Validating MODTRAN for Climate Studies

By Ed Caryl

Some commenters on my previous articles using MODTRAN, here and here, have called into question the validity of using it to investigate the physics behind the CAGW theory. Of course in the 24 year history of MODTRAN, the US Air Force and many other organizations, including climate scientists, have validated the software for their own purposes. The best way to prove that it is valid for the purposes of my articles, is to show comparable spectra from other sources. One commenter claimed the humidity settings were not interpreted correctly. The interface labels do not appear the same in the version used as in his example. His advice was attempted, and the software would not run a fractional entry. The examples below illustrate that my interpretation is correct.

Infrared astronomers are quite familiar with the problem of atmospheric IR. IR astronomy is not easily done on the earth’s surface because the lower atmosphere is nearly opaque to IR. Consequently IR astronomy is done at high altitudes or at very dry locations, or both. The Atacama Desert in Chile, the top of Mauna Kea in Hawaii, the south pole, airborne observatories, or satellites are the favored locations.

The web was searched for any image of the back radiation spectrum. None were found for any temperate or tropical zone. The only spectra found are from the Arctic at -10°C, and the south pole at -30°C. From the spectral shape, it appears that the Arctic data was taken at Summit Camp on the Greenland ice sheet. Those are the next figures with the MODTRAN version for comparison.

Figure 2a and b. The 2a is from Petty (2006), found at skepticalscience here. 2b is from MODTRAN. The scaling is slightly different, the resolution is not as high, the shape is very nearly the same.

Figure 3a and b. Figure 3a from South Pole station here. Figure 3b is from MODTRAN. At sea level, with a clear sky, temperature at 15°C, 50% humidity, and 400 ppm CO2, the spectrum of downward IR, the “back” radiation, looks like this.


12 responses to “Validating MODTRAN for Climate Studies”

  1. RP

    I’m afraid I have not been able to authenticate Figures 2a and 3a with which you are comparing the MODTRAN simulations.

    In the case of 2a, the link to skeptikalscience refers us back merely to the textbook publishing site, so that appears to be a dead end.

    In the case of 3a, your reference link does lead to an original paper by Town et al (2005). However, the paper says that this figure was itself generated by a radiative transfer model (LBLRTM), so this appears to be another model-simulation in fact and not an empirical observation as one might have hoped. Logically one cannot validate one anthropogenic computer code (MODTRAN) by comparing its performance with that of another anthropogenic computer code (LBLRTM). One can only validate it logically by comparing its performance empirically with that of the observed real world.

    We seem to be in danger of getting sucked into a computer-generated virtual reality without realising it here.

    1. Ed Caryl

      I’m beginning to think there are no “real” measurements!

      1. RP

        Me too, Ed! But I don’t want to jump to any conclusions, so will keep on looking.

  2. PeterF


    the similarity of the measured and calculated spectra is really intreaging, but I am now puzzled more than before.

    In your first post you say, quote: “… Sensor altitude 100 km, Looking Down.” This to me means the sensor is at an altitude of 100km, pretty much top of the atmosphere (TOA), and is looking towards the earth. In this present post, however, in Fig 2a I read “(b) Surface looking up” and in the text to Fig 3 you say explicitely “the spectrum of downward IR, the “back” radiation,”. This to me means the sensor is on ground level o km (zero km), and is looking up, away from earth. These are two very different scenarios!

    Sure, in the latter case you can define the humidity at one point, namely around the sensor located on ground level. And the rason why levels of humidity at higher and much levels don’t matter is the strong IR absorption of water vapor (and CO2) within their absorption bands. While I can’t provide numbers, I believe the IR radiation from the ground is absorbed beyond 99% only a few meters up in the air. If you can provide absorption constants it would be nice to calculate this. So what the graphs in this post provide is the IR radiation from the very bottom of the atmosphere.

    I don’t understand what this has to do with the radiation balance at TOA?

    1. DirkH

      “I believe the IR radiation from the ground is absorbed beyond 99% only a few meters up in the air.”

      The mean free path is about 23 m at ground level before the first absorption. In local thermal equilibrium (LTE), thermalization and dethermalization happen with the same frequency, though. (Kirchhoff’s Law)

      The stratosphere is in LTE. Ground level atmosphere is not in LTE so we can’t expect exactly as much re-radiation as absorption but we can assume that both absorption and emission do happen, even if not exactly to the same amount.

      As MODTRAN was developed to emulate observations, its model must take the re-emission into account.
      Basically, about as much LWIR photons are re-emitted as are absorbed, so you end up with a “fog”-like situation.

      1. DirkH
    2. Ed Caryl

      The first post is radiation at the top of atmosphere looking down. The second and third posts are from the ground looking up.

      1. PeterF

        Isn’t it quite a long stretch then to declare a match of spectra for a downwelling radiation situation to be a ‘Validation for Climate Studies’?

  3. Pehr Bjornbom

    I have written a related piece on Modtran that also illustrates some problems in varying the relative humidity profile in the model atmosphere. It will appear early tomorrow here:

  4. Marc77

    It seems to me that reality is more complicated than most models. There are two points that may not be well represented.

    1- The ability of the atmosphere at changing the frequency of photons. It could be done by aerosols, small droplets of water, pressure broadening.

    2- The effective emissivity. I’m not sure what emissivity for. I think the right definition of reflection could go like this: If the amount of energy of an emission is well correlated to the energy of a single absorption, you have a reflection. I don’t know if emissivity accounts for the fact that a molecule that absorbs a photon will be warmer before its heat is fully distributed to the surrounding molecules. Also, when a water molecules absorbs a photon, it is likely to evaporate whith this energy.

  5. Ingemar Nordin

    ”Hello Chris,

    The temperature adjustment that I put into MODTRAN was very ad-hoc, just a toy for teaching undergraduates. No doubt you are correct that the change in temperature with altitude would be more complicated than this. If you think it’s a serious error I can probably dig my way back into that code and the web interface to fix it. But I am guessing that the impacts should be pretty small, at least for the purposes that the model was designed for.

    Feel free to quote me in your discussion thread.


    David Archer”

    I don´t know what to make of this. It is obvious that we cannot draw too dependable conclusions from MODTRAN (after all, it is just an advanced calculator). On the other hand it should tell us a few things about the relation between CO2 and humidity.

    David Archer is a great authority within the IPCC. He cannot make big mistakes, can he?

  6. Calvin

    Does anyone know if ModTran has been radiometricaly validated at high altitudes, looking down, in the range 2.5 to 50 microns? (where most of TOA outoing IR exists). I know that it has been verified from 0.4 to 2.5 microns.


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