# Regridding using xESMF and an existing weights file#

A fairly common request is to use an existing ESMF weights file to regrid a Xarray Dataset (1, 2). Applying weights in general should be easy: read weights then apply them using dot or tensordot on the input dataset.

In the Xarray/Dask/Pangeo ecosystem, xESMF provides an interface to ESMF for convenient regridding, includiing parallelization with Dask. Here we demonstrate how to use an existing ESMF weights file with xESMF specifically for CAM-SE.

CAM-SE is the Community Atmosphere Model with the spectral element dynamical core (Dennis et al, 2011)

The spectral element dynamical core, CAM-SE, is an unstructured grid that supports uniform resolutions based on the equiangular gnomonic cubed-sphere grid as well as a mesh refinement capability with local increases in resolution through conformal mesh refinement. CAM-SE is the default resolution for CESM2 high resolution capabilities … (Lauritzen et al. 2018).

The main challenge is the input dataset has one spatial dimension (ncol), while xESMF is hardcoded to expect two spatial dimensions (lat, lon). We solve that by adding a dummy dimension. At the end, we’ll make this plot of the vertically integrated water vapour transport (IVT)

from IPython.display import Image

Image("../images/cam-se-ivt.png", width=600)

%load_ext watermark

import hvplot.xarray
import matplotlib.pyplot as plt
import numpy as np
import xarray as xr
import xesmf

%watermark -iv