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Photon_transmissivity_2

by Ted Wong (Submitted: 01/30/2008)

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WHAT IS IT?

Greenhouse gases (GHGs) absorb radiation, but they don't absorb all wavelengths equally well. The atmosphere is more transparent to some wavelengths than to others, and while at some concentrations GHGs might block some wavelengths entirely from leaving the atmosphere, other wavelengths might be transmitted to space. Because GHGs generally absorb radiation of some wavelengths with moderate or low efficiency, a greater GHG concentration can reduce the atmosphere's overall transmissivity even if the atmosphere is already saturated for absorption at other wavelengths. Moreover, high GHG concentrations can change the spatial distribution of radiation absorption, concentrating absorption events and therefore heat in the lower atmosphere.

HOW IT WORKS

This model represents just two things: GHG molecules, and photons. In this model, all molecules are the same, and photons have one of three wavelengths. Molecules move around at random. Photons move from left to right at constant velocity. When a molecule and a photon occupy the same place in the model, the photon is absorbed according to a wavelength-specific absorption-probability set by the user. If the photon is absorbed, it disappears and the corresponding molecule increases its heat content (and its color on the screen becomes lighter). If the photon is not absorbed, it continues rightward as if nothing had happened.

HOW TO USE IT

First, set the absorption-probabilities for wach of the three wavelengths. Set the probability for the peak wavelength to be greater than the other two probabilities. Second, set the number of GHG molecules. Finally, let the model go continuously until the first photons have had a chance to traverse the entire length of the space.

THINGS TO NOTICE

How good is the atmosphere at transmitting photons of a particular wavelength? Does it block that wavelength entirely, or does it allow some photons to reach the right-hand edge? Also, how is the heat distributed spatially? Are the warmer, lighter-colored molecules clustered at one end of the atmosphere, or are they spread throughout? If they are clumped, do they eventually spread out -- and how long does that take?

THINGS TO TRY

<ol>
<li>Some people have argued that since the earth's atmosphere already absorbs all longwave radiation at CO2's peak absorption wavelength, increases in atmospheric CO2 can have no effect on climate. Try increasing the number of GHG molecules in the atmosphere. Is the overall transmissivity of the atmosphere affected? What about the spatial distribution of heat?
</li>
<li>
If you were to create a larger climate model, you might want to know something about the relationship between GHG concentration and overall atmospheric transmissivity. Choose some molecule-number and run the model long enough to give some photons the chance to reach the right-hand edge. Then use the "go 20 times" function and see how many photons exit the atmosphere in twenty or forty time-steps. Repeat this several times, enough to get an average. Get averages for several molecule-numbers, eough to graph out a curve of transmissivity vs molecule-number.
</li>
</ol>

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