{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Lecture6

# Lecture6 - Chapter 5 Radiative Properties of Natural...

This preview shows pages 1–4. Sign up to view the full content.

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

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
2 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 system (surface + clouds).
3 5 Lambertian Surface: Reflection is independent of angle (isotropic in “upward” hemisphere) Petty Figure 5.3. Polar plots – distance from

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}