PHY2505%20-%20Lecture%207

1
PHY2505 - Lecture 7

• Thermal Radiation

2
Outline

• Absorption by greenhouse gases
• Thermal infrared radiative transfer
• Line-by-line integration

3
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 Earths emission peak Absorption
cross-section
Liou, FIG 4.1
4
The major absorbers
CO2 V2 at 15um is an active degenerate band at
the peak of Earths blackbody emission curve hot
bands (transition to ground state very
Tdependent) V3 at 4.3 um is less significant
being at the wings
5
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
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-1 CONTINUUM 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
CFCs 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 Earths
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 mcosq
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