Configuring climt¶

A typical climate model allows the user the following kinds of configuration:

• Algorithmic Configuration: Changing the “tuning” parameters of the algorithms underlying various components. For example, the kind of advection used in a dynamical core could be configured.
• Memory/Array Configuration: Deciding the grid shapes, and distribution over multiple processors, if using MPI.
• “Physical” Configuration: Choosing the physical constants that are used during the simulation. For example, gravitational acceleration or planetary rotation rate could be varied.
• Behavioural Configuration: Allows modification of component behaviour. For example, radiative transfer components are called only once every N (>> 1) time steps and the output is kept constant for the remaining N-1 time steps.
• Compositional Configuration: Describing the components that make up the model, the order in which components need to be called, among other things.

Climate models can be configured in many ways, including hard coded configurations, namelists, shell environment variables. These diverse ways of configuring a climate model make it difficult to keep track of all configurations and changes made.

climt aims to keep all configuration in the main run script, but separated logically to ensure the script is still intuitive to read.

Algorithmic Configuration¶

Configuring the algorithm used by each component is done by various keyword arguments passed to the component while creating it. See, for example, the documentation for climt.RRTMGShortwave.

Memory/Array Configuration¶

climt does not yet support MPI, so there is no API yet to handle distributed arrays. However, the shape of arrays used by a model can be set while calling climt.get_default_state(). See, for example, the configuration of arrays in a GCM.

Physical Configuration¶

climt provides an interface to set and reset constants required by various components. The constants are put into different categories (boltzmann_constant is a ‘physical constant’ whereas planetary_rotation_rate is a ‘planetary constant’, for example).

The constants can be reset to their default values so that climt is in a known state at the end of a simulation. In the future, climt will provide a context manager to clean up modified constants at the end of a run.

Interfacial Configuration¶

Wrappers are the preferred way of changing the inputs or outputs of a component to make it apparently work in a different way.

• Piecewise constant output: Computationally expensive modules like radiative transfer are sometimes called only once every few timesteps, and the same values is used for the intermediate timesteps of a model. For example a GCM with a time step of 10 minutes might only call radiation after 1 hour of model time has elapsed. To allow for such behaviour, sympl.UpdateFrequencyWrapper can be used. See how this can be used practically in this example.

• TendencyComponent version: Spectral dynamical cores step the model forward in spectral space, and therefore, they do not play well with Stepper components that step forward the model in grid space. Typically, this is handled by finite differencing the output of Stepper components and providing them as time tendencies. Stepper components can be wrapped with sympl.TimeDifferencingWrapper which returns a component which provides the time differenced tendencies. The time differencing is done using a first order scheme:

$$\frac{dX}{dt} = (X_{out} - X_{in})/\delta t$$.

See how this is used in the Grey GCM.

• Scaled version: Very often, we perform experiments where we want to study the sensitivity of the simulation to a particular quantity or the effect of a certain quantity on the output (mechanism denial). This is in some instances done by scaling the quantity or setting it to zero (which is also a scaling). To allow for this kind of modification, sympl.ScalingWrapper can be used. This is a method available to all kinds of components (Stepper, TendencyComponent, etc.,). See the documentation for this method in the description of the base components in Components.

Compositional Configuration¶

This kind of configuration will allow the automatic building of models given certain components selected by the user. Currently, the user has to write the script to build the model and run it. It is clear that a lot of this code is repetitive and can be replaced by an entity (Which will be called Federation).

Note

This functionality is currently unavailble, and will be present in a future version of climt.