Plotting CESM Data on an Unstructured Grid using Geoviews and Datashader#

This week, during Xdev office hours, Steve Yeager raised a question regarding plotting unstructured grid data within Python. He was interested in plotting output from the Community Atmosphere Model, which supports unstructured grids, essentially a combination of triangles allowing for higher resolution in certain parts of the domain. This can be adventageous when wanting to achieve the benefits of high resolution within the primary domain, while maintaining the global scale of the model. This week, NCAR Science tweeted a great explanation of how revolutionary this capability is in the context of resolving processes over Greenland.

Unstructured grids can be difficult to plot directly within Python since they do not follow the typical lat, lon (x, y) convention. There is some preprocessing that needs to be applied before plotting.

The two main points include:

  • Calculating the vertices

  • Triangulating the vertices

Fortunately, SciPy and Datashader provide utilities for this! We can also use GeoViews/Holoviews to interactively look at the data.

Imports#

The main packages we will use here (outside of the “core” scientific packages), include:

Datashader#

From the datashader documention:

“Datashader is a graphics pipeline system for creating meaningful representations of large datasets quickly and flexibly. Datashader breaks the creation of images into a series of explicit steps that allow computations to be done on intermediate representations. This approach allows accurate and effective visualizations to be produced automatically without trial-and-error parameter tuning, and also makes it simple for data scientists to focus on particular data and relationships of interest in a principled way.”

Holoviews#

From the holoviews documentation:

“HoloViews is an open-source Python library designed to make data analysis and visualization seamless and simple. With HoloViews, you can usually express what you want to do in very few lines of code, letting you focus on what you are trying to explore and convey, not on the process of plotting.”

Geoviews#

From the geoviews documentation

*”GeoViews is a Python library that makes it easy to explore and visualize geographical, meteorological, and oceanographic datasets, such as those used in weather, climate, and remote sensing research.

GeoViews is built on the HoloViews library for building flexible visualizations of multidimensional data. GeoViews adds a family of geographic plot types based on the Cartopy library, plotted using either the Matplotlib or Bokeh packages. With GeoViews, you can now work easily and naturally with large, multidimensional geographic datasets, instantly visualizing any subset or combination of them, while always being able to access the raw data underlying any plot”*

You can install these packages by using the following:

conda install -c conda-forge datashader holoviews geoviews

import cartopy.crs as ccrs
import datashader as ds
import datashader.utils as du
import geoviews as gv
import geoviews.feature as gf
import holoviews as hv
import numpy as np
import pandas as pd
import xarray as xr
from datashader import reductions as rd, transfer_functions as tf
from holoviews.operation.datashader import datashade, rasterize, regrid
from scipy.spatial import Delaunay

hv.extension("bokeh")