  temperature, so humid air will tend to rise by
  natural convection.
• This is a mechanism behind thunderstorms and
  other weather phenomena.
• Relative humidity is often mentioned in weather
  forecasts and reports, as it is an indicator of
  the likelihood of precipitation, dew, or fog.
• In hot summer weather, it also increases the
  apparent temperature to humans (and other
  animals) by hindering the evaporation of
  perspiration from the skin as the relative
  humidity rises. This effect is calculated as the
  heat index or humidex.

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Relative Humidity

• Due to the increasing potential for a higher
  water vapor partial pressure at higher air
  temperatures, the water content of air at sea
  level can get as high as 3 by mass at 30 C
  compared to no more than about 0.5 by mass at 0
  C.
• This explains the low levels (in the absence of
  measures to add moisture) of humidity in heated
  structures during winter, indicated by dry skin,
  itchy eyes, and persistence of static electric
  charges.
• Even with saturation (100 relative humidity)
  outdoors, heating of infiltrated outside air that
  comes indoors raises its moisture capacity, which
  lowers relative humidity and increases
  evaporation rates from moist surfaces indoors
  (including human bodies.)

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