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.