Solar Radiation Absorptions crossing the Atmosphere Il Sole, F. Lelong, 2008, Val Selladi
Riccardo Rigon Absorption and transmission of short wave radiation
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.
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R. Rigon Absorption and transmission of short wave radiation Atmospheric absorption
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 3
R. Rigon Absorption and transmission of short wave radiation Shortwave Radiation budget
Radiation reflected
The solar radiation penetrates the atmosphere, and it is transferred towards the ground, after being reflected and scattered.
Radiation transmitted 4
R. Rigon Absorption and transmission of short wave radiation Shortwave Radiation budget
S It should not be forgot that the radiation budget is an energy budget, for which
the incoming radiation equals the reflected one plus the absorbed plus Radiation the transmitted absorbed
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R. Rigon Absorption and transmission of short wave radiation Shortwave Radiation budget
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Transmitted radiation
Solar radiation reflected back to space Energy absorbed by atmosphere Corrected Solar constant
This budget can be apply to any slice of the atmosphere
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R. Rigon Absorption and transmission of short wave radiation
Coefficients
The following coefficients can also be defined
• is the transmission coefficient, said atmospheric transmissivity
• is the reflection coefficient, said atmospheric reflectivity (albedo)
• is the absorption coefficient, said atmospheric absorptivity
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R. Rigon Absorption and transmission of short wave radiation Shortwave Radiation budget
Energy conservation:
implies that reflectivity, transmissivity and absorptivity sum to one:
Which is, indeed, valid for reflectivity, transmissivity and absorptivity of any other body
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R. Rigon Absorption and transmission of short wave radiation Shortwave Radiation budget
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We just forget for a moment this. It will be splitted into two parts: one depending on diffuse radiation and another on cloud cover
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R. Rigon Absorption and transmission of short wave radiation Shortwave Radiation budget
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Atmosphere is pretty transparent: which means that we can, as a first approximation, neglect it (atmosphere is heated from below)
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R. Rigon Absorption and transmission of short wave radiation Shortwave Radiation budget
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In any case let’s concentrate on the transmitted radiation
This can be decomposed into two parts: direct and diffuse solar radiation
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R. Rigon Absorption and transmission of short wave radiation Shortwave Radiation budget
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Evidently, for simmetry
is also composed by reflected and diffuse solar radiation
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R. Rigon Absorption and transmission of short wave radiation
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
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R. Rigon Absorption and transmission of short wave radiation
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
R. Rigon Absorption and transmission of short wave radiation
Mie Scattering
•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).
incident radiation
diffuse radiation
transmitted radiation
•This phenomenon can be observed, for example, in the presence of clouds.
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R. Rigon Absorption and transmission of short wave radiation
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.
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R. Rigon Absorption and transmission of short wave radiation
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.
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R. Rigon Absorption and transmission of short wave radiation
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.
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R. Rigon Absorption and transmission of short wave radiation Solar radiation transmitted to the ground under clear sky conditions
S Correction due to elevation of the site Finally: Corripio, 2002 Corripio,
Transmittance of the atmosphere
Fraction of direct solar radiation included between the considered 19 wavelengths
R. Rigon Absorption and transmission of short wave radiation
Solar radiation transmitted to the ground under clear sky conditions
S We do not enter in the details of how
and
are determined. Please look, for instance, at Formetta et al., 2012
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R. Rigon