Difference between revisions of "Rayleigh scattering"

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== Formalism ==
 
== Formalism ==
  
We consider a layer K & the channel NW.
+
We consider a layer.
  
 
dP is the difference of pressure between the two levels that define the layer.
 
dP is the difference of pressure between the two levels that define the layer.
  
 
dN is the number of  molecules per m2 & dm is the mass per m2 of the layer
 
dN is the number of  molecules per m2 & dm is the mass per m2 of the layer
 +
 +
dTau is the optical depth for a given wavelength
  
 
<math>m_{molecule}</math> is the mass of one molecule of the considered gas
 
<math>m_{molecule}</math> is the mass of one molecule of the considered gas
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g is the gravity
 
g is the gravity
  
dTau is the optical depth for a given wavelength
+
sigma_mol is the Rayleigh scattering cross section of the molecule
 
 
sigma_mol is the Rayleigh scattering cross section
 
  
 
=== LMDZ formalism ===
 
=== LMDZ formalism ===

Revision as of 18:18, 4 October 2022

About Rayleigh scattering

The following article gives a clear overview on Rayleigh scattering cross sections :

Bodhaine (1999) On Rayleigh Optical Depth Calculations : http://web.gps.caltech.edu/~vijay/Papers/Rayleigh_Scattering/Bodhaine-etal-99.pdf

Have a look especially on equations (2) and (9).

About Rayleigh scattering in LMDZ Generic

References

LMDZ

LMDZ uses formalism from :

Hansen (1974) Light scattering in planetary atmospheres : https://ui.adsabs.harvard.edu/link_gateway/1974SSRv...16..527H/ADS_PDF

Have a look on equations (2.29) to (2.32).

exo_k

Rayleigh routine in exo_k is avalaible here :

http://perso.astrophy.u-bordeaux.fr/~jleconte/exo_k-doc/_modules/exo_k/rayleigh.html#Rayleigh.sigma_mol

Exo_k uses formalism from :

Caldas (2019) Effects of a fully 3D atmospheric structure on exoplanet transmission spectra: retrieval biases due to day–night temperature gradients : https://hal.archives-ouvertes.fr/hal-02005332/document

Have a look on equation (12) & appendix D

Formalism

We consider a layer.

dP is the difference of pressure between the two levels that define the layer.

dN is the number of molecules per m2 & dm is the mass per m2 of the layer

dTau is the optical depth for a given wavelength

\(m_{molecule}\) is the mass of one molecule of the considered gas

g is the gravity

sigma_mol is the Rayleigh scattering cross section of the molecule

LMDZ formalism

In LMDZ0 we have :

dTau = scalep * (tauconsti * tauvari) * dP

tauvari = tauvari(wavelength in micron)

scalep = 100 because P is in mBar in optcv.F90

exo_k formalism

dTau = sigma_mol * dN

sigma_mo = sigma_mol(wavenumber in cm-1)

which gives : dTau = sigma_mol \( \displaystyle \frac{dm}{m_{molecule}} \)

and then : dTau \( \displaystyle = \frac{\text{sigma_mol}}{g * m_{molecule}} dP\)

Relations between LMDZ & Exo_k formalisms

LMDZ & exo_k formalism are linked as following \[ \displaystyle \text{(tauconsti * tauvari)} = \frac{\text{sigma_mol}}{g * m_{molecule}} * \text{scalep}\]

Be careful with units !!! (cm-1 for wavenumbers in exo_k, microns for wavelengths in LMDZ, not to forget the scalep factor in LMDZ)

To be noticed

TAURAY(NW) is calculated in calc_rayleigh.F90.

It is in fact TAUVAR which calculated, and then averaged by the black body function for each channel to give TAURAY \[ \text{TAURAY(NW)} = \frac{\int_{\lambda' \in \text{channel}} \text{TAUVAR} (\lambda') B_{\lambda} \, \mathrm{d}\lambda'}{\int B_{\lambda} \, \mathrm{d}\lambda'} \]

TAUVAR is cut into two parts : TAUCONSTI et TAUVARI with TAUVAR = TAUCONSTI * TAUVARI

The \( \lambda \) dependence is in the TAUVARI