Remapping

It is frequently useful when working with observational datasets or visualizing MPAS data to remap between different global or regional grids and meshes. The pyremap provides capabilities for making mapping files (which contain the weights needed to interpolate between meshes) and using them to remap files or xarray.Dataset objects. Polaris provides a step for producing such a mapping file. Under the hood, pyremap uses the ESMF_RegridWeightGen tool, which uses MPI parallelism. To better support task parallelism, it is best to have each MPI task be a separate polaris step. For this reason, we provide polaris.remap.MappingFileStep to perform remapping.

A remapping step can be added to a task either by creating a polaris.remap.MappingFileStep object directly or by creating a step that descends from the class. Here is an example of using MappingFileStep directly to remap data from a WOA 2023 lon-lat grid to an MPAS mesh. This could happen in the task’s __init__() or configure() method:

from polaris import Task
from polaris.remap import MappingFileStep

class MyTestCase(Task):
    def __int__(self, component):
        step = MappingFileStep(component=component, name='make_map', ntasks=64, 
                               min_tasks=1, method='bilinear')
        # indicate the the mesh from another step is an input to this step
        # note: the target is relative to the step, not the task.
        step.add_input_file(filename='woa.nc',
                            target='woa23_decav_0.25_extrap.20230414.nc',
                            database='initial_condition_database')

        step.add_input_file(filename='mesh.nc',
                            target='../mesh/mesh.nc')
        
        # you need to specify what type of source and destination mesh you
        # will use
        step.src_from_lon_lat(filename='woa.nc', lon_var='lon', lat_var='lat')
        step.dst_from_mpas(filename='mesh.nc', mesh_name='QU60')

Here is an example of creating a subclass to remap from an MPAS mesh to a global lon-lat grid with a given resolution. This is more convenient when you want to use config options to allow users to customize the step. Note that you have to set a source and destination grid before calling polaris.remap.MappingFileStep.runtime_setup(). In the example, the resolution of the lon-lat grid and the remapping method will be set using the config options provided while setting up the polaris task. We call the src_*() and dst_*() methods in the runtime_setup() method to make sure we pick up any changes to the config options that a user might have made before running the task:

from polaris.remap import MappingFileStep


class VizMap(MappingFileStep):
    def __init__(self, component, name, subdir, mesh_name):
        super().__init__(component=component, name=name, subdir=subdir,
                         ntasks=128, min_tasks=1)
        self.mesh_name = mesh_name
        self.add_input_file(filename='mesh.nc', target='../mesh/mesh.nc')

    def runtime_setup(self):
        config = self.config
        section = config['cosine_bell_viz']
        dlon = section.getfloat('dlon')
        dlat = section.getfloat('dlat')
        method = section.get('remap_method')
        self.src_from_mpas(filename='mesh.nc', mesh_name=self.mesh_name)
        self.dst_global_lon_lat(dlon=dlon, dlat=dlat, lon_min=0.)
        self.method = method

        super().runtime_setup()

With either approach, you will need to call one of the src_*() methods to set up the source mesh or grid and one of the dst_*() to configure the destination. Expect for lon-lat grids, you will need to provide a name for the mesh or grid, typically describing its resolution and perhaps its extent and the region covered.

In nearly all situations, creating the mapping file is only one step in the workflow. After that, the mapping file will be used to remap data between the meshes or grids. Steps that want to do this remapping need to have the MappingFileStep (or its subclass) as a dependency (see Adding other steps as dependencies).