you did produce very nice gaphs! They would not even look much different if youj would repeat the calculation including the interaction with some kg of liquid and solid matter within the air column above 1 m^2 of ground and introducing around 1000 m^2 of absorbing/emitting surface into that volume.

This result will show up despite of the fact, that this liquid an solid matter matter is the main heat sink for allmost all LW radiation absorbed by CO2 originally!

]]>the documentation and the user interface are both archaic and arcane

I’ll second that opinion. I’ve downloaded the program, but couldn’t get past the documentation to actually use it. It wasn’t at all clear to me how one constructed the multiple input files necessary to run the program. I suspect MODTRAN would be just as difficult if Archer hadn’t put together a graphical user interface for it.

Your Radiative Transfer textbook is wonderful, by the way.

]]>It uses the HITRAN line database and does all the work of integrating over both wavenumber and altitude for various model atmospheres (including user-supplied). And the computational time isn’t bad as long as one is only interested in a few spectra over fairly narrow intervals.

Unfortunately, while the software can be freely downloaded and is easy to compile with a Fortran compiler, the documentation and the user interface are both archaic and arcane — very difficult for the average amateur to make heads or tails of unless they already have pretty good knowledge of what’s under the hood. In fact, I’m still working on it.

]]>In the calculations in Part Nine, as DeWitt says, I did the calculation with a “brute force” approach.

It was amazingly fast so I didn’t have to think about improving the code to only consider the impact of a line center x cm^-1 away – leading to sensitivity analyses for x.

I did also run all the isotopologues as well – all 315,000 lines each across 350 cm^-1 with a resolution of 0.001 cm^1.

]]>Frank,

The data files from SpectralCalc have a resolution of ~0.01 cm-1. So for a range of 1.4 cm-1 there are ~140 data points. For the range 660-672 cm-1 for a path length of 1m at surface pressure there were 1072 data points. The mean transmittance was 0.8205.

SoD is using something of a brute force approach. At each frequency, he calculates the data for every line in the database. But he’s doing one isotopologue of one molecule. For atmospheric calculations there are 39 (I think) molecules with many isotopologues. CO2 alone has nine. The chlorofluorocarbons, for example, have lots more.

]]>DeWitt: Thanks for you reply. Could I (respectfully) clarify what meant when you said: “The contributions from each line are then summed and the absorptivity calculated.” If I look at the figure in SOD’s reply of 3/9 6:30 above, I see an isolated line at about 661.2 cm-1 that has a half width of about 0.2 cm-1. Are you telling me that line-by-line methods consider 10-100 wavelengths across this one peak (say from 660.5 to 661.9 cm-1) and calculate the absorbance and emission at each of these wavelengths as they pass through the atmosphere and as the peak narrows with decreasing pressure and temperature? Then the same thing is done with each of the overlapping lines at 667-670 cm-1 that aren’t resolved, but which have the parameters in the database needed to define each peak?

How does anyone get a good feeling for how well the whole process works? I saw in the paper I linked that a Voigt profile can fit the experimental data for a single line at a single pressure and temperature extremely well (until, as you point out, you get to the far wings which are only important for strong peaks). How well do the best-fit parameters from one pressure and temperature perform at a different temperature and pressure?

Are there any experiments that integrate energy transmitted through a range of wavelengths (say the 660-672 cm-1 range shown above) at a variety of temperature and pressure combinations? I suppose this is unnecessary; if I trust the ability of parameters and theory to reproduce the behavior of any one line, I should trust any ensemble of 30 or 30,000 lines. However, dealing with a range of wavelengths at once gives one an easy way to estimate the total error/uncertainty in the process.

]]>How about doing some calculations for water vapor?

]]>Line-by-line calculations are done by wavelength or frequency. At each step, the line table is searched for lines that might have significant absorption at that point. The contributions from each line are then summed and the absorptivity calculated. One of the larger uncertainties is from the far wings of intense lines. The Lorenz profile for pressure broadening is only an approximation and may not work all that well far from the line peak.

Self absorption is saturation to all intents and purposes. It means that the optical depth of the source is thick and emission isn’t linear with concentration any more.

That paper was published in 1998 and references the 1996 version of the HITRAN database. The current version of HITRAN is 2008. The problems in the paper you linked to have probably been addressed by now.

]]>If I think about a single strong isolated line, could radiative forcing originate from the “wings” of the line when the center is saturated? (Is this related to “self-absorption?) When line-by-line calculations are performed, is there one calculation for each line, or many calculations across the profile of each line?

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