Radiation Balance
1
Radiation Balance
Reading Assignment:
• A&B:
Ch. 3
(p. 60-73)
• CD:
tutorial: energy balance concepts
interact. ex.: shortwave & longwave rad.
• LM:
Lab. 5
• Radiation = Mode of Energy transfer
• observes the conservation principle
• radiation emitted from Earth/atmosphere: terrestrial,
longwave radiation
• radiation emitted from sun: solar, shortwave radiation
• when solar radiation is absorbed in the Earth/atmosphere,
part or most of it is re-emitted as longwave radiation
• “Balance” ⇒ conservation of energy:
storage change = input – output
The Radiation Balance can be expressed in a budget
equation, composed of different terms that each represent a
radiation transport or conversion process
• Conservation of energy
Q* = (K↓ - K↑) + (L↓ - L↑)
= K* + L*
Units: W m-2
Radiation Balance
2
Q* - net all wave radiation
• net radiative energy that is absorbed and then
transformed into a different form (non-radiative) ⇒
becomes
• energy available to be partitioned in the energy
balance, i.e. to
• heat the air,
• heat the ground or
• evaporate water (see below)
K↓ - incoming shortwave radiation.
• emitted by the sun, transmitted to location of the
balance (e.g., top of atmosphere or surface)
• dependent on solar altitude, transmissive property of
the atmosphere above
K↑ - outgoing shortwave radiation.
• outgoing shortwave radiation (reflected!)
• Depends on K↓ and the albedo (α):
K↑ = α K↓
K* - net shortwave radiation (K*= K↓ - K↑)
Energy that is absorbed may be reradiated at longer
wavelengths.
Radiation Balance
3
L↓ - incoming longwave radiation:
• Depends on sky temperature and sky emissivity (εs)
L↓ = εs σTs4
Ts; εs: summary effect of all layers of the atmosphere;
depend on cloud cover, humidity, temperature
structure.
Can be calculated from radiosonde data
L↑ - outgoing longwave radiation:
• Depends on surface emissivity and surface temperature
L↑ = ε0 σT04
ε0: see list of values in Lab Manual, lab#5
L* - net longwave radiation (L*= L↓ - L↑)
Global Average
(i) Shortwave radiation (A&B, Fig. 3-4)
• total reflected: 30% (= global albedo)
• total absorbed: 70%
Radiation Balance
4
(ii) Long-wave radiation (A&B, Fig. 3-5)
• total lost to space: 70% (= absorbed solar)
• L↓ at surface (from atmosphere): greenhouse effect =
additional energy source for surface
• atmospheric net loss: compensated from K↓(absorbed) &
convection from surface
(iii) All wave net radiation (A&B, Fig. 3-7)
• in the atmosphere and at the surface: non-zero net radiation
⇒ other forms of energy transport must compensate
• balance can be formed at any level:
o top-of-the-atmosphere (TOA)
o atmosphere
Radiation Balance
5
o surface
• these numbers are long-term global averages, average
cloud cover, temperature, etc.
• considerable spatial and temporal (weather, seasons,
climate!) variations exist
Global Distribution (spatial variations)
Incoming solar radiation
ERBE Video
• Spatially variable because of Earth-sun geometry
• Maximum solar radiation receipt at the top of the earth's
atmosphere at right angles ⇒
• Solar constant: 1367 W m-2
(i) January incoming solar radiation: K↓Jan
Question: Is this the radiation at the surface or the top
of the atmosphere?
[Hint:
look at isopleths across continents.]
Radiation Balance
6
January net shortwave radiation at the surface
Note: here, negative values refer to K* taken up by the surface
(different sign convention from the one we use)
(iii) January net longwave radiation at the surface
Note: here, positive values refer to L* deficit at the surface
(different sign convention from the one we use)
Radiation Balance
(iv) Net Radiation: Q* - spatially variable
To prevent runaway heating in Q* surplus (net gain)
region (latitudes < 40° N/S), and runaway cooling in Q*
deficit (net loss) regions (latitudes > 40° N/S), energy is
transported from the surplus to the deficit regions
(poleward transport) by:
• ocean currents (~1/3)
• warm/cold winds (sensible heat) (~1/3)
•• m
mo
oiissttu
urree iin
n aaiirr ((llaatteen
ntt h
heeaatt)) ((~~11//33))
7
Radiation Balance
8
5. Temporal Variations
Observations at long-term research location:
Observations at one location
• Morgan Monroe State Forest http://www.indiana.edu/~co2/
Biosphere-Atmosphere Exchange Project
The 46 m (150 ft.) MMSF AmeriFlux Tower
with Energy Balance Instrumentation
Radiation Balance
9
(i)Clear Summer Day, July 12 1999
1200
1000
Q*
Kdn
800
Kup
Lup
-2
Flux (Wm )
Ldn
600
400
200
0
193.00
193.25
193.50
193.75
194.00
-200
Time
• What will happen on a:
(ii) Cloudy Summer Day - overcast, stratus
1200
1000
Q*
Kdn
Kup
Ldn
Lup
Flux (Wm-2)
800
600
400
200
0
193.00
193.25
193.50
193.75
194.00
-200
Time
(draw the changes into the figure)
Radiation Balance
10
(iii) Cloudy Summer Day - partly cloudy, cumulus
1200
1000
Q*
Kdn
Kup
Ldn
Lup
Flux (Wm-2)
800
600
400
200
0
193.00
193.25
193.50
193.75
194.00
-200
Time
(draw the changes into the figure)
(iv) Cloudy Winter Day - what happens if it snows?
1200
1000
Q*
Kdn
Kup
Ldn
Lup
600
-2
Flux (Wm )
800
400
200
0
193.00
193.25
193.50
193.75
194.00
-200
Time
(draw the changes into the figure)