Atmospheric Radiative Transfer PHYS 721 “The ocean sunglint in a dusty/polluted day” Picture by Yoram J. Kaufman - 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

Frequency ( =  /2  ) Wavelength ( =c/ ) The EM spectrum Our domain of interest

Two important BB laws Wien’s 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: F BB =  B T 4

You’ll often see normalized plots of the Planck function (see also last solved problem of the notes) Normalization of Planck functions

Results from the TSI instrument on Sorce 1357 W/m

TSI SORCE

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.

Solar Spectrum at different levels:

Interactions between Aerosols and Molecules with Radiation: (a) Black Body Curves (  m) Aerosol Extinction Coef. (m -1 ) 5780 K 255 K N O R M A LI Z A D F L u X ABSORPTION%ABSORPTION% Large Aerosols Small Aerosols

Absorption spectra of atmospheric gases CH 4 CO 2 N2ON2O H2OH2O O 2 & O 3 atmosphere ABSORPTIVITY WAVELENGTH (micrometers) IR Windows Infrared Visible UV H 2 O dominates >15 µm

Average Solar Radiation intercepted by Earth and Distributed over its Surface Surface Area 4  R 2 Solar Constant So R Area Intercepting Solar Radiation =  R 2  R 2 So = 4  R 2 = So/4

Simple Climate Model: Earth as a Black Body and no Atmosphere In equilibrium  ·T e 4 = T e = +5.8 o C All the solar radiation is absorbed and re-emitted by the surface So = 1370 W/m 2 = So/4 = W/m 2  = 5.669x10 -4 Wm -2 deg -4

A· In equilibrium  ·Te 4 = (1-A) · (1-A) A=0.3 Simple Climate Model: Earth with Albedo = 0.3 and no Atmosphere T e = -17 o C

Simple Climate Model: Earth with Atmosphere and Albedo = 0.3 So A· In equilibrium  ·Te 4 = 2·(1-A) · Absorption and emission in the atmosphere: greenhouse gases, clouds, aerosols… Atmospheric Scattering: Molecules, aerosols, clouds, and surface. (1-A) A=0.3 T e = +30 o C

A TOA < A SUP Warming AEROSOL plus Surface Albedo Effect A TOA = A SUP Balance A TOA > A SUP Cooling Large contrast in radiative forcing due to the combination of surface and aerosol properties Smoke – Instantaneous Direct Radiative Forcing over Varying Surface Albedo (Cuiaba – Brazil) for  = 1

R particles = R SUP Balance between Absorption and Scattering R particles << R SUP Surface Darkening or Warming R particles > R SUP Surface Brightening or Cooling Large contrast in radiative forcing due to the combination of surface and aerosol properties Smoke – Instantaneous Direct Radiative Forcing over Varying Surface Albedo (Cuiaba – Brazil) for  = 1 AEROSOL plus Surface Albedo Effect

AEROSOL DIRECT RADIATIVE FORCING R particles = R SUP Balance between Absorption and Scattering R particles << R SUP Surface Darkening or Warming R particles > R SUP Surface Brightening or Cooling Large contrast in radiative forcing due to the combination of surface and aerosol properties Smoke – Instantaneous Direct Radiative Forcing over Varying Surface Albedo (Cuiaba – Brazil) for  = W/m 2

Hansen, 2000: Separation of the BC forcing from other aerosol types Jacobson, Radiative Forcing: BC = Wm 2 CH4 = Wm 2 CO2 = Wm 2 Andreae, 2001: 1/3 of carbon- cycle resources should go to Black Carbon studies “The Dark Side of Aerosols” (Andreae, A. 2001) or The Dark Side of the Aerosol Forcing Aerosols containing black carbon Aerosols not containing black carbon Hansen et al. [2000] 50 yrs climate change scenario