Lecture6 - Chapter 5 Radiative Properties of Natural...

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1 1 Chapter 5 Radiative Properties of Natural Surfaces 2 Chapter 5. Radiative Properties of Natural Surfaces Absorptivity (a, unitless): Fraction of incident radiation that is absorbed. Reflectivity (r, unitless): Fraction of incident radiation that is reflected. Since these depend on wavelength, refer to them as a λ and r λ . Since they depend on angle of incident radiation, write them as fn( θ , Φ ). For opaque surface (no transmission): a λ ( θ , Φ ) + r λ ( θ , Φ ) = 1 If the reflection is isotropic (independent of θ and Φ ) then the reflected monochromatic (spectral) irradiance can be written as: F λ ,r = r λ F λ ,o [W m -2 nm -1 ] (F λ ,o = incident, F λ ,r = reflected) And the absorbed irradiance is: F λ ,a = F λ ,o -F λ ,r = F λ ,o -r λ F λ ,o = (1 – r λ ) F λ ,o = a λ F λ ,o [W m -2 nm -1 ] Petty 5.1 Petty 5.2 Petty 5.3
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3 Petty Figure 5.2. Shortwave reflectance spectra of various natural surface types. Where, in the visible, does chlorophyll absorb? www.arborsci.com/CoolStuff/cool22.htm 4 Graybody Approximation Approximate the reflectivity of a surface as independent of wavelength, so the surface appears “gray”. The effective graybody reflectivity, r_bar (unitless), and absorptivity, a_bar (unitless), are defined as (Petty 5.4): r a F F r i r = 1 Here F r is the total reflected irradiance (flux) in some broad wavelength interval (W m -2 ). F i is the total incident irradiance (flux) over this interval (W m -2 ). The graybody approximation is commonly used in problems dealing with the Earth’s radiation budget. Slightly more complex – Divide the reflectivity and absorptivity into shortwave and longwave: lw i lw r lw lw sw i sw r sw sw a F F r a F F r = = = = 1 1 , , , , r = “albedo” . For most terrestrial surfaces, r lw ~ 0, but r sw is variable. Average shortwave albedo ~ 0.1 for the Earth’s surface, and 0.3 for the Earth
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This note was uploaded on 01/19/2011 for the course ATOC 5235 taught by Professor Randell during the Fall '10 term at Colorado.

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Lecture6 - Chapter 5 Radiative Properties of Natural...

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