python-ale

Interface to MOM6 regridding/remapping in Python

Installation

MOM6 is bundled as a git submodule, so be sure to run git submodule update --init --recursive before building/installing. At the minimum, you'll need a version of Python with the development headers, a Fortran compiler, and the netCDF library, with its Fortran bindings.

With the prerequisites in place, you'll be able to run pip install . to build and install the extension into your current Python environment. As usual for libraries like this, an isolated virtual environment is a nice place to do development.

How it works

MOM6 was built upon FMS (the Flexible Modelling System), and calls out to that for processes like domain decomposition, diagnostics, and file I/O. Within the model, there is an infra layer, that translates higher-level requests into the specific FMS routines that implement that feature. The intention was for the infra layer to be pluggable with different frameworks, so as to not depend explicitly on FMS. In practice, the abstracted routines still map quite closely onto FMS.

Because we don't need the full FMS to be able to call the ALE routines from MOM6, we implement our own infra layer, consisting of shims for a lot of the parallel- and diagnostic-related functionality. We can then build an FMS-independent MOM6 shared library, which we link against a small driver module that initialises the minimum control structures necessary to access ALE.

An example

Here's a minimal example for setting up a domain, loading a restart, and performing regridding according to that restart. Note that the parameters should correspond to those in the MOM_input that was used to generate the restart in the first place.

import pyale

params = {
    "NIGLOBAL": 160,
    "NJGLOBAL": 800,
    "GRID_CONFIG": "mercator",
    "ISOTROPIC": True,
    "SOUTHLAT": -70.275597151116,
    "LENLAT": 140.5511943022,
    "LENLON": 40,
    "TOPO_CONFIG": "file",
    "TOPO_VARNAME": "topog",
    "MAXIMUM_DEPTH": 4000.0,
    "NK": 75,
}
cs = pyale.mom_init_cs(params)
pyale.load_mom_restart(cs, "MOM.res.nc")
zstar_cs = pyale.mom_init_regrid(cs, params, "ZSTAR")
h_new = pyale.do_regrid(cs, zstar_cs)

Available regridding schemes

It's possible to create multiple regridding control structures using mom_init_regrid. Here are the possible schemes:

• ZSTAR, the stretch geopotential coordinate
• SIGMA, the terrain following coordinate
• SIGMA_SHELF_ZSTAR, a hybrid of ZSTAR and SIGMA (for ice shelf cavities)
• RHO, the continuous isopycnal coordinate
• HYCOM1, the HyCOM-like coordinate
• HYBGEN, the HyCOM hybgen coordinate
• SLIGHT, the SLight coordinte (stretched coordinate above continuous isopycnal
• ADAPTIVE, the AG coordinate