# GCM-1D: A tool to simulate an Earth-like atmosphere

We here present a new tool that was developed to easily simulate the climate of planets similar to Earth (i.e., terrestrial but not giant planets). It is accessible here.

This model is based on the LMD (Laboratoire de Météorologie Dynamique) Global Climate Model (GCM), that is a complex 3-D numerical model of climate, which solves equations of thermodynamics, radiative transfer and hydrodynamics (Wordsworth et al. 2011, Charnay et al. 2013, Leconte et al. 2013). This complex 3-D model has been simplified to a 1-D code (Turbet et al. 20162017, 2018), which is therefore much faster to run and can now be used online in an interactive fashion.

This new 1-D code evolves the climate on planets and exoplanets (i.e., planets orbiting around other stars than our Sun) as a function of time (for several orbital periods) with a good accuracy. The planet's atmosphere is represented by a 1-D column composed of 20 cells from 0 to 65 km. The atmosphere is assumed to be composed of a mix of N2, CO2 and water. The water content in each atmospheric layer is worked out self-consistently by taking into account that water is evaporated from the surface, condenses in the atmosphere and precipitates, and that there is a vertical mixing of water vapour between the different atmospheric layers.

This new fast model can, for example, be used to simulate the climate on Earth (e.g. its temperature, water vapour profile, ...) as a function of time (because it can vary slightly along its slightly eccentric orbit) but also any terrestrial planet, with a gravity, water content or atmospheric pressure that would differ from the Earth. Some parameters can even be modified while running the simulation to see live changes, e.g. what would happen to the Earth temperature if the Sun would, all of a sudden, stop irradiating us? Or what would happen if the Earth was further away or more eccentric, or with more CO2? This simple model can tackle all of these exciting questions and so can you.

To use the 1-D model, first access to the online tool here.

Then, the first step is to fill in the planet's parameters that you want to simulate. Give it a gravity and the average quantity of water at the surface (average depth of water in metre) as well as the oceanic coverage (in %) and atmospheric pressure at the surface of the planet.

You then have to set up the other set of adaptive parameters (that are part of a different panel) that can be updated while running the simulation (i.e., a change of these parameters will automatically update all the plots that are being shown in the simulation from the old set of parameters to this new set).
The parameters that are adaptive are: the stellar flux received at 1 au (astronomical unit), the distance planet-star (in au), the eccentricity of the planet, the orbital period of the planet (in days), the amount of CO2 in the atmosphere (in ppm) and the stellar type of the host star.

We note that the orbital period can seem redondant as the planet's distance and stellar type (from which a stellar mass can be deduced) are known and thus the orbital period can be calculated. However, letting this period as a free parameter allows us to handle a wider variety of stellar masses and let the possibility to test variations when changing only this one parameter while holding other fixed (even if it can be unphysical). This is the same for the flux at 1au that is a free parameter while the stellar type should be selected.

Press start to run your simulation with the parameters you've chosen. While running, you can also modify the adaptive parameters (given above) to converge towards a solution corresponding to the new set of parameters you've modified from where you had previously converged. You can always restart the simulation in case anything goes wrong.

The different plots that are displayed are:
1) The temporal evolution of the surface temperature (in K)
2) The instantaneous atmospheric temperature profile as a function of altitude (in K)
3) The instantaneous water vapour mixing ratio profile as a function of altitude (in kg/kg of air and using a logscale)
4) The temporal evolution of the absorbed stellar radiation (in green) versus the outgoing thermal radiation emitted by the atmosphere (in blue). The incoming stellar radiation is shown in red.
5) The position of the planet on its orbit as a function of time.
6) The outgoing thermal radiation spectrum (in red) for wavelengths between 2.5 microns and 1mm. It shows the flux emitted by the planet at the top of its atmosphere. Thus, it takes into account the flux emitted from the surface and by each atmospheric layer that is not absorbed by the upper layers. Therefore, it depends on the quantity of water and CO2 in the atmosphere as well as the temperature profile.
7) The outgoing stellar radiation spectrum for shorter wavelengths (between 0.29 and 5.13 microns). This spectrum shows the stellar flux that manages to first cross the atmosphere, then is reflected and recross the atmosphere a second time without being absorbed by neither the surface nor the atmosphere.