Regridding

This page describes the regridding capabilities of GCPy. GCPy currently supports regridding of data from GEOS-Chem restarts and output NetCDF files. Regridding is supported across any horizontal resolution and any grid type available in GEOS-Chem, including lat/lon (global or non-global), global standard cubed-sphere, and global stretched-grid. GCPy also supports arbitrary vertical regridding across different vertical resolutions.

Regridding for Plotting in GCPy

When plotting in GCPy (e.g. through compare_single_level() or compare_zonal_mean()), the vast majority of regridding is handled internally. You can optionally request a specific horizontal comparison resolution in compare_single_level() and compare_zonal_mean(). Note that all regridding in these plotting functions only applies to the comparison panels (not the top two panels which show data directly from each dataset). There are only two scenarios where you will need to pass extra information to GCPy to help it determine grids and to regrid when plotting.

Pass stretched-grid file paths

Stretched-grid parameters cannot currently be automatically determined from grid coordinates. If you are plotting stretched-grid data in compare_single_level() or compare_zonal_mean() (even if regridding to another format), you need to use the sg_ref_path or sg_dev_path arguments to pass the path of your original stretched-grid restart file to GCPy. If using single_panel(), pass the file path using sg_path. Stretched-grid restart files created using GCPy contain the specified stretch factor, target longitude, and target latitude in their metadata. Currently, output files from stretched-grid runs of GCHP do not contain any metadata that specifies the stretched-grid used.

Pass vertical grid parameters for non-72/47-level grids

GCPy automatically handles regridding between different vertical grids when plotting except when you pass a dataset that is not on the typical 72-level or 47-level vertical grids. If using a different vertical grid, you will need to pass the corresponding grid parameters using the ref_vert_params or dev_vert_params keyword arguments.

Automatic regridding decision process

When you do not specify a horizontal comparison resolution using the cmpres argument in compare_single_level() and compare_zonal_mean(), GCPy follows several steps to determine what comparison resolution it should use:

If both input grids are lat/lon, use the highest resolution between them (don’t regrid if they are the same resolution).
Else if one grid is lat/lon and the other is cubed-sphere (standard or stretched-grid), use a 1x1.25 lat/lon grid.
Else if both grids are cubed-sphere and you are plotting zonal means, use a 1x1.25 lat/lon grid.
Else if both grids are standard cubed-sphere, use the highest resolution between them (don’t regrid if they are the same resolution).
Else if one or more grids is a stretched-grid, use the grid of the ref dataset.

For differing vertical grids, the smaller vertical grid is currently used for comparisons.

Regridding Files

You can regrid existing GEOS-Chem restart or output diagnostic files between lat/lon and cubed-sphere formats using gcpy.file_regrid. gcpy.file_regrid can either be called directly from the command line using python -m gcpy.file_regrid or as a function (gcpy.file_regrid.file_regrid()) from a Python script or interpreter. The syntax of file_regrid is as follows:

def file_regrid(fin, fout, dim_format_in, dim_format_out, cs_res_out=0, ll_res_out='0x0',
sg_params_in=[1.0, 170.0, -90.0], sg_params_out=[1.0, 170.0, -90.0]
):
"""
Regrids an input file to a new horizontal grid specification and saves it
as a new file.
"""

Required Arguments:

fin : str: The input filename

fout : str: The output filename (file will be overwritten if it already exists)

dim_format_in : str: Format of the input file’s dimensions (choose from: classic, checkpoint, diagnostic), where classic denotes lat/lon and checkpoint / diagnostic are cubed-sphere formats

dim_format_out : str: Format of the output file’s dimensions (choose from: classic, checkpoint, diagnostic), where classic denotes lat/lon and checkpoint / diagnostic are cubed-sphere formats

Optional arguments:

cs_res_out : int

The cubed-sphere resolution of the output dataset. Not used if dim_format_out is classic

Default value: 0

ll_res_out : str

The lat/lon resolution of the output dataset. Not used if dim_format_out is not classic

Default value: ‘0x0’

sg_params_in : list[float, float, float]

Input grid stretching parameters [stretch-factor, target longitude, target latitude]. Not used if dim_format_in is classic

Default value: [1.0, 170.0, -90.0] (No stretching)

sg_params_out : list[float, float, float]

Output grid stretching parameters [stretch-factor, target longitude, target latitude]. Not used if dim_format_out is classic

Default value: [1.0, 170.0, -90.0] (No stretching)

There are three dimension formats available for regridding: classic (GEOS-Chem Classic lat/lon format), checkpoint (GCHP restart file format), and diagnostic (GCHP output file format). You can regrid between any of these formats using file_regrid, as well as between different resolutions and/or grid-types within each dimension format (e.g. standard cubed-sphere checkpoint to stretched-grid checkpoint). Note that although the cs_res_out and ll_res_out parameters are technically optional in the function, you must specify at least one of these in your call to file_regrid.

As stated previously, you can either call file_regrid.file_regrid() directly or call it from the command line using python -m gcpy.file_regrid ARGS. An example command line call (separated by line for readability) for regridding a C90 cubed-sphere restart file to a C48 stretched-grid with a stretch factor of 3, a target longitude of 260.0, and a target latitude of 40.0 looks like:

python -m gcpy.file_regrid             \
      -i initial_GEOSChem_rst.c90_standard.nc   \
      --dim_format_in checkpoint      \
      -o sg_restart_c48_3_260_40.nc       \
      --cs_res_out 48            \
      --sg_params_out 3.0 260.0 40.0      \
      --dim_format_out checkpoint