===== Radiative Forcing Experiments with Modtran =====

Reproduce with: {{ :modtranexp.tar.gz |}}

Also at [[https://github.com/bfiedler/modtran/tree/main | github]]

==== Using Modtran online at U. Chicago ====

{{:300ppm20km600ppm20kmt125rh_pt.png |}}
We use the website [[https://climatemodels.uchicago.edu/modtran/ | Modtran Infrared Light in the Atmosphere ]] for the calculations.  All calculations are //1976 U.S. Standard Atmosphere// and //no clouds//.

We assume a certain profile of atmospheric temperature is in 
equilibrium with the specified atmospheric constituents. The upward irradiance at
a certain level (either 20 km or 70 km) is noted for this initial equilibrium profile. 
We seek to find the change in the temperature profile that could restore the upward irradiance after constituents are changed (such as doubling carbon dioxide). We could call that profile the new
equilibrium profile.

For all experiments here, the temperature profile is  //1976 U.S. Standard Atmosphere//, with an optional specified uniform temperature offset (increase or decrease) as specified by the GUI.  To the left is
the initial assumed equilibrium profile, in blue, with 300 ppm of CO<sub>2</sub>. The red is the predicted new equilibrium, after carbon dioxide is doubled and water vapor increases to preserve the specified relative humidity that was in the initial profile.  The required temperature offset to restore equilibrium was found to be 1.25 K.

==== Upward spectral irradiance at 20 km ====

For reasons explained below, experiments to explore adjusted radiative equilibrium are best
done by monitoring the upward irradiance to 20 km, rather than the top of the atmosphere.

^ Going from 300 ppm to 0 ppm CO<sub>2</sub> would increase the outward irradiance $F\uparrow$ by 28.90 W m<sup>-2</sup>. The atmosphere, of course, would have to cool.^
|{{ :300ppm20km0ppm20kmf.png }}|

^ Going from 300 ppm to 600 ppm CO<sub>2</sub> would decrease the outward irradiance $F\uparrow$ by 3.14 W m<sup>-2</sup>. The atmosphere, of course, would have to warm.^
|{{ :300ppm20km600ppm20kmf.png }}|

^ Alternatively, we could increase methane from 1.7 to 13.6 ppm to get a similar radiative forcing.^
| {{ :300ppm20km300ppm20kmch136f.png }} |

==== Adjusted equilibrium temperatures ====

Doubling CO<sub>2</sub> and adjusting the temperature profile uniformly, to leave $F\uparrow$ unchanged.

^ With specific humidity constant ^
| {{ :300ppm20km600ppm20kmt88f.png }} |

^ With specific humidity increasing to maintain initial relative humidity. ^
| {{ :300ppm20km600ppm20kmt125rhf.png }} |

==== Experiments at 70 km ====

^At 70 km, there is a spike in the CO<sub>2</sub> from emission from the warm upper stratosphere.^
|{{ :300ppm20km300ppm70kmf.png }}|

^ Double CO<sub>2</sub>. Enhanced stratospheric cooling.^
{{ :300ppm70km600ppm70kmf.png }}

^Smaller temperature increase is needed to restore equilibrium. Increasing stratospheric temperature is not a good approximation for what happens in restored equilibrium. The stratopsphere would cool. ^
| {{ ::300ppm70km600ppm70kmt105rhf.png}} |

==== The GUIs ====



^ 300ppm20km ^ 600ppm20km ^ 
|{{ ::gui300ppm20km.png }} | {{ ::gui600ppm20km.png }} |
^ 600ppm20kmT88 ^ 600ppm20kmT125RH ^
|{{ ::gui600ppm20kmT88.png }} |  {{ ::gui600ppm20kmT125RH.png }} |

^ 0ppm20km ^ 300ppm20kmch136 ^
|{{ ::gui0ppm20km.png }} | {{ :gui300ppm20kmch136.png }} |

^ 600ppm70kmt105rh ^
|{{ :gui600ppm70kmt105rh.png }}|