Atmospheric Radiative Transfer

1. 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 day”Picture by Yoram J. Kaufman

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

4. 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: FBB=BT4

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

7. Results from the TSI instrument on Sorce 1362 W/m2 1357 2007 2003

8. TSI SORCE 2007

9. 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.

10. http://climate.gsfc.nasa.gov/viewImage.php?id=158

11. Solar Spectrum at different levels: http://lasp.colorado.edu/sorce/instruments/sim/sim_science.htm

12. Small Aerosols Large Aerosols 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 %

13. Absorption spectra of atmospheric gases Visible Infrared UV CH4 N2O O2 & O3 CO2 H2O ABSORPTIVITY atmosphere WAVELENGTH (micrometers) IR Windows H2O dominates >15 µm

14. Surface Area 4pR2 R Area Intercepting Solar Radiation = pR2 Solar Constant So Average Solar Radiation intercepted by Earth and Distributed over its Surface pR2So = 4pR2<So> <So> = So/4

15. Simple Climate Model:Earth as a Black Body and no Atmosphere In equilibrium s·Te4 = <So> Te = +5.8oC <So> So = 1370W/m2 <So> = So/4 = 342.5W/m2 s = 5.669x10-4Wm-2deg-4 All the solar radiation is absorbed and re-emitted by the surface

16. Simple Climate Model:Earth with Albedo = 0.3 and no Atmosphere In equilibrium s·Te4 = (1-A) ·<So> <So> Te = -17oC (1-A)<So> A·<So> (1-A)<So> A=0.3

17. Simple Climate Model:Earth with Atmosphere and Albedo = 0.3 In equilibrium s·Te4 = 2·(1-A) ·<So> (1-A)<So> So Te = +30oC Absorption and emission in the atmosphere: greenhouse gases, clouds, aerosols… A·<So> (1-A)<So> A=0.3 Atmospheric Scattering: Molecules, aerosols, clouds, and surface. (1-A)<So>

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

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

20. AEROSOL DIRECT RADIATIVE FORCING Smoke – Instantaneous Direct Radiative Forcing over Varying Surface Albedo (Cuiaba – Brazil) for t = 1 Rparticles << RSUP Surface Darkening or Warming -95 W/m2 Rparticles = RSUP Balance between Absorption and Scattering +10 Rparticles > RSUP Surface Brightening or Cooling Large contrast in radiative forcing due to the combination of surface and aerosol properties

21. “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]