Radiation: WHY CARE ???

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

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Sun
Earth
Y-axis Spectral radiance, aka monochromatic
intensity units watts/(m2sterwavelength)
Blackbody curves provide the envelope to Sun,
earth emission
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• 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

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Wiens 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
• gt max. wavelength Sun 0.475 micron (blue) ,
  max wavelength Earth 14 micron.

Explains spectral Distribution of radiation
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Energy absorbed from Sun establishes Earths mean
T
Energy inenergy out FsunpiR2earth
4piR2earth(1.-albedo)(sigmaT4earth)
global albedo 0.3 gt Tearth
255 K
Fsun 1368 W m-2 _at_ earth
This Wiens law explains why earths radiation
is in the infrared
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Sun
Earth
visible
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Depth of penetraion into earths atmosphere of
solar UV
1 Angstrom 10-10 m.
Photoionization _at_ wavelengths lt 0.1 micron (1000
angstroms) Photodissociation _at_ wavelengths lt 0.24
microns O2 -gt 2O Ozone dissociation _at_wavelengths
lt 0.31 micron
Visible spectrum 0.39 to 0.76 micron
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To understand Earths emission need.. Kirchoffs
Law emissivity absorptivity, for a given
wavelength
Also called Local Thermodynamic Equilibrium
(LTE) Holds up to 60 km
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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 gt Outgoing IR at surface, with
absorbing atmosphere gt outgoing IR with no
atmosphere
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ManabeStrickler, 1964
Note ozone, surface T
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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
Extinctionscatteringabsorption
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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 2piscatterer
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)
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Rayleigh scattering solar scattering off of gases
proportional to (1/???
R0.1 ?m
R10-4 ?m
Gas (air)
aerosol
Solar scattering
Cloud drops
Mie scattering 1 lt x lt 50
R1 ?m
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Mie scattering solar scattering off of cloud
water and ice microwave scattering off of
precipitation
Index of refraction is complex real part
scattering imagery componentabsorption
mreal1.33 for water, 1.3 for ice
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water
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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
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Secondary rainbox at 51 degrees
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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