Further delaying a repeated promise to progress upwards in prior posts in this series, it seems important to address an obvious question: why, when you look up and down from say 10 meters, can you see deviations from the blackbody spectrum indicating radiation from greenhouse gasses looking up? In other words, if we are going to say that no signal is seen looking down because such radiation is instantly and completely absorbed and thermalized, why doesn’t this happen to the radiation from above?

Always in the back of your mind there is the possibility that Modtran is just wrong, or somehow inadequately designed for this up and down exercise.

modtran-up-and-down-full-bore-10-meterHere is Modran full bore, as I call it, with all the greenhouse gasses; looking down with the red background run and up with the blue looking up at ten meters. At first glance one is inclined to say the signal from above is all water. We can check that.


What we have done here is leave the full bore signal looking up from ten meters in red as the background run compared to water only looking up from the same level in blue.

Interesting. In the CO2 bands from wave numbers 600-750, the water only signal comes from a much higher and colder altitude. This information is obscured in the full bore view because the virtual spectrometer registers the higher energy near surface thermalized radiation along the Planck curve to about WN 700 and then the “tops” of the water bands.

Through the atmospheric window the energy and altitude differences are much smaller and the blue water only signal barely “pokes out” below the full bore signal except in the ozone bands where the full bore lines “jump up”.


Using the same comparison between full bore and ozone you get the idea…ozone behaves peculiarly because there is significant surface ozone besides the better known stratospheric zone.


Just to complete the train of thought we compare full bore with CO2 only. CO2 agrees with water in the deviation from blackbody in the ozone bands.

After all this, how do we answer the initial question: why is radiation from above not absorbed and thermalized just as radiation from below? Why do we see a deviation from the blackbody temperature looking up but not down?


Getting back to the first graphic of full bore up and down from 10 meters, the answer is that the radiation from above is coming in at a lower energy than the radiation from below. The vibrations that dominate the molecular excitations in the infrared spectrum are quantized. The higher quanta/ higher energy molecules both below 10 meters and above are transparent to lower energy photons from above. They just sail right through.



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1 Response to MODTRAN Up and Down III

  1. Pingback: MODTRAN Up and Down IV, Upwards | geosciencebigpicture

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