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PHY2505 - Lecture 7

PHY2505 - Lecture 7. Thermal Radiation. Outline. Absorption by greenhouse gases Thermal infrared radiative transfer Line-by-line integration. Radiance (Wm -2 cm -1 sr -1 ). Absorption by greenhouse gases.

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PHY2505 - Lecture 7

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  1. PHY2505 - Lecture 7 Thermal Radiation

  2. Outline Absorption by greenhouse gases Thermal infrared radiative transfer Line-by-line integration

  3. Radiance (Wm-2 cm-1 sr-1) Absorption by greenhouse gases The Earth emits thermal radiation which is absorbed and trapped by gases in the atmosphere leading to a warmer surface than would exist if there were no atmosphere. The contribution to global warming due to a particular absorbing gas depends on Abundance Absorption lines relative to Earth’s emission peak Absorption cross-section Liou, FIG 4.1

  4. Radiance (Wm-2 cm-1 sr-1) The major absorbers CO2: V2 at 15um is an active degenerate band at the peak of Earth’s blackbody emission curve: hot bands (transition to ground state: very T–dependent) V3 at 4.3 um is less significant being at the wings

  5. Radiance (Wm-2 cm-1 sr-1) The major absorbers • O3: • V1,V3 at 9.6um are in the atmospheric window region • V2 at 14.27um - effect is masked due to strong CO2 band • if absorption is already total – adding more gas will have no heating effect!

  6. Radiance (Wm-2 cm-1 sr-1) The major absorbers H2O: V2 at 6.25um significant absorption V1,V3 at 2.7um in wings of B(288K) emission curve Also 0-1000cm-1CONTINUUM – a mystery!

  7. Other absorbing gases Liou, FIG 4.3 ASIDE: Here you see the normalisation effect of expressing radiance as a Blackbody “temperature” B(T)=sTB4/p – useful for enhancing signals in the wings..

  8. Significance of new anthropogenic pollutants 13 atoms, asymmetric 12 atoms, symmetric 4 atoms, asymmetric 4 atoms, symmetric 2 different atoms CFC’s and other pollutants could have a surprisingly significant effect on global warming due to large cross section: complex molecules (strong interaction with radiation field: asymmetric and many vibrational modes) absorption lines close to peak of Earth’s emission B(288K) Air monitoring by spectroscopic techniques, Ed. M.W.Sigrist, 1994, p337

  9. Modelling thermal radiative transfer Radiative transfer equation Plane parallel approx Thermal equilibrium m=cosq Surface Cumulative sum over all layers I(t,m)=Boexp(-t1-t2-t3)+ B1exp(-t2-t3) + B2exp(-t3) + B3 Boexp(-t1-t2)+ B1 exp(-t2)+B2 Boexp(-t1)+ B1 Bo

  10. Line-by-line model Use a spectral database for line position, strength and width information Use a Voigt line model to calculate effects of pressure and temperature on line shape For each line in spectral region, calculate Sum contribution from each line, from each gas, in each spectral interval – VERY COMPUTATIONALLY EXPENSIVE - Relies on accuracy of database

  11. The HITRAN database http://cfa-www.harvard.edu/HITRAN/ Absorption intensity per unit length/ per molecule/per cross sectional area

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