ATMOSPHERIC RADIATION

1. ATMOSPHERIC RADIATION

2. RADIATION & FUNDAMENTAL RELATIONSHIPS Electromagnetic energy at wavelength () has associated frequency (f) and photon energy (E): Also often use wavenumbers notation: h=6.62x10-34 Js c=3.0x108 m/s From: Heald

3. EMISSION OF RADIATION • Radiation is energy transmitted by electromagnetic waves; all objects emit radiation • One can measure the radiation flux spectrum emitted by a unit surface area of object: Here DF is the radiation flux emitted in [l, l+Dl] is the flux distribution function characteristic of the object Total radiation flux emitted by object: From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

4. BLACKBODY RADIATION(黑體輻射) • 黑體(blackbodies )就是能吸收100% 入射輻射的物體 • For blackbodies,fl is given by the Planck function: Function of T only! Often denoted B(l,T) F = sT 4 • = 2p 5k 4/15c2h3is the Stefan-Boltzmann constant lmax = hc/5kTWien’s law lmax From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

5. Kirchhoff’s law • The Planck blackbody formulation for the emission of radiation is generalizable to all objects using Kirchhoff’s law. • This law states that if an object absorbs radiation of wavelength λ with an efficiency ε λ, then it emits radiation of that wavelength at a fraction ε λ of the corresponding blackbody emission at the same temperature. From: Jacob

6. SOLAR RADIATION SPECTRUM: blackbody at 5800 K From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

7. TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra for different T Scene over Niger valley, N Africa From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

8. = 255 K RADIATIVE EQUILIBRIUM FOR THE EARTH Solar radiation flux intercepted by Earth = solar constant FS = 1370 W m-2 Radiative balance c effective temperature of the Earth: where A is the albedo (reflectivity) of the Earth From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

9. GREENHOUSE EFFECT:absorption of terrestrial radiation by the atmosphere • Major greenhouse gases: H2O, CO2, CH4, O3, N2O, CFCs,… • Not greenhouse gases: N2, O2, Ar, … From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

10. SIMPLE MODEL OF GREENHOUSE EFFECT IR VISIBLE • Energy balance equations: • Earth system Incoming solar Reflected solar Transmitted surface • Atmospheric layer Solution: To=288 K e f=0.77 T1 = 241 K Atmospheric emission Atmospheric layer (T1) abs. eff. 0for solar (VIS) f for terr. (near-IR) Atmospheric emission Surface emission Earth surface (To) Absorption efficiency 1-A in VISIBLE 1 in IR From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

11. RADIATIVE AND CONVECTIVE INFLUENCESON ATMOSPHERIC THERMAL STRUCTURE In a purely radiative equilibrium atmosphere T decreases exponentially with z, resulting in unstable conditions in the lower atmosphere; convection then redistributes heat vertically following the adiabatic lapse rate From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

12. EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

13. TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra emitted from different altitudes at different temperatures From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

14. Fin Fout RADIATIVE FORCING OF CLIMATE CHANGE Reflected solar radiation (surface, air, aerosols, clouds) IR terrestrial radiation ~ T4; absorbed/reemitted by greenhouse gases, clouds, absorbing aerosols Incoming solar radiation EARTH SURFACE • Stable climate is defined by radiative equilibrium: Fin = Fout • Instantaneous perturbation e Radiative forcing DF = Fin– Fout Increasing greenhouse gases gDF > 0 positive forcing • The radiative forcing changes the heat content H of the Earth system: eventually leading to steady state where Tois the surface temperature and l is a climate sensitivity parameter • Different climate models give l = 0.3-1.4 K m2 W-1, insensitive to nature of forcing; • differences between models reflect different treatments of feedbacks http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

15. CO2 climate sensitivity • CO2 climate sensitivity has a component directly due to radiative forcing by CO2, and a further contribution arising from feedbacks, positive and negative. "Without any feedbacks, a doubling of CO2 (which amounts to a forcing of 3.7 W/m2) would result in 1 °C global warming, which is easy to calculate and is undisputed. The remaining uncertainty is due entirely to feedbacks in the system, namely, the water vapor feedback, the ice-albedo feedback, the cloud feedback, and the lapse rate feedback; addition of these feedbacks leads to a value of the sensitivity to CO2 doubling of approximately 3 °C ± 1.5 °C, which corresponds to a value of λ of 0.8 K/(W/m2).

16. CLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSE Positive feedback from water vapor causes rough doubling of l From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

17. CLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDS Clouds reflect solar radiation (DA > 0) g cooling; …but also absorb IR radiation (Df > 0) g warming WHAT IS THE NET EFFECT? sTcloud4 < sTo4 sTcloud4≈ sTo4 Tcloud≈ To convection sTo4 sTo4 To HIGH CLOUD: WARMING LOW CLOUD: COOLING From: Jacob http://acmg.seas.harvard.edu/people/faculty/djj/book/powerpoints/index.html

18. IPCC [2013]