2 Modify an Existing Model

Tutorial 2: Modify an Existing Model

This tutorial walks through the steps to modify an existing model. There are several existing models publically available on Github, and for the purposes of this tutorial we will use MimiDICE2010, available on Github here.

Working through the following tutorial will require:

If you have not yet prepared these, go back to the main tutorial page and follow the instructions for their download.

Futhermore, this tutorial uses the DICE model as an example. Downloading DICE uses similar steps to those described for FUND in Tutorial 1 Steps 1 and 2.


There are various ways to modify an existing model, and this tutorial aims to introduce the Mimi API relevant to this broad category of tasks. It is important to note that regardless of the goals and complexities of your modifications, the API aims to allow for modification without alteration of the original code for the model being modified. Instead, you will download and run the new model, and then use API calls to modify it. This means that in practice, you should not need to alter the source code of the model they are modifying. Thus, it is easy to keep up with any external updates or improvements made to that model.

Possible modifications range in complexity, from simply altering parameter values, to adjusting an existing component, to adding a brand new component. These take advantage of the public API listed here, as well as other functions listed in the Mimi Documentation.

Parametric Modifications: The API

Several types of changes to models revolve around the parameters themselves, and may include updating the values of parameters and changing parameter connections without altering the elements of the components themselves or changing the general component structure of the model. The most useful functions of the common API in these cases are likely update_param!/update_params!, disconnect_param!, and connect_param!. For detail on these functions see the API reference here.

When the original model calls set_param!, Mimi creates an external parameter by the name provided, and stores the provided scalar or array value. The functions update_param! and update_params! allow you to change the value associated with this external parameter. Note that if the external parameter has a :time dimension, use the optional argument update_timesteps=true to indicate that the time keys (i.e., year labels) associated with the parameter should be updated in addition to updating the parameter values.

update_param!(mymodel, :parametername, newvalues) # update values only 

update_param!(mymodel, :parametername, newvalues, update_timesteps=true) # also update time keys

Also note that in the code above,newvalues must be the same size and type (or be able to convert to the type) of the old values stored in that parameter.

If you wish to alter connections within an existing model, disconnect_param! and connect_param! can be used in conjunction with each other to update the connections within the model, although this is more likely to be done as part of larger changes involving components themslves, as discussed in the next subsection.

Parametric Modifications: DICE Example

Step 1. Download DICE

The first step in this process is downloading the FUND model, which is now made easy with the Mimi registry. Assuming you have already done the one-time run of the following to connect your julia installation with the central Mimi registry of Mimi models

pkg> registry add https://github.com/mimiframework/MimiRegistry.git

You simply need to add the DICE2010 model with

add MimiDICE2010

You have now successfully downloaded DICE to your local machine.

Step 2. Run DICE

The next step is to run DICE. If you wish to first get more aquainted with the model itself, take a look at the provided online documentation. Now run DICE using the provided API for the package:

using MimiDICE2010
m = construct_dice()

Note that these steps should be relatively consistent across models, where a repository for ModelX should contain a primary file ModelX.jl which exports, at minimum, a function named something like getModelX or construct_ModelX which returns a version of the model, and can allow for model customization within the call.

In this case, the function construct_dice has the signature


Thus there are no required arguments, although the user can input params, a dictionary definining the parameters of the model.

Step 3. Altering Parameters

In the case that you wish to alter an exogenous parameter, you may use the update_param! function. For example, in DICE the parameter fco22x is the forcings of equilibrium CO2 doubling in watts per square meter, and exists in the components climatedynamics and radiativeforcing. If you wanted to change this value from its default value of 3.200 to 3.000 in both components,you would use the following code:

update_param!(m, :fco22x, 3.000)

A more complex example may a situation where you want to update several parameters, including some with a :time dimension, in conjunction with altering the time index of the model itself. DICE uses a default time horizon of 2005 to 2595 with 10 year increment timesteps. If you wish to change this, say, to 2000 to 2500 by 10 year increment timesteps and use parameters that match this time, you could use the following code:

First you upate the time dimension of the model as follows:

const ts = 10
const years = collect(2000:ts:2500)
nyears = length(years)
set_dimension!(m, :time, years)

Next, create a dictionary params with one entry (k, v) per external parameter by name k to value v. Each key k must be a symbol or convert to a symbol matching the name of an external parameter that already exists in the model definition. Part of this dictionary may look like:

params = Dict{Any, Any}()
params[:a1]         = 0.00008162
params[:a2]         = 0.00204626
params[:S]          = repeat([0.23], nyears)

Now you simply update the parameters listen in params and re-run the model with

update_params!(m, params, update_timesteps=true)

Note that here we use the update_timesteps flag and set it to true, because since we have changed the time index we want the time labels on the parameters to change, not simply their values.

Component and Structural Modifications: The API

Most model modifications will include not only parametric updates, but also strutural changes and component modification, addition, replacement, and deletion along with the required re-wiring of parameters etc. The most useful functions of the common API, in these cases are likely replace_comp!, add_comp! along with Mimi.delete! and the requisite functions for parameter setting and connecting. For detail on the public API functions look at the API reference here.

If you wish to modify the component structure we recommend you also look into the built-in helper components adder, ConnectorCompVector, and ConnectorCompMatrix in the src/components folder, as these can prove quite useful.

Component and Structural Modifications: DICE Example

This example is in progress and will be built out soon.