First run tutorial

This tutorial runs the runaway greenhouse example that ships with JANUS. By the end you will have a plot of outgoing longwave radiation (OLR) versus surface temperature for a pure steam atmosphere, validated against four published datasets.

pip install -r examples/requirements.txt

1. What the example does

examples/demo_runaway_greenhouse.py increases the surface temperature of a pure H₂O atmosphere from 200 K to 2800 K in 20 steps, computing the OLR at each step using the full JANUS pipeline (moist pseudoadiabat + SOCRATES radiative transfer). The result is the classic runaway greenhouse curve, plotted against reference data from Kopparapu+2013, Goldblatt+2013, Hamano+2015, and Selsis+2023.

2. Run it

From the root of the JANUS repository:

python examples/demo_runaway_greenhouse.py

The run takes a few minutes, since each of the 20 temperature steps calls SOCRATES twice. You should see progress logged to the terminal:

INFO - Start
INFO - Running JANUS...
INFO - T_surf = 200 K
...
INFO - T_surf = 2800 K
INFO - Making plot
INFO - Done!

Output files are written to output/:

output/runaway_demo.pdf
output/runaway_demo.png

3. Understanding the output

The plot shows OLR as a function of surface temperature.

The plateau: OLR barely changes between ~350 K and ~1600 K, staying near 277 W m⁻². This is the Simpson–Nakajima radiation limit: the photosphere is trapped within the saturated water vapour region, so increasing the surface temperature does not increase the emitting temperature. Any planet receiving more stellar radiation than this limit cannot radiate fast enough and enters a runaway greenhouse; its oceans evaporate entirely.

The steep rise: above ~1800 K the deep atmosphere becomes supercritical (water has no condensed phase above 647 K). The condensing region disappears, the photosphere couples back to the surface, and OLR rises steeply into the post-runaway regime.

The JANUS curve should sit close to Hamano+2015 and Selsis+2023. Small differences between models come from different spectral line databases and lapse rate assumptions.

4. The other example

examples/demo_instellation.py increases the stellar instellation instead of the surface temperature, showing how the energy balance changes with orbital distance. Run it the same way:

python examples/demo_instellation.py

5. Next steps

• Modify vol_mixing in the script to add a CO₂ or N₂ background gas and see how it shifts the radiation limit
• Change mean_distance in the config located at src/janus/data/tests/config_runaway to move the planet closer or further from its star
• See the physical model overview for the equations behind the pseudoadiabat and radiative transfer