Transcript Evolution of The Biosphere - University of Northern

Chapter 5: Atmospheric Structure and
Energy Balance
(I) Characteristics of the Atmosphere
 Thickness, air pressure, density
 Air pressure and density decrease with altitude
 90% of its mass (5.1 x 1018 kg) is within 16 km (10 mi) of the
surface (about 0.0025 times the radius of the Earth)
97% of air in first 29 km or 18 mi; 99% 32 km (18 mi); 99.9% 47km (30mi)
 Atmospheric motions can therefore be considered to occur
“at” the Earth’s surface
 The greatest and most important variations in its composition involve
water in its various phases
 Water vapor
 Clouds of liquid water
 Clouds of ice crystals
 Rain, snow and hail
Composition of the Atmosphere
Dry Air
TRACE GASES
Argon (.93%) and
Carbon Dioxide (.03%)
Ozone (.000004%)
Solid particles (dust, sea salt,
pollution) also exist
Water vapor is constantly being
added and subtracted from the
atmosphere, and varies from near
0% (deserts) to 3-4% (warm,
tropical oceans and jungles)
Vertical Structure of the Atmosphere
4 distinct layers
determined by
the change of
temperature
with height
Extends to 10 km in the extratropics, 16 km in the tropics
Contains 80-90% of the atmospheric mass, and 50% is
contained in the lowest 5 km (3.5 miles)
It is defined as a layer of temperature decrease
The total temperature change with altitude is about 72°C
(130°F), or 6.5°C per km (lapse rate)
• It is the region where all weather occurs, and it is kept
well stirred by rising and descending air currents
• The transition region of no temperature change is the
“tropopause”: it marks the beginning of the next layer
Vertical Structure of the Atmosphere
4 distinct layers
determined by
the change of
temperature
with height
Extends to about 50 km
It is defined as a layer of temperature increase and
is stable with very little vertical air motion – a good place to fly!
Why does temperature increase?
The major heating is the UV of sunlight absorbed by O3.. When the sunlight
travel down, the UV will become less and less available, so the temperature
increase with height…
• The transition region to the next layer is the “stratopause”
Atm. vertical structure
• Air pressure p at sea
level is 1 atm. = 1.013
bar = 1013 mb
• p decr. with altitude by
factor of 10 every 16 km.
• T decr. with altitude in
troposphere,
rises in stratosphere
drops in mesosph.
rises in thermosph.
Temperature
(II) Radiation Energy
Objectives:
• Electromagnetic (EM) radiation & spectrum
• Energy flux
• Blackbody radiation -- Wien’s Law &
Stefan-Boltzmann Law
• Planetary energy balance
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EM radiation
wavelength
later t
• EM radiation includes visible light,
ultraviolet, infrared, microwaves.
• wavelength 
• period T, frequency  = 1/T
• wave speed or phase speed c = /T =  
• Speed of light in vacuum: c = 3.00108 m/s
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•c = /T
=>  = cT = c/ 
longer period waves => longer
?
wavelength
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Energy flux
• Power = energy per unit time (watt W = J/s)
• Flux F = power per unit area (W/m2)
less flux
high lat.
=> less F
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EM spectrum
• EM radiation classified by their wavelength
or freq.
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Inverse-square law
• Solar flux S falls off
as
r0 
S  S 0  
r 
2
e.g. if r = 2r0
=> S = S0/4
S
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Blackbody radiation
• Absolute temperature in degrees Kelvin (K)
• 0 K = -273°C (coldest possible T)
• All bodies emit EM radiation
• e.g. humans emit mainly infrared (IR)
• “Blackbody” emits (or absorbs) EM rad.
with 100% efficiency.
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Wien’s Law
Planck function (blackbody rad. curve)
max = const./T
Rad. flux
Temp. T in K
const. = 2898 m
max refers to the Wavelength of
energy radiated with greatest
intensity.
max
wavelength
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Blackbody rad. curves for Sun & Earth
max = const./T
Temp. T in K
const. = 2898 m
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Stefan-Boltzmann Law
Planck function (blackbody rad. curve)
F = T4
Rad. flux
 = 5.67 x 10-8 W/m2/K4
total F = area
under curve
wavelength
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Planetary energy balance
• Earth is at steady state:
Energy emitted by Earth =
Energy absorbed
• E emitted = (area of Earth)   Te4
= 4 Re2   Te4
..(1)
(Te= Earth’s effective rad. temp., Re= Earth’s radius)
• E absorbed = E intercepted - E reflected
• Solar E intercepted = S Re2 (solar flux S)
• Solar E reflected = AS Re2 (albedo A)
• E absorbed = (1-A) S Re2
• (1) => 4 Re2   Te4 = (1-A) S Re2
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Magnitude of greenhouse effect
•  Te4 = (1-A) S/4
• Te = [(1-A) S/(4  )]1/4 (i.e. fourth root)
• Te = 255K = -18°C, very cold!
• Observ. mean surf. temp. Ts = 288K = 15°C
• Earth’s atm. acts as greenhouse, trapping
outgoing rad.
• Ts - Te = Tg, the greenhouse effect
• Tg = 33°C
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Greenhouse effect of a 1-layer atm.
•Energy balance at Earth’s surface:
Ts4 = (1-A)S/4 + Te4
..(1)
•Energy balance for atm.:
Ts4 = 2 Te4
.. (2)
S/4
Te
AS/4
Te4
Atm.
(1-A)S/4
Ts
Ts4
Earth
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Te4
Subst. (2) into (1):
Te4 = (1-A)S/4 ..(3) (same eq. as in last lec.)
Divide (2) by ; take 4th root:
Ts = 21/4 Te = 1.19 Te
For Te = 255K, Ts = 303K. (Observ. Ts = 288K)
Tg = Ts - Te = 48K,
15K higher than actual value.
• Overestimation: atm. is not perfectly
absorbing all IR rad. from Earth’s surface.
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(III) Modelling Energy Balance
Objectives:
• Effects of clouds
• Earth’s global energy budget
• Climate modelling
• Climate feedbacks
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Cumulus
Cumulonimbus
Stratus
Cirrus
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Climatic effects of clouds
• Without clouds, Earths’ albedo drops from
0.3 to 0.1.
By reflecting solar rad., clouds cool Earth.
• But clouds absorb IR radiation from
Earth’s surface (greenhouse effect) =>
warms Earth.
• Cirrus clouds: ice crystals let solar rad.
thru, but absorbs IR rad. from Earth’s sfc.
=> warm Earth
• Low level clouds (e.g. stratus): reflects
solar rad. & absorbs IR => net cooling of
Earth
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• IR rad. from clouds at
T4
• High clouds has much
lower T than low clouds
=> high clouds radiate
much less to space than
low clouds.
=> high clouds much
stronger greenhouse
effect.
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Earth’s global energy budget
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Climate Modelling
•“General
circulation
models” (GCM):
Divide atm./oc.
into 3-D grids.
Calc. variables
(e.g. T, wind,
water vapor,
currents) at grid
pts.
=> expensive.
•e.g. used in
double CO2 exp.
GFDL, Princeton
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• Weather forecasting also uses atm. GCMs.
Assimilate observ. data into model. Advance
model into future => forecasts.
• Simpler: 1-D (vertical direction) radiativeconvective model (RCM):
Doubling atm. CO2 => +1.2°C in ave.sfc.T
• Need to incorporate climate feedbacks:
• water vapour feedback
• snow & ice albedo feedback
• IR flux/Temp. feedback
• cloud feedback
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Water vapour feedback
• If Ts incr., more evap. => more water vapour
=> more greenhouse gas => Ts incr.
• If Ts decr., water vap. condenses out => less
greenhouse gas => Ts decr.
• Feedback factor f = 2.
• From RCM: T0 = 1.2°C (without feedback)
=> Teq = f T0 = 2.4°C.
Ts
(+)
Atm. H2O
Greenhouse effect
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Snow & ice albedo feedback
• If Ts incr. => less snow & ice => decr.
planetary albedo => Ts incr.
• As snow & ice are in mid-high lat. => can
only incorp. this effect in 3-D or 2-D models,
not in 1-D RCM.
Ts
(+)
snow &
ice cover
planetary albedo
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IR flux/Temp. feedback
• So far only +ve feedbacks => unstable.
• Neg. feedback: If Ts incr. => more IR rad.
from Earth’s sfc. => decr. Ts
Ts
(-)
Outgoing IR flux
•But this feedback loop can be overwhelmed
if Ts is high & lots of water vap. around
=> water vap. blocks outgoing IR
=> runaway greenhouse (e.g. Venus)
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Uncertainties in cloud feedback
• Incr. Ts => more evap. => more clouds
• But clouds occur when air is ascending, not
when air is descending. If area of
ascending/descending air stays const.
=> area of cloud cover const.
• High clouds or low clouds? High clouds
warm while low clouds cool the Earth.
• GCM’s resolution too coarse to resolve
clouds => need to “parameterize” (ie.
approx.) clouds.
• GCM => incr. Ts => more cirrus clouds =>
warming => positive feedback.
=> Teq = 2 -5°C for CO2 doubling
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