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1. Ch. 3 Radiation

   Text: Wallace and Hobbs, Ch. 4, Radiative Transfer p113-152, Liou, An introduction to atmospheric radiation Chs. 3, 4 Reading assignment: Manabe & Wetherald, 1967 Forster & Ramaswamy et al. 2007 (p. 131-145)
2. Central Questions: Why is the earth’s atmosphere largely transparent to solar radiation, but opaque to longwave radiation? What are physics laws that govern the earth’s radiative energy balance? What control absorption of the radiation in the atmosphere and earth’s surface? What are the physical basis for remote sensing of earth and atmospheric temperature, humidity, aerosols and chemical composition?
3. 3.1 The spectrum of radiation: Radiation can be viewed as a ensemble of electromagnetic waves propagating at the speed of light, c*=2.998X108m/s. The wave length (l), wave frequency( ) and wave number u obey the following relationships: Terrestrial radiation
4. 3.2 Radiance, irradiance, and flux Radiance: monochromatic or spectral intensity, I: the radiative energy transferred in a specific direction through a unit area (normal to the direction considered) per unit time at a specific wavelength (or wave number). Q: Does Il vary with incident angle and wavelength?
5. The Planck’s function: What is the planck’s function? The spectrum and wavelength of maximum radiative energy emitted by a blackbody is determined solely by its temperature.
6. The Wien’s displacement Law The wavelength of peak emission for a blackbody is inversely related to the surface temperature of the blackbody, lm=2897/T Where lm is wavelength in unit of mm, T is temperature in unit of K. Q: If you were an alien travel through the solar system. a) You first spot the earth and measured that the wavelength of maximum emission at 11.4 mm. What would be the effective temperature of earth’s atmosphere? (effective temperature is the temperature corresponding to the maximum emission); b) As you continued to travel, you measured the wavelength of peak emission of the Mars at 13.4 mm. What would be the effective temperature of Mar’s atmosphere?
7. The Wien’s displacement Law The wavelength of peak emission for a blackbody is inversely related to the surface temperature of the blackbody, lm=2897/T Q: If you were an alien first spot the earth and you measured that the wavelength of maximum emission at 11.4 mm. What is the effective temperature of earth’s atmosphere? (effective temperature is the temperature corresponding to the maximum emission) Based on Wien’s law, T=2897/lm=2897/11.4=254K for the earth T=2897/13.4=216K for Mars’ atmosphere.
8. Can a planet emit visible radiation? Fresh volcano lava can look bright red in dark. From this color (l=0.65mm), can you infer the minimum temperature in earth’s mantle?
9. The Stefan-Boltzman Law: The blackbody flux density (irradiance) integrated over all wavelengths using the Planck function, F, is proportional to T4. F =s T4 where s=5.67X10-8 Wm-2K-4, T: temperature in K. The Stefan-Boltzman law is commonly used in estimate radiative energy balance because its simple relation with T.
10. Non-blackbody: Earth’s atmosphere and surface is not exactly a blackbody. Earth emits on average about 61% of the absorbed radiation. Emissivity, el=Il/Bl(T) The ratio of emitted radiation intensity vs. that corresponding to the blackbody radiation. Reflectivity, Rl=Il(reflected)/Il(incident) The ratio of reflected radiation intensity vs. that of incident radiation. Transmissivity, Tl=Il(transmitted)/Il(incident)
11. The Kirchhoff’s law: the emissivity must equal to the absorptivity to maintain radiative equilibrium at each and all wavelengths. El=al Kirchhoff’s law is applicable to gases below altitude of 60 km, where frequency of molecular collision >> frequency of molecular absorb and emit radiation.
12. 3.3 Absorption by gas molecules Absorption by gas molecules occurs at discrete value: A molecule in the atmosphere can absorb a photon only when an atom makes a transition from one state to another state and only if the energy level of the photon corresponds to the difference between the energy of two allowable states of the molecule. In ionosphere, extreme ultraviolet (<0.1 mm) is absorbed by photoionization. In middle and upper stratosphere, ultraviolet (0.24mm) is sufficiently energetic to cause photodissociation of ozone, and being absorbed. In lower stratosphere and troposphere, the longwave radiation energy level match those changes caused by vibration-rotational energy transitionof the greenhouse gases’ molecules,thus being absorbed and emitted. If no transitions correspond to the energy of visible wavelengths, then it passes through the atmosphere without absorption.
13. Nuclear Configuration and Permanent Dipole Movement Symmetric, linear molecules (Both ends look the same) Nonlinear or bent Have pure rotation bands
14. What determine radiation of a gas molecule?
15. What control absorption and emission lines by the gases in the atmosphere? Vibration and rotational transitions Vibration Radiation: due to atoms’ vibrate about their equilibrium position relative to each other is called vibration energy. Vibration energy is generally greater than 600 cm-1 (15mm), emitted by gases such as O3 and H2O that have dipole moments due to their asymmetric change distribution. Rotational radiation: due to atoms rotation or revolve about an axis through its center of gravity. Rotational energy changes are relatively small with a minimum on order of 1cm-1. Thus, pure rotational lines are in the microwave or far-infrared spectra. Vibration transition never occurs along, but is coupled with rotational transition, producing a group of lines known as the vibration-rotational band in the intermediate infrared spectrum. Gases with symmetrical charge distributions, such as N2 and O2, have weak magnetic dipole moments and allow radiation in ultraviolet, and to a least extent, in visible spectra.
16. Nitrous Oxide What control the absorption lines of the atmosphere? Methane Rotation transitions require photons with a wavelength < 20 mm. CO2, H2O provide strong absorption near 15 mm, critical for longwave absorption in the upper troposhereand stratosphere. Why? The combination of vibration and rotational transitions allow the molecules to absorb and emit photons at large numbers of closely spaced absorbing bands. Ozone Absorption (100%) Water Vapor Carbon Dioxide UV IR Total Atmo
17. Broadening of the absorption lines: Discrete energy transition due to rotational or combined rotational and vibration state of the molecules of greenhouse gases (CO2, CH4, N2O, O3, CO, CFC) cause absorption lines. These absorption lines are broadened by a) the collisions among air molecules (the pressure broadening) below 20 km, b) gas molecules move toward to away from energy sources (Doppler broadening) above 50 km. Broadening are determined by a
18. Rotational band Spectra of outgoing radiation from Earth observed by IRIS on Nimbus 3 15 mm band 9.6 mm band 6.7 mm band z T(z) 280K 220K 320K T Wavenumber (cm-1)
19. Summary-1: Why is the earth’s atmosphere largely transparent to solar radiation, but opaque to longwave radiation? The infrared radiation energy of the photon matches the energy level the molecules of the greenhouse gases can absorb in makes an atmotransition from one state to another state What are physics laws that govern the earth’s radiative energy balance?
20. Summary-2-key concepts: What are differences between radiance, irradiance, and radiative flux? Radiance is the radiative energy transmitted in a specific direction at a specific wavelength per unit area and time interval. The unit is W/m2/Sr. It is wavelength, zenith and azimuth angle dependent. Irradiance: radiance integrated over all incident angles (directions). It is wavelength dependent. Radiative flux: irradiance integrated for all wavelengths.
21. Summary-3-Key physics laws: Planck’s function: Wien’s law Stefan-Boltzman law: F =s T4 Kirchhoff law: El=al
22. Summary-4: What is blackbody? What determines the emission of radiative energy for a blackbody? A blackbody is an object absorbs all incident radiation and emit all the absorbed radiation. The wavelength of maximum emission and total radiative flux emitted by a blackbody is a function of its surface temperature, following the Planck’s function, Wien’s Law and Stefan-Boltzman’s law.
23. Summary-5 What control absorption of the radiation in the atmosphere and what are the physical basis for remote sensing of earth and atmospheric temperature, humidity, aerosols and chemical composition? Different green houses gases absorb radiation at different wavelengths (frequencies), radiative emission at different temperature has different wavelength.