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Radiation: WHY CARE ??? the ultimate energy source, driver for the general circulation

Radiation: WHY CARE ??? the ultimate energy source, driver for the general circulation usefully applied in remote sensing (more and more). Sun. Earth. Y-axis: Spectral radiance, aka monochromatic intensity units: watts/(m^2*ster*wavelength).

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Radiation: WHY CARE ??? the ultimate energy source, driver for the general circulation

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  1. Radiation: WHY CARE ??? • the ultimate energy source, driver for • the general circulation • usefully applied in remote sensing (more • and more)

  2. Sun Earth Y-axis: Spectral radiance, aka monochromatic intensity units: watts/(m^2*ster*wavelength) Blackbody curves provide the envelope to Sun, earth emission

  3. All objects radiate • Blackbody: absorbs all, reflects none, emits isotropically • Blackbody radiation observed first, only later described • (Max) Planck function • Integrated over all wavelengths: E=T4 ; x 10-8 W m-2 K-4; E is called irradiance, flux density. Units of W/m^2

  4. Wien’s Law • wavelength of the peak emission from dE/d(wavelength) = 0 • Wavelengthmax (in microns) = 2897/T (in Kelvin) • For Sun, = 6000 K, for Earth = 255 K • => max. wavelength Sun = 0.475 micron (blue) , max wavelength Earth ~ 14 micron. Explains spectral Distribution of radiation

  5. Energy absorbed from Sun establishes Earth’s mean T Energy in=energy out Fsun*pi*R2earth = 4*pi*R2earth*(1.-albedo)*(sigma*T4earth) global albedo ~ 0.3 => Tearth = 255 K Fsun= 1368 W m-2 @ earth This + Wien’s law explains why earth’s radiation is in the infrared

  6. Sun Earth visible

  7. Depth of penetraion into earth’s atmosphere of solar UV 1 Angstrom= 10-10 m. Photoionization @ wavelengths < 0.1 micron (1000 angstroms) Photodissociation @ wavelengths < 0.24 microns: O2 -> 2O Ozone dissociation @wavelengths < 0.31 micron Visible spectrum 0.39 to 0.76 micron

  8. To understand Earth’s emission need….. Kirchoff’s Law: emissivity = absorptivity, for a given wavelength Also called Local Thermodynamic Equilibrium (LTE) Holds up to 60 km

  9. High solar transmissivity + low IR transmissivity = Greenhouse effect 1. 2. Consider multiple isothermal layers, each in radiative equilibrium. Each layer, opaque in the infrared, emits IR both up and down, while solar is only down Top of atmosphere: Fin = Fout incoming solar flux = outgoing IR flux At surface, incoming solar flux + downwelling IR = outgoing IR => Outgoing IR at surface, with absorbing atmosphere > outgoing IR with no atmosphere

  10. Manabe&Strickler, 1964: Note ozone, surface T

  11. Radiation transmits through an atmospheric layer According to: • I = intensity • = air density r = absorbing gas amount k =mass extinction coeff. rk = volume extinction coeff. Path length ds Inverse length unit Extinction=scattering+absorption

  12. Whether/how solar radiation scatters when it impacts gases,aerosols,clouds,the ocean surface depends on 1. ratio of scatterer size to wavelength: Size parameter x = 2*pi*scatterer radius/wavelength Sunlight on a flat ocean Sunlight on raindrops X large X small Scattering neglected IR scattering off of air, aerosol Microwave scattering off of clouds Microwave (cm)

  13. Rayleigh scattering: solar scattering off of gases proportional to (1/ R=0.1m R=10-4 m Gas (air) aerosol Solar scattering Cloud drops Mie scattering: 1 < x < 50 R=1m

  14. Mie scattering: solar scattering off of cloud water and ice microwave scattering off of precipitation Index of refraction is complex: real part = scattering imagery component=absorption mreal=1.33 for water, 1.3 for ice

  15. water

  16. Mie scattering: algorithms for spherical drops work very well. Calculated radiance depends on drop size, wavelength, indx of refraction Backward scattering Back towards viewer Forward scattering In direction of light

  17. Secondary rainbox at 51 degrees

  18. Glory: around the shadow of your head, or an airplane, At the anti-solar point. - need small drops “Heiligenschein” Corona: often seen around the moon

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