# Using Intake-ESM’s New Derived Variable Functionality#

Last week, Anderson Banihirwe added a new feature to intake-esm, enabling users to add “derived variables” to catalogs! This is an exciting addition, and although this has not been included in a release yet, you are welcome to test out the functionality!

Keep in mind that this is an initial prototype and the API is likely to change.

## What is a “Derived Variable”#

A “derived variable” in this case is a variable that doesn’t itself exist in an intake-esm catalog, but can be computed (i.e., “derived”) from variables that do exist in the catalog. Currently, the derived variable implementation requires variables on the same grid, etc.; i.e., it assumes that all variables involved can be merged within the same dataset.

An example of a derived variable could be temperature in degrees Fahrenheit. Often times, climate model models write temperature in Celsius or Kelvin, but the user may want degrees Fahrenheit! This is a really simple example; derived variables could include more sophsticated diagnostic output like aggregations of terms in a tracer budget or gradients in a particular field.

A traditional workflow for “derived variables” might consist of the following:

• Apply some function to the loaded datasets

• Plot the output

This has shortcomings in the context of an automated process. For example, imagine the following.

for

But what if we could couple those first two steps? What if we could have some set of **variable definitions**, consisting of variable requirements, such as dependent variables, and a function which derives the quantity. This is what the derived_variable funtionality offers in intake-esm! This enables users to share a "registry" of derived variables across catalogs!

Let's get started with an example!


## Installing the Development Version of Intake-ESM#

Since this has not yet been released in a version of Intake-ESM, you can install the development version using the following:

pip install git+https://github.com/intake/intake-esm.git


## Imports#

import holoviews as hv
import hvplot
import hvplot.xarray
import intake
import numpy as np
from distributed import Client
from intake_esm.derived import DerivedVariableRegistry
from ncar_jobqueue import NCARCluster

hv.extension('bokeh')