Transcript Radiation & Energy Budgets

Chapter 2
temperature, radiation & energy
Temperature vs. Heat
• Temperature: A measure of internal energy (in
this case, 1575oF).
•
Heat: Thermal energy transferred between
systems at different temperatures.
Energy Transfer
• Conduction, convection, and advection
require molecules
• Radiation is an electromagnetic
phenomenon, and is able to pass
through the vacuum of space.
(a)
conduction: molecular vibration
(b) convection:
eddy transfer
1
(b) convection
2
1
1
(b) convection
2
3
icecold
advection: mass transfer
cool
Pop quiz
• It is a balmy winter day in Chicago. This is because of warm air …………. by
winds from the Gulf of Mexico.
–
–
–
–
conduction;
advection;
convection;
radiation.
• You can burn your hand holding it above a candlelight because of …
– Convection!
radiation
the solar spectrum
l
Blue has a shorter wavelength than red
colors in the sky …
• Why is the clear sky blue?
• Why are sunsets red?
Scattering of Visible Light
K : scattering efficiency
K ~ l-4
Rayleigh scattering:
molecules of size r << l
l: wavelength
K(blue) / K(red) = (lred / lblue)4
= (0.64m / 0.47m)4
= 3.5
 blue is scattered more than red
Scattering of Visible Light
Mie scattering: haze, dust
rl
little color variation
Three forms of light scattering:
Rayleigh : r << l
Mie
:r~l
geometric : r >> l
Geometric scattering:
r >> l
(water droplets, ice crystals)
light is reflected or refracted
Question:
• How much of the solar radiation reaching the earth, is
reflected into space?
30%
Planet Earth’s Albedo: 30%
Albedo: the fraction of solar
radiation that is reflected or
scattered back into space
How bright is
the moon?
Mars
Moon
Venus
visible view
radar view
volcanoes and dark lava rocks
….below a thick CO2 atmosphere
with sulphuric acid clouds
the Earth’s albedo is far from constant
now
The albedo of the ocean is very low
Zenith angle, 

7%
Interpret the global mean albedo
The solar radiation budget on earth
30%
4%
20%
100%
6%
19%
51%
puzzle
the sun shines every day
every day, the earth cumulates more solar radiation
radiation = energy = heat
so the earth should become warmer every day
Answer: the Earth
emits radiation as well!
micrometer
micrometer
We all emit IR radiation!
radiation
• solar radiation (0.5 mm)
terrestrial radiation (10 mm)
solar
terrestrial
Now we can connect to the concept of greenhouse gases
Terrestrial radiation emitted
• Each surface emits radiation, at capacity (‘blackbody’)
• The most likely type of radiation emitted depends on temperature T (K):
b
–
lmax 
T
–
Wien’s displacement law (b= 2900)
lmax is the wavelength at which the radiation peaks (mm)
• The amount of radiation emitted (W) increases with the 4th power of T:
–
W  sT
4
Stefan Boltzman’s equation [s= 5.67 10-8 W/(m2 K) ]
• The atmosphere will absorb some of the radiation emitted by the Earth surface.
We are closer to the concept of greenhouse gases
Absorption of radiation by the atmosphere
0%
small window
Absorption
100%
big window
0.01
0.05
0.1
0.5
1
5
10
50
100
50
100
Wavelength (micrometer)
Visible
I nfrared
Radiation Intensity
Ultra Violet
0.01
0.05
0.1
0.5
1
Wavelength (micrometer)
5
10
If we had no atmosphere …
… the global mean temperature would be 0°F
Our atmosphere acts as a greenhouse, and causes the
air temperature to be 33 K (59°F) above the Earth’s
‘radiative equilibrium’ temperature
with an atmosphere
T = 59°F (15°C)
without atmosphere
T = 0°F (-18°C)
Pop quiz
1. Is the greenhouse effect of the Earth’s atmosphere:
–
–
manmade (mainly due to the burning of fossil fuels);
or mostly natural and existed before human history ?
2. What is the ratio of the manmade to the natural greenhouse
warming?
1. Answer: about 1:33, but rising
(Source: Climate Research Unit, Univ. of East Anglia, UK)
A petroleum geologist told me this …
• In the last 100 years or so, we have been burning a lot of coal and oil
and gas, fossil fuels. That produces heat. That heat adds up and
spreads globally. That causes the global warming.
• 3. Is his argument right or false? Why?
• 4. What (else) does cause global warming?
• Answer (3): False. The heat generated by burning of fossil fuels is
insignificant compared to other terms in the global energy balance.
The heat that was generated by cars and industry years ago has long
been dissipated into space as terrestrial radiation.
• Global warming is largely due to the greenhouse gases contained in
the burnt fossil fuels (mainly CO2). These gases alter the Earth’s
radiative balance.
How long does it take for the Earth to cool,
if the Sun suddenly went out?
• Without the oceans, the Earth would cool from the current average (59ºF)
to freezing (32ºF) in 7 days.
• The oceans store a lot of heat. Depending on the rate at which this is
released, the cooling down to freezing would probably take some 59 days.
• The heat associated with the burning of all fossil fuels in the past century
corresponds with all the solar radiation received by the Earth in just 4
days !
reminder: the solar radiation budget
30%
4%
20%
100%
6%
19%
51%
The Earth surface is emitting IR radiation, but then some of it is
absorbed by the atmosphere.
The Earth’s energy budget
+70
energy gained by the atmosphere 130
NET infrared radiation
lost at the earth surface -117+96=-21
=> There is net deficit of 30 units in the atmosphere, and a net excess of 30 units at the surface
Global energy balance
• At the top of the atmosphere, outgoing terrestrial radiation is
balanced by incoming solar radiation.
• At the earth surface, the net longwave radiation emitted (21%) is
insufficient to offset the net solar radiation (51%) received.
• The atmosphere continuously cools by radiation: the net longwave
radiation lost (49%) exceeds the net solar radiation (19%) received
• So what prevents the earth surface from heating up & the
atmosphere from cooling down?
Non-radiative atmospheric heating:
Conduction + convection = sensible heating
Condensation, freezing = latent heating
The lower atmosphere is heated from below….
Evaporation
takes energy
Oceans continuously heat up by net radiation uptake.
They are ‘air-conditioned’ by evaporation at the surface.
evaporation
trade
winds
evaporation over the ocean
Satellite IR image shows cold anvils on top of thunderstorms
evaporation
Thunderstorms!
Inter-tropical
convergence zone
evaporation
The Earth’s energy budget
-30 net radiation
-30
+30 net radiation
=100%
Fig 2.20 in the textbook. The units are NOT % of the incoming radiation at the top of
the atmosphere, but rather in W/m2
Solar constant = 1380 W/m2
Global mean surface energy balance:
net rad = net SW rad + net LW rad
R  H + LE
R = Sn+ Ln
and
R = 51 –21
= 30
R = 7 + 23
= 30
Why are the tropics warmer than polar regions?
net outgoing
terrestrial
radiation
net incoming solar radiation
Why are the tropics warmer than
polar regions?
•
•
•
•
•
net radiation R is positive in the tropics,
negative at poles.
 heat transfer:
–
–
atmospheric currents (especially near fronts)
ocean currents
in winter, the high-latitude radiation deficit is
even larger,
therefore the pole-to-equator temperature
difference is larger,
therefore the currents need to transport
more heat poleward
There are two reasons why the solar radiation at the
surface is weaker
when the Sun is lower in the sky
What are these reasons?
Why is the sun stronger
when it is higher in the sky?
normal
oblique
(1) Because normal insolation provides
more energy, per unit area, than does
oblique insolation.
Atmospheric attenuation:
{scattering + absorbance}
(2) Because oblique insolation is more
attenuated than is direct insolation.
Air Mass traversed is double at 60º
Seasonal variation of the net radiation R at the surface
W/m2
What explains the seasons?
What explains the seasons?
Sun above equator
Sun above 23½ºS
Sun above 23½ºN
try this animation!
Sun above equator
Fig. 2.17
total insolation, all day long, at various latitudes
June 21: summer solstice
Attenuation removes a great amount
of solar energy at the pole.
December 21: winter solstice
Axial tilt has plunged the North
Pole into 24-hour darkness.
Axial Tilt of Earth, 21 June
Tilted by 23.5 from the
perpendicular
Solar angle v season
Length of day as function of
time of year and latitude
40°N
Fig. 2.16 in textbook
Fraction of solar constant
Energy Balance at the Earth’s Surface
Net radiation: R = Sn+ Ln
R = H + LE
R warms the surface causing convective currents (H), and
R evaporates water (LE)
Energy Balance at the Earth’s Surface
Pop quiz
• Sensible heat flux H versus latent heat flux LE.
Which one is true?
–
–
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–
a: over the ocean LE > H;
b: over a dry desert surface, at noon, H > LE;
c: as a global average, LE > H;
d: all of the above.
H vs. LE Globally
• Over oceans, 90% of R is used to evaporate water (LE),
only 10% used to warm the air (H) by conduction or
convection.
• On land, H  LE.
• Globally, LE = 23 units (77%), H = 7 units.
100
60
H
LE
40
20
0
Arctic
Pacific
Indian
Atlantic
Which bar represents:
Australia
South America
Antarctica
Antarctica
Australia
Africa
S. America
N. America
Asia
-20
Europe
Kly per year
Energy flux
80
These bars respresent
different continents
Arctic
Pacific
Indian
Atlantic
Antarctica
Australia
Africa
S. America
N. America
Asia
Europe
Energy flux
Kly per year
100
80
60
40
H
LE
20
0
-20
Local energy balance
Inside which one is it warmer on a sunny day? Why?
– a white styrofoam cooler, lid closed;
– a white styrofoam cooler, lid off;
– a styrofoam cooler painted black on the inside, lid off;
– a styrofoam cooler, painted black on the inside, lid off, but covered by a glass pane;
– a metal toolbox, painted black on the inside, covered by a glass pane.
– a metal toolbox, painted black on the inside, covered by a glass pane, and buried in the
ground so that the top is level with the surface.
results
• 9 Sept 2003, Prexy lawn, 1:15 pm. Sunny day. Air temperature: 81°F
–
–
–
–
a: a white styrofoam cooler, lid closed: 78°F
b: a white styrofoam cooler, lid off: 88°F
c: a styrofoam cooler painted black on the inside, lid off: 103°F
d: a styrofoam cooler, painted black on the inside, lid off, but covered by a glass pane:
189°F
– e: a metal toolbox, painted black on the inside, lid off, but covered by a glass pane:
124°F
– f: a metal toolbox, painted black on the inside, lid off, but covered by a glass pane,
half-buried: 115°F
Summary of chapter 2
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Electromagnetic radiation
Heat transfer (convection, conduction, advection)
Scattering and absorption of radiation by the atmosphere
Shortwave (solar) and longwave (terrestrial) radiation
The natural greenhouse effect
Global energy balance (solar radiation, terrestrial radiation, and
heat transfer)
Seasonal/regional variations of the surface energy balance
End of Chapter 2