ESM 203: Energy balance and atmospheric circulation

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ESM 203 Energy balance and atmospheric
circulation

• Jeff Dozier Thomas DunneFall 2007

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Solar radiation at top of atmosphere(MJ m2day1)
Dingman, Figure 3-6
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Wavelengths of radiation and the atmosphere

• Ultraviolet (lt0.4 ?m) absorbed by stratospheric
  ozone (less ozone ? more UV)
• Visible (0.40.7 ?m) scattered by air molecules,
  dust, soot, salt, clouds
• Scattering by air greater for shorter wavelengths
  (blue).
• Near-infrared (0.7-3.0 ?m) (from Sun) scattered
  less, but absorbed by water vapor, especially at
  1.4 and 1.9 ?m, and by clouds
• Middle infrared (3-5 ?m) (from Sun and Earth) and
  thermal infrared (gt5 ?m) (from Earth) absorbed
  by clouds, water vapor, carbon dioxide, methane,
  ozone, and other greenhouse gases
• Some windows (3.5-4.0?m and 10.5-12.5?m) when
  no clouds

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The simplest climate modelenergy balance with a
non-absorbing atmosphere

• S0 solar radiation
• a planetary average albedo
• F? infrared radiation
• T planetary surface temperature
• ? ? 0.90-0.95, s 5.67108
• S0 1370 W m2 normal to Sun
• Divide by 4 to average over Earth, 342.5 Wm2
• Albedo ? 0.27-0.33 (see Charlson)
• Thus T ? 255K 18C

• Solar radiation absorbed by whole Earth
  infrared radiation emitted by whole planet
• i.e., net all-wave radiation 0

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Interpretation

• If the atmosphere didnt absorb radiation, the
  global average temperature should be about 18C.
• Near the surface, average air temperature is
  measured to be about 16C.
• The discrepancy must be due to the role of the
  atmosphere in absorbing energy and storing it
  near the surface.
• This interaction between solar radiation and the
  atmosphere begins the processes of energy
  transfer that create climate

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Variation of atmospheric temperature with
elevation reflects absorption of radiation
emitted from surface and absorbed by atmospheric
gases
? lt 0.1µm absorbed by N2, O2, N, O
? lt 0.2µm absorbed by O2
O3 absorbs ? lt 0.31 µm and ? 8 µm
? gt 0.31µm warms surface, which radiates and
warms atmosphere
Graedel, T. E. and P. J. Crutzen (1995)
Atmosphere, Climate and Change
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Most of the atmospheric constituents that absorb
out-going long-wave radiation (relatively large
asymmetric molecules) although natural, are
augmented by pollutant gases. If we change
these concentrations, expect more outgoing
radiation to be absorbed and the atmospheric
temperature to rise, especially in the lower
parts of the atmosphere.
Graedel, T. E. and P. J. Crutzen (1995)
Atmosphere, Climate and Change
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Mean annual global energy balance for Earths
atmosphere
Graedel, T. E. and P. J. Crutzen (1995)
Atmosphere, Climate and Change
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Hartes more realistic energy-balance model, but
still 1-D (Homework 1)
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Hartes 1-D climate model with atmosphere

• A simple model of this type allows us to
  anticipate the general nature of changes in
  atmospheric temperature if various controlling
  factors were to change
• e.g. solar radiation, albedo, or the absorbing
  capacity of the atmosphere caused by changes in
  concentrations of absorbing gases.
• We are concerned about such changes because we
  have come to recognize that
• surface albedo has changed due to regional-scale
  vegetation changes
• there are feedback effects between climate and
  albedo because of snow and ice
• several greenhouse gases have changed over recent
  Earth history.

• Layered atmosphere, most infrared absorption in
  lower layer.
• Some solar absorption in upper atmosphere.
• Sensible and latent heat from surface up into
  atmosphere.
• Latent heat estimated from global average of
  precipitation.
• Also includes energy released from human
  activities, although negligible.
• This is a steady state model
• no time element
• in contrast with a transient model.

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Structure of energy balance models in general

• They have compartments
• Energy (and mass) fluxes into and out of each
  must balance
• Temperature affects some of the fluxes, so T can
  adjust to make them balance
• Fluxes are
• Radiative
• Convective (vertical) and advective (horizontal)
• Both sensible and latent
• Conductive (not important in atmosphere)

Top boundary
Layers (e.g., atmosphere)
Surface (lower boundary)
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Lean 2005, Physics Today
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We have discussed controls on global average
temperatures, but what controls climate?

• Solar radiation
• Orbital controls
• Latitude
• Clouds
• Albedo
• Atmospheric emissivity
• Absorption and scattering of solar radiation
• Atmospheric composition (water vapor, CO2, CH4)
• Absorption of solar radiation
• Atmospheric emissivity
• Cryosphere
• Albedo
• Water storage

• Aerosols
• Albedo
• Absorption and scattering of solar radiation
• Condensation nuclei
• Land surface
• Albedo
• Evaporation
• Temperature
• Oceans
• Albedo
• Evaporation
• Energy transfer by ocean currents vertical
  mixing

Which ones do humans alter?
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(Karl Trenberth 2003)
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Variability in net radiation (http//cimss.ssec.wi
sc.edu/wxwise/homerbe.html)
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Variability in planetary albedo
(http//cimss.ssec.wisc.edu/wxwise/homerbe.html)
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Cloud effects on albedo and net radiation
(http//cimss.ssec.wisc.edu/wxwise/homerbe.html)
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But the picture is not static --- Radiation
imbalance varies with latitude it drives
circulation of the atmosphere and ocean, which
reduce these latitudinal differences
excess
deficit
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Effects of heating the atmosphere (mainly from
below)

• The density of the atmosphere depends on
  temperature, water vapor content, and pressure
• Heating and evaporation of water from surface
  lower air density relative to surrounding air and
  cause air to rise
• Denser air moves in below the rising low-density
  air (low-pressure air since pressure is the force
  due to the overlying column of air)
• Spatially unequal heating causes air to rise in
  some places and to descend in other, cooler
  places
• Relate to your Santa Barbara experience

Duxbury, A.C. Duxbury, A. B. (1989) An
Introduction to the Worlds Oceans
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Force balances, hydrostatic if only the
pressure-gradient force and gravity were acting
a good approximation locally, such as over Santa
Barbara region

• Hydrostatic pressure equalsweight of air above
• in warm air, pressuredecreases more slowly with
  ma molar mass of air, R gas
  constantheight, so warm (low pressure) areas at
  surface are locally high pressure areas aloft

(Higher P)
(Lower P)
pressure gradient
pressure gradient
(Lower P)
(Higher P)
Land (warm)
Water (cool)
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Effects of spatially variable heating on a
uniform, non-rotating Earth

• Heating at equator and cooling near poles would
  cause a single convection cell in the atmosphere
  if Earth were covered with a uniform surface and
  if Earth did not rotate

Duxbury, A.C. Duxbury, A. B. (1989) An
Introduction to the Worlds Oceans
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Movement across Earths rotating surface

• A substance that moves across Earths rotating
  surface moves from a place where the planet is
  rotating at one speed to a position where it is
  rotating at a different speed (measured in m/s,
  not in radians/s or deg/s)
• Ignoring friction, if an air parcel moves
  directly north in N hemisphere, it begins with a
  faster W?E velocity than does the place it is
  heading for.
• Also, as it moves inward relative to Earths
  axis of rotation the airs rotation speeds up
  because its angular momentum (m?r, where ? is
  angular velocity) must remain constant., like a
  spinning skater.

• From the point of view of an observer on the
  surface, the air appears to move to the right

Duxbury, A.C. Duxbury, A. B. (1989) An
Introduction to the Worlds Oceans
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Coriolis force

• Displacement of the air parcel is to the right in
  N hemisphere appears to be subject to a
  Coriolis force that increases with increasing
  latitude and air speed
• Using same reasoning, imagine the effect on air
  moving south (toward faster rotating surface) in
  the N hemisphere
• In the S hemisphere, the sense of the force is
  reversed (i.e. to left) whether moving north or
  south

Duxbury, A.C. Duxbury, A. B. (1989) An
Introduction to the Worlds Oceans
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Coriolis force

• If air is moving due E or W, it still moves to
  the right in the N. hemisphere.
• The reason is a little more complicated than the
  N-S movement, -- it has to do with the component
  of the centrifugal force that acts parallel to
  the surface being aligned to the right of the
  initial motion in N. hemisphere, etc.
• The devoted student may wish to consult a
  textbook of atmospheric science or oceanography,
  or .

Duxbury, A.C. Duxbury, A. B. (1989) An
Introduction to the Worlds Oceans
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Coriolis force on a rotating disk
Definitions velocity of rotation v ?r,?
angular velocity (rad sec-1)r radius of
rotationangular momentum vr ?r2 Conserving
angular momentum with change ?rr0-r
requires ?r02 (???)r2 (??v/r)r2,where ?v
is relative velocity caused by ?r Solving, ?v
?(?r02/r - r)
r0
r
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Effect of latitude, Coriolis force on a sphere
N
true direction of Coriolis force
the horizontal (only important) component,
proportional to sin(latitude)
latitude
Equator
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Coriolis force magnitude and direction
Right in Northern Hemisphere
Left in Southern Hemisphere
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Coriolis force and cooling of air raised at
equator disrupt the simple circulation of a
non-rotating Earth

• Air rising at Equator moves N and increasingly to
  E while cooling (densifying)
• By time it has reached 30N, some of it sinks
  and flows back along surface to S (and therefore
  W) and to N (E)
• Remainder of air aloft continues towards pole,
  where it sinks and flows S (W) meeting the
  NE-flowing air at the surface
• Reverse in S hemisphere

Duxbury, A.C. Duxbury, A. B. (1989) An
Introduction to the Worlds Oceans
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Schematic zonal circulation is complicated by
unmixed boundaries between cold and warm air,
creating fronts
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Simplified zonal pattern of surface winds

• Where winds converge, air must rise, and thus
  pressure is lowered
• Where winds diverge, they must be supplied by
  sinking air, and the pressure must be relatively
  high

Duxbury, A.C. Duxbury, A. B. (1989) An
Introduction to the Worlds Oceans
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Simple zonal picture of pressure distribution is
complicated by

• Seasonal changes in solar heating of continents
  and oceans
• Distribution of continents
• Why are interiors of continents alternately
  locations of relatively high and low pressure?
• Note low pressures that drive monsoonal flow in
  India, Africa, and SW US.

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Surface pressure, January and July
From Columbia University
January1000 mb height
July1000 mb height
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Force balances, geostrophic

• Gradients of pressure (the pressure-gradient
  force) drive air flow
• Geostrophic balance between pressure gradient
  and Coriolis forces where friction is negligible
  (aloft)
• geostrophic wind blows parallel to isobars
• Geostrophic friction
• at surface, wind slowed by friction,is not
  parallel to isobars but still moves to right of
  PGF direction

F
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Geostrophic winds in upper atmosphere flowing
around the high- and low-pressure cells
Duxbury, A.C. Duxbury, A. B. (1989) An
Introduction to the Worlds Oceans
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Forces on air parcel in atmosphere

• Gravity ?g (force per unit volume is density of
  air ? gravitational acceleration)
• Pressure gradient force spatial variability in
  air pressure, from high to low, ?P/distance
• Coriolis force right in northern hemisphere,
  left in southern, magnitude depends on latitude
  (zero at equator) and wind speed
• Friction small except near surface
• Centrifugal force where winds are turning
  rapidly, such as in a hurricane
• Wind speed and direction balance all these forces.

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Links to some animations showing circulation

• Winds and clouds (NCAR)
• Water vapor and precipitation (NCAR) (long)