# Overview¶

ESMPy is a Python interface to the Earth System Modeling Framework (ESMF) regridding utility. ESMF is software for building and coupling weather, climate, and related models. ESMF has a robust, parallel and scalable remapping package, used to generate remapping weights. It can handle a wide variety of grids and options: logically rectangular grids, unstructured meshes and sets of unconnected points; regional or global grids; 2D or 3D; and pole and masking options. ESMF also has capabilities to read grid information from NetCDF files in a variety of formats, including the Climate and Forecast (CF) V1.6 (a.k.a GridSpec), UGRID, ESMF Unstructured and SCRIP conventions.

ESMPy provides a `Grid`

to represent single-tile logically
rectangular coordinate data, a `Mesh`

for unstructured
coordinates, and a `LocStream`

for collections of
unconnected points like observational data streams.
ESMPy supports bilinear, nearest neighbor, higher order patch recovery,
first-order conservative and second-order conservative regridding. There is
also an option to ignore unmapped destination points, mask out points on either
the source or destination, choose straight line or great circle paths when using
spherical coordinates and extrapolate data to points outside of the destination
domain. Regridding on the sphere takes place in 3D Cartesian space, so the pole
problem is not an issue as it commonly is with many Earth system grid remapping
softwares. `Grid`

and `Mesh`

objects can be created in 2D or 3D space, and 3D conservative regridding is
fully supported.

Regridding, also called remapping or interpolation, is the process of changing the grid underneath field data values while preserving the qualities of the original data. Different kinds of transformations are appropriate for different problems. Regridding may be needed when communicating data between Earth system modeling components such as land and atmosphere, or between different data sets to support analysis or visualization.

Regridding can be broken into two stages. The first stage is generation of an interpolation weight matrix that describes how points in the source grid contribute to points in the destination grid. The second stage is the multiplication of values on the source grid by the interpolation weight matrix to produce the appropriate values on the destination grid. ESMPy provides access to both stages through two separate interfaces.

There are many different interpolation methods, suitable for different problems. In ESMPy, the basic bilinear option is a two dimensional variant of linear interpolation. The higher order patch recovery is a second degree polynomial regridding method, which uses a least squares algorithm to calculate the polynomial. This method gives better derivatives in the resulting destination field than the bilinear. There are two nearest-neighbor methods which map the points from one grid to the nearest corresponding point on the other grid, from either source to destination or vice versa. These are useful for extrapolation or categorical fields.

The first-order conservative regridding is a method designed to preserve the integral of the field across the interpolation from source to destination. It uses the proportion of the area of the overlapping source and destination cells to determine appropriate weights. The second-order conservative method also preserves the integral, but uses the source gradient to give a smoother result than the first-order conservative. All of these methods can be broken down to a simple sparse matrix multiplication operation between interpolation weights and data values.

# Important Links¶

The ESMPy home page has all of the latest information on the ESMPy project including release notes, known bugs, supported platforms, and download information.

Please see the ESMF home page for more information on ESMF in general.

Fast Parallel Grid Remapping for Unstructured and Structured Grids gives a nice overview of the ESMF remapping functionality.

The ESMF Regridding Status page gives a good overview of the functionality that is available through various interfaces to ESMF regridding.

The ESMF_RegridWeightGen application is a command-line version of the functionality that is available through ESMPy.

Please contact esmf_support@list.woc.noaa.gov with any questions or problems.