Climate-Radiation Balance
Whether and Climate
? Weather is the condition of the atmosphere in
terms of temperature,humidity,winds and
precipitation.
? Climate is defined as the average weather
over various time periods ranging from a few
weeks to decades to geological timescales.
? Whether and climate are ultimately
determined by location,topography,and the
exchange of radiant energy between the Sun,
Earth,and space.
Factors affecting the earth’s climate
? Incoming solar irradiance
? The earth system
– Surface
– Atmosphere (greenhouse gases,particles,
etc)
– Clouds
Solar irradiation,Solar constant
Solar constant,The amount of solar energy
received per unit of area outside the Earth’s
atmosphere,normal to the direction of
propagation of the light.
22 /1370//1370 mw a t t smsJS
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Radiation Budget
? The Earth's climate system constantly tries to
maintain a balance between the energy from the
Sun that is absorbed by the Earth,and the energy
that goes from Earth back out to space,We refer
to this process as Earth's "radiation budget."
? This radiation budget allows the Earth to maintain
the moderate temperature range essential for life
as we know it.
? The components of the Earth system that are
important to the radiation budget are the planet's
surface,atmosphere,and clouds.
Radiation Budget at the Top of the Atmosphere
Albedo (a)
? The fraction of solar energy that is reflected
back to space is called the albedo.
? Different parts of the Earth (surface and
cloud cover) have different albedo,
? Over the whole Earth,about 30 percent of
incoming solar energy is reflected back to
space.
What contributes to the global albedo?
? Clouds account for two-thirds (67%) of the
global albedo
? Backscattering from gaseous molecules and
particles in the air,20%
? Earth’s surface,13%
Albedo for various types of surface
Surface albedo
Clouds (average over all types) 0.35-0.40
Snow 0.60-0.80
Ocean 0.06-0.20
Deserts 0.25-0.40
Agricultural land 0.10-0.20
Forest 0.10-0.20
The incoming solar energy per unit area
at the earth’s surface
? Solar energy strikes earth at 1370 watts/m2 to a
disk area pr2 (r,earth radius).
? Earth radiates from its entire surface,which is 4
pr2,
? The energy Earth receives at the top of the
atmosphere is therefore,So/4.
? A fraction (a) of solar energy is reflected back to
the space
The incoming solar energy per unit area at the Earth’s
surface:
2/240
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How does the outgoing energy of the earth
relate with the Earth’s temperature?
? Blackbody radiation
? Definition,At a given temperature,there is a
maximum amount of energy that can be
emitted per unit time per unit area of a body,
The maximum amount of radiation for a
certain temperature is called blackbody
radiation.
How the outgoing energy of the earth relates
with the Earth’s temperature? (Continued)
4
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Stefan-Boltzman law
E,radiated energy per unit
time per unit area
Te,Blackbody temperature
tc o n sB o l tz m a nS t e f a nKmwxk ta n_:/1067.5 428 ?? ??
Spectral distribution of solar and terrestrial radiation
Txnmp e a k /109.2)( 6??
The Corresponding blackbody temperature for
the Earth and the Sun
?max Te (K)
Sun 483nm 6,000 K
Earth 10,000nm 288 k
Txnmp e a k /109.2)( 6??
Approximate the earth surface temperature
Balancing incoming and outgoing energy
for the Earth-Atmosphere system
Incoming = outgoing
4)1(
4 e
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4/1)]1(
4[ ak
ST o
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a Te (K)
0 279
0.15 268
0.3 255
Temperature of the Earth-Air
system seen from the space
Why the blackbody temperature of the earth-
atmosphere system seen from the space is 33 k
colder than the average temperature at the earth
surface?
? Earth-Atmosphere system,255k
? Earth surface average temperature,288k
? Answer,Greenhouse effect,
The atmosphere traps much of the heat emanating
from Earth’s surface and radiates it back,raising the
surface temperature.