Py4CATS --- FAQ
Forecasted Asked Questions
Typically extracting lines form Hitran or Geisa in a certain spectral range will provide linelists of a dozen molecules or so.
Some of these molecules are probably very ``exotic'', i.e. they might have very small concentrations
and hence will "normally" not contribute significantly to the total optical depth.
However, especially heavy molecules such as ClONO2 will have thousands or even ten thousands of lines and would slow down the line-by-line calculation considerably.
Therefore its up to the user to decide what molecules should be taken into account for cross section, absorption coefficient, and optical depth:
First extract the line parameters from Hitran or Geisa and check the list of line parameter files
(you can do this graphically with plot_atlas *.vSEan);
then continue to optical depths with only the 'important' molecules.
In order to compute cross sections, absorption coefficients, and optical depths for some spectral range
xlo ... xhi,
lines in an extended spectral range
xlo - d ... xhi + d
should be considered, where d is typically some wavenumbers cm-1.
In some way the -w (see also previous item) is kind of 'physics option', whereas the -W
is more related to the numerics and approximation,
and it is only relevant for the 'multigrid' cross section algorithms:
Near the line center the cross section for an individual line is evaluated on a fine wavenumber grid
with a spacing essentially determined by the line width (hwhm=half width @ half maximum):
dv = hwhm / n
where n is the sampling rate defined by the -s option (default 5.0).
Outside this line center region the (Voigt) line profile is evaluated only on a coarse grid with spacing
increased by a factor two, four, or eight (according to the -g option).
The extension of the center region is defined by nWidths, the -W option,
typically ± 25 hwhm.
© fgs (+49-8153-28-1234)
Tue Feb 23, 2010; 11:21