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6 f-radiation-absorptions

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Parameterisation of Atmospheric absorptions of solar radiation. As used in the class of Hydrology at the University of Trento

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6 f-radiation-absorptions

  1. 1. Riccardo Rigon IlSole,F.Lelong,2008,ValdiSella Solar Radiation Absorptions crossing the Atmosphere
  2. 2. R. Rigon Atmosphere is a gray body • The blackbody is an ideal object that absorb all the radiative energy it receives • Real objects (bodies, “gray bodies”) are not capable of absorbing all radiation. • To understand the difference between a blackbody and a gray body we need to analyse the interactions between a surface and the electromagnetic radiation incident onto it. 2 Absorption and transmission of short wave radiation
  3. 3. R. Rigon Atmospheric absorption 3 Radiation passes quite freely through the Earth’s atmosphere and it warms the surfaces of seas and oceans. A portion of between 45% and 50% of the incident radiation onto the Earth reaches the ground Absorption and transmission of short wave radiation
  4. 4. R. Rigon Radiation transmitted Radiation reflected Shortwave Radiation budget The solar radiation penetrates the atmosphere, and it is transferred towards the ground, after being reflected and scattered. 4 Absorption and transmission of short wave radiation
  5. 5. R. Rigon the incoming radiation equals the reflected one plus the absorbed plus the transmitted 5 Shortwave Radiation budget S It should not be forgot that the radiation budget is an energy budget, for which Radiation absorbed Absorption and transmission of short wave radiation
  6. 6. R. Rigon 6 S Energy absorbed by atmosphere Transmitted radiation Corrected Solar constant Solar radiation reflected back to space This budget can be apply to any slice of the atmosphere Shortwave Radiation budget Absorption and transmission of short wave radiation
  7. 7. R. Rigon • is the reflection coefficient, said atmospheric reflectivity (albedo) • is the transmission coefficient, said atmospheric transmissivity • is the absorption coefficient, said atmospheric absorptivity Coefficients The following coefficients can also be defined 7 Absorption and transmission of short wave radiation
  8. 8. R. Rigon Energy conservation: Which is, indeed, valid for reflectivity, transmissivity and absorptivity of any other body implies that reflectivity, transmissivity and absorptivity sum to one: 8 Shortwave Radiation budget Absorption and transmission of short wave radiation
  9. 9. R. Rigon We just forget for a moment this. It will be splitted into two parts: one depending on diffuse radiation and another on cloud cover 9 S Shortwave Radiation budget Absorption and transmission of short wave radiation
  10. 10. R. Rigon Atmosphere is pretty transparent: which means that we can, as a first approximation, neglect it (atmosphere is heated from below) 10 S Shortwave Radiation budget Absorption and transmission of short wave radiation
  11. 11. R. Rigon In any case let’s concentrate on the transmitted radiation This can be decomposed into two parts: direct and diffuse solar radiation 11 Shortwave Radiation budget S Absorption and transmission of short wave radiation
  12. 12. R. Rigon Evidently, for simmetry is also composed by reflected and diffuse solar radiation 12 Shortwave Radiation budget S Absorption and transmission of short wave radiation
  13. 13. R. Rigon 5 Diffuse radiation comes from scattering Incident solar radiation strikes gas molecules, dust particles, and pollutants, ice, cloud drops and the radiation is scattered. Scattering causes diffused radiation. Two types of light diffusion can be distinguished: Mie scattering Rayleigh scattering Absorption and transmission of short wave radiation
  14. 14. R. Rigon Rayleigh Scattering •The impact of radiation with air molecules smaller than λ/π causes scattering (Rayleigh scattering) the entity of which depends on the frequency of the incident wave according to a λ-4 type relation. •In the atmosphere, the wavelengths corresponding to blue are scattered more readily than others. incident radiation diffuse radiation transmitted radiation 14 Absorption and transmission of short wave radiation
  15. 15. R. Rigon •When in the atmosphere there are particles with dimensions greater than 2 λ/π (gases, smoke particles, aerosols, etc.) there is a scattering phenomenon that does not depend on the wavelength, λ, of the incident wave (Mie scattering). •This phenomenon can be observed, for example, in the presence of clouds. Mie Scattering 15 incident radiation diffuse radiation transmitted radiation Absorption and transmission of short wave radiation
  16. 16. R. Rigon Diffused Light Scattering selectively eliminates the shorter visible wavelengths, leaving the longer wavelengths to pass. When the Sun is on the horizon, the distance travelled by a ray within the atmosphere is five or six times greater than when the Sun is at the Zenith and the blue light has practically been completely eliminated. 16 Absorption and transmission of short wave radiation
  17. 17. R. Rigon Tilt of the Earth’s axis and atmospheric effects The tilt of the earth’s axis and atmospheric effects together affect the amount of radiation that reaches the ground. 17 Absorption and transmission of short wave radiation
  18. 18. R. Rigon 18 One way to take into account of absorption Would be to run a full model of atmospheric transmission (e.g. Liou, 2002). However hydrologists prefer to use parameterizations, and the concept of atmospheric transmissivity. Absorption and transmission of short wave radiation
  19. 19. R. Rigon Solar radiation transmitted to the ground under clear sky conditions Finally: Fraction of direct solar radiation included between the considered wavelengths Transmittance of the atmosphere Correction due to elevation of the site Corripio,2002 19 S Absorption and transmission of short wave radiation
  20. 20. R. Rigon We do not enter in the details of how and are determined. Please look, for instance, at Formetta et al., 2012 Solar radiation transmitted to the ground under clear sky conditions 20 S Absorption and transmission of short wave radiation

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