2. Radiation Balance

3. In atmosphere, radiation can be…
transmitted
absorbed
reflected

4. 1. transmission Radiation passes through atmosphere unimpeded.
Shortwave and longwave “windows”

5. 2. absorption Energy is transferred to absorber; absorber emits energy
Energy emitted in photons (energy bundles).
Each level/orbit represents different
amount of energy.
Atmospheric gases selectively absorb
and emit only at certain wavelengths.
Shells or orbits correspond to energy levels; when an atom receives E, electron moves to a different E level; when it goes back to previous state, it gives off energy
Photons: discrete bundles of energy

6. 3. reflection Energy re-directed; not absorbed
Our eyes detect reflected visible wavelengths.
Albedo is the reflective quality of a surface
Percent of incoming radiation reflected

7. Earth’s average albedo, March

8. Albedo is an important variable in global climate change
“A drop of as little as 0.01 in Earth’s albedo would have a major warming influence on climate—roughly equal to the effect of doubling the amount of carbon dioxide in the atmosphere, which would cause Earth to retain an additional 3.4 Wm-2 ”.

9. Albedos of various surfaces:
Earth’s surface (31%)
Cumulonimbus clouds (90%)
Stratocumulus clouds 0.6 (60%)
Cirrus clouds (40 – 50%)
Fresh snow – 0.9 (80 – 90%)
Melting snow – 0.6 (40 – 60%)
Sand – 0.35 (30 – 35%)
Grain crops – 0.25 (18 – 25%)
Deciduous forest – 0.18 (15 – 18%)
Coniferous forest – 0.15 (9 – 15%)
Tropical rainforest – 0.15 (7 – 15%)
Water bodies – (6 – 10%) increases at low sun angles

10. Scattering / diffuse radiation
A form of reflection

11. Types of scattering:
Rayleigh
Mie
Nonselective

12. 1. Rayleigh Happens when diameter of gas is
< 1/10th diameter of wavelength
of incoming radiation
favors smaller wavelengths
Scatters forward and back

15. Longer path through atmosphere at decreasing angle of sun; other wavelengths have been scattered away; leaving long wavelengths (red)

16. Optical path at point of tangency is 20 x as long as at SSP.

17. 2. Mie Caused by aerosols: Scatter forward
particles in atmosphere
microscopic but larger than gas molecules (pollen, dust, smoke, small water droplets )
Scatter forward
Do not favor short wavelengths; scatter all visible wavelengths

18. Pollution: high aerosol content
Grey sky : aerosols scatter entire visible range towards surface

21. 3. nonselective
No wavelength preference; particles much larger than wavelength
Big water droplets; large dust particles
E.g., fog and clouds reflect all wavelengths of light, appear white or grey

24. Radiation Balance
Balance maintained by earth and atmosphere between incoming and outgoing radiation
Mention that this is averaged over entire globe over entire year.

25. Imagine shortwave solar radiation entering the top of the atmosphere as total we start with.
70% is absorbed by earth/atmosphere
30% is reflected by earth/atmosphere
(albedo = 30%)
“100%”
incoming

26. 70% absorbed by: Ground (47%) Gases, dust in atmosphere Clouds (23%)
Shortwave!

27. 30% reflected by Ground (7%) Clouds (17%) Scattered by atmosphere (6%)
Albedo of earth/atmosphere = 30%
Shortwave!

29. 23
Shortwave
absorption
Point out 47 shortwave absorbed by earth; 23 by atmosphere 47

30. Earth absorbs far more solar radiation than atmosphere.
Consider absorption:
Earth absorbs far more solar radiation than atmosphere.

31. Why aren’t we boiling up?
Because energy is transferred between atmosphere and earth.
Shortwave solar radiation is absorbed and longwave radiation is emitted.
Earth’s surface emits 116 % longwave !

32. 116 units (%) of longwave emitted from earth surface to atmosphere 104 absorbed by atmosphere 12 transmitted to space 12
Space
104
Atmosphere
116
Earth’s surface:
ABSORBS SHORTWAVE, EMITS LONGWAVE
Surface

33. Longwave emission from earth12
104 23
116 47

35. Agents in atmosphere that absorb longwave :
(clouds, water vapor, carbon dioxide, ozone, other greenhouse gases)
Their energy level is raised; emit longwave 12
Space
104
Atmosphere
116
Earth’s surface:
ABSORBS SHORTWAVE, EMITS LONGWAVE
Surface

36. Atmosphere (clouds, water vapor, greenhouse gases)
absorb and emit longwave:
98 emitted back to earth
58 emitted to space 58
Space
Atmosphere
Surface 98
Notice: Amount re-emitted ( ) exceeds amount absorbed (104)

38. 12
Longwave emission
from atmosphere
104 23 58 98
116 47

40. Net radiation Difference between amount emitted and amount absorbed.
Shortwave
1. For atmosphere:
Net shortwave radiation =
amount emitted ? None!
amount absorbed ?

41. 23
Shortwave
absorption
Point out 47 shortwave absorbed by earth; 23 by atmosphere 47

42. Net radiation Difference between amount emitted and amount absorbed.
Shortwave
1. For atmosphere:
Net shortwave radiation =
amount emitted ? None!
amount absorbed = 23
Net shortwave radiation for atmosphere = + 23

43. Net radiation Shortwave 2. For surface: Net shortwave radiation =
amount emitted ? None!
amount absorbed ?

44. 23
Shortwave
absorption
Point out 47 shortwave absorbed by earth; 23 by atmosphere 47

45. Net radiation Shortwave 2. For surface: Net shortwave radiation =
amount emitted ? None!
amount absorbed = 47
Net shortwave radiation for surface = + 47

46. Net radiation B. Longwave 1. For atmosphere: Net longwave radiation =
amount emitted ?

47. 12
Longwave emission
from atmosphere
104 23 58 98
116 47

48. Net radiation Difference between amount emitted and amount absorbed.
B. Longwave
1. For atmosphere:
Net longwave radiation =
amount emitted : = -156
amount absorbed ?

49. Longwave emission from earth12
104 23
116 47

50. Net radiation Difference between amount emitted and amount absorbed.
B. Longwave
1. For atmosphere:
Net longwave radiation =
amount emitted : = -156
amount absorbed : +104
Net longwave radiation for atmosphere = 104 – 156 = -52

51. Net radiation B. Longwave 2. For surface: Net longwave radiation =
amount emitted ?

52. Longwave emission from earth12
104 23
116 47

53. Net radiation B. Longwave 2. For surface: Net longwave radiation =
amount emitted : - 116
amount absorbed ?

54. 12
Longwave emission
from atmosphere
104 23 58 98
116 47

55. Net radiation B. Longwave 2. For surface: Net longwave radiation =
amount emitted : -116
amount absorbed : +98
Net longwave radiation for surface = 98 – 116 = -18

56. Net all wave radiation includes long and shortwave.
Atmosphere:
= - 29
Surface :
= + 29

58. The balancing act…..
The net deficit of the atmosphere equals the net surplus of the earth’s surface.

59. But, there’s more…
If radiation were only means of transferring energy, our feet would scorch and our heads would freeze.

60. Energy transfer mechanisms (other than radiation):
Conduction: transfer of heat from one molecule to another by collision
Only a few cm of air above surface are heated by conduction
Convection : transfer of heat from one area to another by physical mixing
Warm air near surface transfers heat upward by mixing

61. conduction and convection…
Temperature gradient in upper few centimeters of soil
Energy conducted downward during day; upward at night
Temperature gradient in laminar boundary layer of air (few mm. thick)
A third process transferring heat from surface to atmosphere is EVAPORATION, a latent heat transfer mechanism.

62. Sensible Heat: can be sensed with thermometer.
Types of heat energy:
Sensible Heat: can be sensed with thermometer.
Latent Heat : heat released or absorbed during phase changes (solid-liquid-gas)
Energy used to change phase is not lost
energy evaporating water is “held” in water vapor to be released in reverse process
Some of energy received at surface is used to evaporate water rather than to raise surface temp. even more.

63. 24 5

64. Surplus of 29 units of net all wave at surface
5 are transferred to atmosphere by sensible heat transfer of conduction and convection
Sensible heat travels by conduction through laminar boundary layer and is dispersed upward by convection
24 are transferred to the atmosphere as latent heat (evaporation)
Evaporation of water makes energy available to atmosphere that otherwise would have warmed surface

66. “Natural” Greenhouse effect
Maintains earth’s mean surface temperature at 59°F (15°C)
Otherwise -4°F (-20°C)
Caused by counterradiation:
greenhouse gases in atmosphere absorb longwave emitted from earth’s surface, some of which is radiated back to earth.

67. Greenhouse analogy
Glass of greenhouse allows shortwave radiation IN, but does not allow escape of longwave
BUT…UNLIKE the atmosphere, a greenhouse prevents loss of heat by convection