Joining partitioned ROMS files

In addition to requiring partitioned input files, ROMS produces partitioned output files, which need to be joined to be analyzed. We can join these files using the join_netcdf function.

Writing some partitioned example files

As in the page in partitioning, we will create and partition some example files to demonstrate the use of the joining tool:

from roms_tools import Grid

grid = Grid(
    nx=300,
    ny=150,
    size_x=23000,
    size_y=12000,
    center_lon=-161.0,
    center_lat=14.4,
    rot=-3.0,
)

grid.plot()

filepath_grid="my_roms_grid.nc"
grid.save(filepath_grid)

2025-11-21 13:36:49 - INFO - Writing the following NetCDF files:
my_roms_grid.nc

[PosixPath('my_roms_grid.nc')]

from roms_tools import partition_netcdf
partition_netcdf(filepath_grid, np_xi=5, np_eta=3, output_dir="to_join/")

[PosixPath('to_join/my_roms_grid.00.nc'),
 PosixPath('to_join/my_roms_grid.01.nc'),
 PosixPath('to_join/my_roms_grid.02.nc'),
 PosixPath('to_join/my_roms_grid.03.nc'),
 PosixPath('to_join/my_roms_grid.04.nc'),
 PosixPath('to_join/my_roms_grid.05.nc'),
 PosixPath('to_join/my_roms_grid.06.nc'),
 PosixPath('to_join/my_roms_grid.07.nc'),
 PosixPath('to_join/my_roms_grid.08.nc'),
 PosixPath('to_join/my_roms_grid.09.nc'),
 PosixPath('to_join/my_roms_grid.10.nc'),
 PosixPath('to_join/my_roms_grid.11.nc'),
 PosixPath('to_join/my_roms_grid.12.nc'),
 PosixPath('to_join/my_roms_grid.13.nc'),
 PosixPath('to_join/my_roms_grid.14.nc')]

Joining the example files

Each of these files has a subset of the original grid in it. To reassemble them into a coherent whole, we use the join_netcdf tool. The tool takes a wildcard pattern or list of files, and infers the original layout. We can also provide an optional output_path, or ROMS-Tools will take the common root filename by default:

from roms_tools import join_netcdf

%%time
joined_grid_path = join_netcdf("to_join/my_roms_grid.??.nc")

Saved joined dataset to: to_join/my_roms_grid.nc
CPU times: user 353 ms, sys: 96.5 ms, total: 449 ms
Wall time: 1.32 s

Comparing with the original grid, we can see the partitions have joined correctly:

joined_grid = Grid(filename="to_join/my_roms_grid.nc")

joined_grid.plot()