Radiation Balance

1. Radiation Balance

2. In atmosphere, radiation can be… • transmitted • absorbed • reflected

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

4. 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.

5. 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

6. Earth’s average albedo, March

7. 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 ”.

8. Clip about polar vortex

9. Albedos of various surfaces: • Earth’s surface 0.31 (31%) • Cumulonimbus clouds 0.9 (90%) • Stratocumulus clouds 0.6 (60%) • Cirrus clouds 0.4 -0.5 (40 – 50%) • Fresh snow 0.8 – 0.9 (80 – 90%) • Melting snow 0.4 – 0.6 (40 – 60%) • Sand 0.3 – 0.35 (30 – 35%) • Grain crops 0.18 – 0.25 (18 – 25%) • Deciduous forest 0.15 – 0.18 (15 – 18%) • Coniferous forest 0.09 – 0.15 (9 – 15%) • Tropical rainforest 0.07 – 0.15 (7 – 15%) • Water bodies 0.06 – 0.10 (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. Optical path at point of tangency is 20 x as long as at SSP.

17. 2. Mie • Caused by aerosols: • 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

20. 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

23. Radiation Balance • Balance maintained by earth and atmosphere between incoming and outgoing radiation. • Averaged over entire earth over entire year.

24. “100%” incoming 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%)

25. 70% absorbed by: • Ground (47%) • Gases, dust in atmosphere • Clouds 70% (23%) Shortwave!

26. 30% reflected by • Ground (7%) • Clouds (17%) • Scattered by atmosphere (6%) 30% Albedo of earth/atmosphere = 30% Shortwave!

28. 23 Shortwave absorption 47

29. Earth absorbs far more solar radiation than atmosphere.

30. Energy is transferred between atmosphere and earth. When Shortwave solar radiation is absorbed, longwave radiation is emitted. • Earth’s surface emits 116 % longwave !

31. 116 units (%) of longwave emitted from earth surface to atmosphere 104absorbed by atmosphere 12 transmitted to space 12 Space 104 Atmosphere 116 Earth’s surface: ABSORBS SHORTWAVE, EMITS LONGWAVE Surface

32. Longwave emission from earth 12 104 23 116 47

34. 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

35. 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 (98 + 58=156) exceeds amount absorbed (104)

37. 12 Longwave emission from atmosphere 104 23 58 98 116 47

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

40. 23 Shortwave absorption 47

41. 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

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

43. 23 Shortwave absorption 47

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

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

46. 12 Longwave emission from atmosphere 104 23 58 98 116 47

47. Net radiation • Difference between amount emitted and amount absorbed. B. Longwave 1. For atmosphere: Net longwave radiation = amount emitted : -58 + -98 = -156 amount absorbed ?

48. Longwave emission from earth 12 104 23 116 47

49. Net radiation • Difference between amount emitted and amount absorbed. B. Longwave 1. For atmosphere: Net longwave radiation = amount emitted : -58 + -98 = -156 amount absorbed : +104 Net longwave radiation for atmosphere = 104 – 156 = -52

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