Atmospheric Radiative Transfer

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PHYS 721
http//userpages.umbc.edu/martins/PHYS721/
Atmospheric Radiative Transfer

• Motivation, applications and issues
• Definitions and Radiation Quantities
• Thermal Emission/Absorption Basics
• Solar and Terrestrial Spectra
• From Single to Multiple Scattering
• The Radiative Transfer Equations Theory and
  Solution Methods
• Absorption and Emission by Gas Molecules
• Radiation and Climate Issues

The ocean sunglint in a dusty/polluted
dayPicture by Yoram J. Kaufman
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The EM spectrum
Frequency (??/2?)
Wavelength (?c/?)
Our domain of interest
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Two important BB laws

• Wiens law Wavelength (frequency, etc.) of
  maximum emission
• ?max(µm)3000/T
• Location of maximun depends on representation
  (see solved problem at end of notes). Equal
  wavelength intervals do not correspond to equal
  frequency intervals

Stefan-Boltzmann law Total (wavelength-integrated
) emitted flux FBB?BT4
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Normalization of Planck functions
Youll often see normalized plots of the Planck
function (see also last solved problem of the
notes)
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Results from the TSI instrument on Sorce
1362
W/m2
1357
2007
2003
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TSI SORCE
2007
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Special Note on TIM TSI Data

• The TIM's measured value of TSI at 1 AU is lower
  than that reported by other TSI-measuring
  instruments an upcoming solar minimum value of
  1361 W/m2 is estimated from the current TIM data.
  This is due to unresolved differences between TSI
  instruments. The TIM measures TSI values 4.7 W/m2
  lower than the VIRGO and 5.1 W/m2 lower than
  ACRIM III.
• This difference exceeds the 0.1 stated
  uncertainties on both the ACRIM and VIRGO
  instruments. Differences between the various data
  sets are solely instrumental and will only be
  resolved by careful and detailed analyses of each
  instrument's uncertainty budget. We report only
  the TSI measurements from the TIM, and make no
  attempt to adjust these to other TSI data
  records.
• The TIM TSI data available are based on
  fundamental ground calibrations done at CU/LASP,
  NIST, and NASA. On-orbit calibrations measure the
  effects of background thermal emission,
  instrument sensitivity changes, and electronic
  gain. The TIM TSI data products have been
  corrected for instrument sensitivity and
  degradation, background thermal emission,
  instrument position and velocity, and electronic
  gain. The TIM relies on several component-level
  calibrations, as no calibration source or
  detector is available with the level of accuracy
  desired for this instrument -- a level of
  accuracy nearly 10 times better than that
  previously attempted for space-based radiometry.

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http//climate.gsfc.nasa.gov/viewImage.php?id158
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Solar Spectrum at different levels
http//lasp.colorado.edu/sorce/instruments/sim/sim
_science.htm
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Interactions between Aerosols and Molecules with
Radiation
Aerosol Extinction Coef. (m-1)
(a) Black Body Curves
NORMALIZAD FLuX
255 K
5780 K
? (?m)
ABSORP T I ON
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Absorption spectra of atmospheric gases
Visible
Infrared
UV
CH4
N2O
O2 O3
CO2
H2O
ABSORPTIVITY
atmosphere
WAVELENGTH (micrometers)
IR Windows
H2O dominates gt15 µm
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Average Solar Radiation intercepted by Earth and
Distributed over its Surface
pR2So 4pR2ltSogt ltSogt So/4
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Simple Climate Model Earth as a Black Body and
no Atmosphere
In equilibrium sTe4 ltSogt Te 5.8oC
ltSogt
So 1370W/m2 ltSogt So/4 342.5W/m2 s
5.669x10-4Wm-2deg-4
All the solar radiation is absorbed and
re-emitted by the surface
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Simple Climate Model Earth with Albedo 0.3
and no Atmosphere
In equilibrium sTe4 (1-A) ltSogt
ltSogt
Te -17oC
(1-A)ltSogt
AltSogt
(1-A)ltSogt
A0.3
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Simple Climate Model Earth with Atmosphere and
Albedo 0.3
In equilibrium sTe4 2(1-A) ltSogt
(1-A)ltSogt
So
Te 30oC
Absorption and emission in the atmosphere
greenhouse gases, clouds, aerosols
AltSogt
(1-A)ltSogt
A0.3
Atmospheric Scattering Molecules, aerosols,
clouds, and surface.
(1-A)ltSogt
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AEROSOL plus Surface Albedo Effect
Smoke Instantaneous Direct Radiative Forcing
over Varying Surface Albedo (Cuiaba Brazil) for
t 1
ATOA lt ASUP Warming
ATOA ASUP Balance
ATOA gt ASUP Cooling
Large contrast in radiative forcing due to the
combination of surface and aerosol properties
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AEROSOL plus Surface Albedo Effect
Smoke Instantaneous Direct Radiative Forcing
over Varying Surface Albedo (Cuiaba Brazil) for
t 1
Rparticles ltlt RSUP Surface Darkening or Warming
Rparticles RSUP Balance between Absorption and
Scattering
Rparticles gt RSUP Surface Brightening or Cooling
Large contrast in radiative forcing due to the
combination of surface and aerosol properties
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AEROSOL DIRECT RADIATIVE FORCING
Smoke Instantaneous Direct Radiative Forcing
over Varying Surface Albedo (Cuiaba Brazil) for
t 1
Rparticles ltlt RSUP Surface Darkening or Warming
-95
W/m2
Rparticles RSUP Balance between Absorption and
Scattering
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Rparticles gt RSUP Surface Brightening or Cooling
Large contrast in radiative forcing due to the
combination of surface and aerosol properties
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The Dark Side of Aerosols(Andreae, A.
2001)or The Dark Side of the Aerosol Forcing
50 yrs climate change scenario

• Hansen, 2000 Separation of the BC forcing from
  other aerosol types
• Jacobson, 2001 - Radiative Forcing
• BC 0.55 Wm2
• CH4 0.47 Wm2
• CO2 1.56 Wm2
• Andreae, 2001 1/3 of carbon-cycle resources
  should go to Black Carbon studies

Aerosols containing black carbon
Aerosols not containing black carbon
Hansen et al. 2000
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