Overview of the Generic PCM

What is a GCM

A Global Climate Model (GCM) like the Generic PCM calculates the temporal evolution of the different variables (temperature, pressure, mass, mixing ratio of tracers/gas/clouds, winds, etc.) that control or describe the planetary meteorology and climate at different points of a 3D “grid” that covers the entire atmosphere.

To do that, the model solves 3D hydrodynamic equations that describe how atmospheric winds are created by various physical forcings (radiative heating/cooling, etc.) and how these winds transport and redistribute the various atmospheric fields (temperature, tracers, etc.). However, it can be shown that to a good level of approximation, the various forcings can be computed using only local conditions in a column of atmosphere, whereas long-range couplings between various points of the planet are mediated through atmospheric transport.

Physics / Dynamics interface ; credit: Emmanuel Marcq

For these reasons, the 3D model is split in two components:

• a 3D dynamical core abble to integrate in time the general equations for atmospheric circulation when the various local forcings are known. This component (including source code) is common to all terrestrial-type atmospheres, and applicable in certain cases to the upper atmospheres of gas giant planets.
• a physical package that is specific to the planet in question (as it takes into account dedicated parametrizations representative of ongoing physical processes) and which computes atmospheric processes in each atmospheric column (which are sub-grid-scale from the point of view of the dynamical core).

In practice, we have several dynamical cores available with the model: LMDZ (global longitude/latitude grid), DYNAMICO (icosaedric grid), and WRF (rectangular box for local/regional studies)

From an initial state, the model calculates the temporal evolution of these variables, one timestep after another:

• At instant t, variables X_t (temperature for example) are known at every grid point of the atmosphere.
• The physical model calculates the local time derivatives of the various variables -- called the tendencies -- ((δX/δt)₁, (δX/δt)₂, etc.) due to each physical phenomenon: for example, heating due to absorption of solar radiation and infrared cooling, mixing due to turbulence in the planetary boundary layer, condensation/evaporation of water vapor, etc. These tendencies are calculated via parameterization of the processes involved that try to capture the physical behavior of the atmopshere at the subgrid scale.
• These tendencies are passed to the dynamical core that integrates the model forward in time to the next time step, t + δt, by solving the hydrodynamical equations.
• This new state can be used to recompute the tendencies for the next time step and so on.

Philosophy of the Generic PCM

The accuracy and predictive power of any GCM is often dictated by the choice made for the physical phenomena that are included and for the way they are parametrized. To be accurate, many Earth-centric climate models use very complex parametrizations that are heavily tuned on Earth observations. Such parametrizations can, however, behave very poorly far from the parameter space where they have been validated, thus reducing their predictive power for exotic atmospheres.

As the goal of the Generic PCM is to model the climate of very different planets with the same model, a particular emphasis has been put on using simple parametrizations based as much as possible on ab-initio physical equations and conservation principles to avoid unphysical behaviors in extreme regimes.

GCM inputs and outputs

input/output data

The input files are the files needed to initialize the model (state of the atmosphere at instant t0 as well as a dataset of boundary conditions). The output files are “historical files”, archives of the atmospheric flow history as simulated by the model: the “diagfi files”, the “stats files”, the daily averages, and so on. The "parameter files" are the files describing the physical parameters of the planet, as well as the physical parameterizations used in the model. The figure (on the right) shows the main architecture of the Generic PCM.

start and startfi files

GCM simulations needs a list of initial conditions/parameters to be run. This includes, for example, the initial temperature field, wind field, cloud field, position of water reservoirs on the surface, etc.

In the Generic PCM, initial conditions/parameters are stored in two files named "start.nc" and "startfi.nc". At each new run, the GCM reads these two files to set up the proper set of initial conditions.

start vs restart files

At the end of each GCM simulation, the Generic PCM produces several output files, and notably two files named "restart.nc" and "restartfi.nc".

These two files store all the final conditions of the model, which includes, for example, the initial temperature field, wind field, cloud field, position of water reservoirs on the surface, etc.

These "restart.nc" and "restartfi.nc" have the exact same architecture than the "start.nc" and "startfi.nc" files, and can thus be used as initial conditions for a new Generic PCM simulation.

parameter (.def) files

The parameter files (listed below) are files required by the Generic PCM to run.

List of needed .def files:

• callphys.def: Physical parameters used in the simulation
• run.def: Numerical parameters used in the simulation
• gases.def: List of gases in the simulations
• traceur.def: Numerical tracers in the simulations
• z2sig.def: Pseudo-altitudes (in km) at which the user wants to set the vertical levels
• diagfi.def (optional): List of wanted outputs

input data

The Generic PCM requires several inputs in order to run. This includes, for instance:

• opacity tables
• aerosols optical properties
• stellar spectrum

These data can be modified/adapted by the users depending on the planet to be simulated.

output files

At the end of each GCM simulation, the Generic PCM produces several output files, and notably a file named "diagfi.nc". This file is an archive of the atmospheric/surface fields (temperatures, winds, clouds, etc.) along the GCM simulation. This is the file that contains most of the information needed to analyze/understand the content of a GCM simulation.

nb: The Generic PCM can also produce extra output files, such as the "diagspec.nc" files (that keep track of the spectral radiative fluxes in the GCM), the "stats.nc" (that keep track of statistical information along the GCM simulation), etc.

Our team of developers (by alphabetical order)

Active contributors

• Deborah Bardet
• Siddharth Bhatnagar
• Benjamin Charnay (permanent, LESIA)
• Guillaume Chaverot
• Noé Clément
• Aurélien Falco
• François Forget (permanent, LMD)
• Gabriella Gilli
• Sandrine Guerlet (permanent, LMD)
• Mathilde Houelle
• Yassin Jaziri
• Sébastien Lebonnois (permanent, LMD)
• Jeremy Leconte (permanent, LAB)
• Maxence Lefèvre
• Arthur Le Saux
• Alice Maurel
• Maxime Maurice
• Gwenael Milcareck
• Ehouarn Millour (permanent, LMD)
• Aymeric Spiga (permanent, LMD)
• Lucas Teinturier
• Martin Turbet (permanent, LMD)

Collaborators and previous contributors

• Tanguy Bertrand
• Antoine Bierjon
• Alexandre Boissinot
• Emeline Bolmont
• Francis Codron
• Antony Delavois
• Thomas Dubos
• Vincent Eymet
• Frédéric Hourdin
• Franck Lefèvre
• Laura Kerber
• Jean-Baptiste Madeleine
• Emmanuel Marcq
• Diogo Quirino
• Franck Selsis
• Jan Vatant d'Olonne
• Margaux Vals
• Romain Vandemeulebrouck
• Robin Wordsworth