# Overview: Why Sympl?¶

Traditional atmospheric and Earth system models can be hard to read and change for many reasons. Sympl tries to learn from the past experience of these models to speed up research and improve accessibility.

At its core, Sympl defines a model in terms of a state that gets changed in sequence by components of a model (like the radiation scheme, or dynamical core). Each of those components as well-defined and documented inputs and outputs, and code in Sympl will automatically handle unit and dimensionality conversions (such as dimension orderings) to give components the inputs they need.

Sympl defines a framework of Python object interfaces (APIs) that can be combined to create a model. This has many benefits:

• Objects can use code written in any language that can be called from Python, including Fortran, C, C++, Julia, Matlab, and others.
• Each object, such as a radiation parameterization, has a clearly documented interface and can be understood without looking at any other part of a model’s code. Certain interfaces have been designed to force model code to self-document, such as having inputs and outputs as properties of a scheme.
• Objects can be swapped out with other compatible objects. For example, Sympl makes it trivial to change the type of time stepping used on a prognostic scheme.
• Code can be re-used between different types of models. For instance, an atmospheric general circulation model, numerical weather prediction model, and large-eddy simulation could all use the same RRTM radiation object.
• Already-existing documentation for Sympl can tell your users how to configure and run your model. You will likely spend less time writing documentation, but end up with a better documented model. As long as you write docstrings, you’re good to go!

Sympl also contains a number of commonly used objects, such as time steppers and NetCDF output objects.

## So is Sympl a model?¶

Sympl is not a model itself. In particular, physical parameterizations and dynamical cores are not present in Sympl. This code instead can be found in other projects that make use of Sympl.

Sympl is meant to be a community ecosystem that allows researchers and other users to use and combine components from a number of different sources. By keeping model physics/dynamics code outside of Sympl itself, researchers can own and maintain their own models. The framework API ensures that models using Sympl are clear and accessible, and allows components from different models and packages to be used alongside one another.

## Then where’s the model?¶

Check out CliMT as an example. We highly recommend reading the paper on Sympl and CliMT.

In Sympl, the “model” in the traditional sense of Fortran models is essentially your run script. You use a Python run script instead of a Fortran module like main.f90. This may sound scary, but the idea is that the python run script is as easy (or easier) to understand than a configuration file. Sympl makes choices that force those run scripts to be easier to understand. You can see examples in the above paper.

A “model developer” in the traditional sense would write a toolkit package containing model components that are used by those run scripts. See CliMT for an example of this. That toolkit should also come with example run scripts using its components.

In a way, when you configure the model you are writing the model itself. This is reasonable in Sympl because the model run script should be accessible and readable by users with basic knowledge of programming (even users who don’t know Python). By being readable, the model run script tells others clearly and precisely how you configured and ran your model.

If someone wants to write a model in the traditional way, where their Python run script is never changed and instead you configure the model by changing a configuration file, they can do that, too! Read about the particular model you’re using for details.

## The API¶

In a Sympl model, the model state is contained within a “state dictionary”. This is a Python dictionary whose keys are strings indicating a quantity, and values are DataArrays with the values of those quantities. The one exception is “time”, which is stored as a timedelta or datetime-like object, not as a DataArray. The DataArrays also contain information about the units of the quantity, and the grid it is located on. At the start of a model script, the state dictionary should be set to initial values. Code to do this may be present in other packages, or you can write this code yourself. The state and its initialization is discussed further in Model State.

The state dictionary is evolved by TendencyStepper and Stepper objects. These types of objects take in the state and a timedelta object that indicates the time step, and return the next model state. TendencyStepper objects do this by wrapping TendencyComponent objects, which calculate tendencies using the state dictionary. We should note that the meaning of “Stepper” in Sympl is slightly different than its traditional definition. Here an “Stepper” object is one that calculates the new state directly from the current state, or any object that requires the timestep to calculate the new state, while “TendencyComponent” objects are ones that calculate tendencies without using the timestep. If a TendencyStepper or Stepper object needs to use multiple time steps in its calculation, it does so by storing states it was previously given until they are no longer needed.

The state is also calculated using DiagnosticComponent objects which determine diagnostic quantities at the current time from the current state, returning them in a new dictionary. This type of object is particularly useful if you want to write your own online diagnostics.

The state can be stored or viewed using Monitor objects. These take in the model state and do something with it, such as storing it in a NetCDF file, or updating an interactive plot that is being shown to the user.