6. Limited Area Model (LAM) Grids: Predefined and User-Generated Options

In order to set up the workflow and experiment generation of the UFS SRW App, the user must choose between three predefined FV3-LAM grids, or generate a user-defined grid. At this time, full support will only be provided to those using one of the three predefined grids supported in this release. However, preliminary information is provided at the end of this chapter that describes how users can leverage the SRW App workflow scripts to generate their own user-defined grid. This feature is not fully supported at this time and is ‘use at your own risk’.

6.1. Predefined Grids

The UFS SRW App release includes three predefined LAM grids that users can choose from prior to generating a workflow/experiment configuration. To select a predefined grid, the PREDEF_GRID_NAME variable within the config.sh script needs to be set to one of the following three options:

  • RRFS_CONUS_3km

  • RRFS_CONUS_13km

  • RRFS_CONUS_25km


Fig. 6.1 The boundary of the RRFS_CONUS_3km computational grid (red) and corresponding write-component grid (blue).

The predefined grids are named after the prototype 3-km continental United States (CONUS) grid being tested for the Rapid Refresh Forecast System (RRFS), which will be a convection-allowing, hourly-cycled, FV3-LAM-based ensemble planned for operational implementation in 2024. To allow for use of High Resolution Rapid Refresh (HRRR) data to initialize the SRW App, all three supported grids were created to fit completely within the HRRR domain. Three resolution options were provided for flexibility related to compute resources and physics options. For example, a user may wish to use the 13-km or 25-km domain when running with the FV3_GFS_v15p2 suite definition file (SDF), since that SDF uses cumulus physics that are not configured to run at 3-km. In addition, users will have much fewer computational constraints when running with the 13-km and 25-km domains.

The boundary of the RRFS_CONUS_3km domain is shown in Figure 6.1 (in red). Note that while it is possible to initialize the FV3-LAM with coarser external model data when using the RRFS_CONUS_3km domain, it is generally advised to use external model data that has a resolution similar to that of the native FV3-LAM (predefined) grid. In addition, this grid is ideal for running the FV3_RRFS_v1alpha suite definition file (SDF), since this SDF was specifically created for convection-allowing scales, and is the precursor to the operational physics suite that will be used in the RRFS.

As can be seen in Figure 6.1, the boundary of the write-component grid (in blue) sits just inside the computational domain (in red). This extra grid is required because the post-processing utility (UPP) is currently unable to process data on the native FV3 gnomonic grid (in red). Therefore, model data are interpolated to a Lambert conformal grid (the write component grid) in order for UPP to read in and correctly process the data.

The RRFS_CONUS_13km grid (Fig. 6.2) also covers the full CONUS, but due to its coarser resolution, and the need to remain within the HRRR domain, areas of the contiguous United States, such as Northern Washington, Southern Texas, and the Florida Keys, are closer to the boundaries of the grid than in the RRFS_CONUS_3km grid. This grid is meant to be run with the FV3_GFS_v15p2 SDF.


Fig. 6.2 The boundary of the RRFS_CONUS_13km computational grid (red) and corresponding write-component grid (blue).

The final predefined CONUS grid (Fig. 6.3) uses a 25-km resolution and is meant mostly for quick testing to ensure functionality prior to using a higher-resolution domain. However, for users who would like to use this domain for research, the FV3_GFS_v15p2 SDF is recommended.


Fig. 6.3 The boundary of the RRFS_CONUS_25km computational grid (red) and corresponding write-component grid (blue).

6.2. Creating User-Generated Grids

While the three predefined grids available in this release are ideal for users just starting out with the SRW App, more advanced users may wish to create their own grid for testing over a different region and/or with a different resolution. Creating a user-defined grid requires knowledge of how the SRW App workflow functions, in particular, understanding the set of scripts that handle the workflow and experiment generation. It is also important to note that user-defined grids are not a supported feature of the current release, however information is being provided for the benefit of the FV3-LAM community.

With those caveats in mind, this section provides instructions for adding a new grid to the FV3-LAM workflow that will be generated using the “ESGgrid” method (i.e. using the regional_esg_grid code in the UFS_UTILS repository, where ESG stands for “Extended Schmidt Gnomonic”). We assume here that the grid to be generated covers a domain that (1) does not contain either of the poles and (2) does not cross the -180 deg –> +180 deg discontinuity in longitude near the international date line. Instructions for domains that do not have these restrictions will be provided in a future release.

The steps to add such a grid to the workflow are as follows:

  1. Decide on the name of the grid. For the purposes of this documentation, the grid will be called “NEW_GRID”.

  2. Add NEW_GRID to the array valid_vals_PREDEF_GRID_NAME in the ufs-srweather-app/regional_workflow/ush/valid_param_vals.sh file.

  3. In the file ufs-srweather-app/regional_workflow/ush/set_predef_grid_params.sh, add a stanza to the case statement case ${PREDEF_GRID_NAME} in for NEW_GRID. An example of such a stanza is given below along with comments describing the variables that need to be set.

To run a forecast experiment on NEW_GRID, start with a workflow configuration file for a successful experiment (this file is named config.sh and is located in the directory ufs-srweather-app/regional_workflow/ush) and change the line for PREDEF_GRID_NAME to the following:


Then, generate a new experiment/workflow using generate_FV3LAM_wflow.sh in the usual way.

The following is an example of a stanza for “NEW_GRID” to be added to set_predef_grid_params.sh:

# Stanza for NEW_GRID.  This grid covers [provide a description of the
# domain that NEW_GRID covers, its grid cell size, etc].

# The method used to generate the grid.  This example is specifically
# for the "ESGgrid" method.

# The longitude and latitude of the center of the grid, in degrees.

# The grid cell sizes in the x and y directions, where x and y are the
# native coordinates of any ESG grid.  The units of x and y are in
# meters. These should be set to the nominal resolution we want the
# grid to have. The cells will have exactly these sizes in xy-space
# (computational space) but will have varying size in physical space.
# The advantage of the ESGgrid generation method over the GFDLgrid
# method is that an ESGgrid will have a much smaller variation in grid
# size in physical space than a GFDLgrid.

# The number of cells along the x and y axes.

# The width of the halo (in units of grid cells) that the temporary
# wide-halo grid created during the grid generation task (make_grid)
# will have.  This wide-halo grid gets "shaved" down to obtain the
# 4-cell-wide halo and 3-cell-wide halo grids that the forecast model
# (as well as other codes) will actually use.  Recall that the halo is
# needed to provide lateral boundary conditions to the forecast model.
# Usually, there is no need to modify this parameter.

# The default physics time step that the forecast model will use. This
# is the (inverse) frequency with which (most of) the physics suite is
# called. The smaller the grid cell size is, the smaller this value
# needs to be in order to avoid numerical instabilities during the
# forecast.  The values specified below are used only if DT_ATMOS is
# not explicitly set in the user-specified experiment configuration
# file config.sh.  Note that this parameter may be suite dependent.
  if [ "${CCPP_PHYS_SUITE}" = "FV3_GFS_v15p2" ]; then
  elif [ "${CCPP_PHYS_SUITE}" = "FV3_RRFS_v1alpha" ]; then

# Default MPI task layout (decomposition) along the x and y directions and blocksize.
# The values specified below are used only if they are not explicitly set in the user-specified
# experiment configuration file config.sh.

# The parameters for the write-component (aka "quilting") grid. This
# is the grid to which the output fields from the forecast are
# interpolated. The output fields are not specified on the native grid
# but are instead remapped to this write-component grid because the
# post-processing software (UPP; called during the run_post tasks) is
# not able to process fields on the native grid.  The variable
# "QUILTING", which specifies whether or not to use the
# write-component grid, is by default set to "TRUE".
  if [ "$QUILTING" = "TRUE" ]; then

# The number of "groups" of MPI tasks that may be running at any given
# time to write out the output.  Each write group will be writing to
# one set of output files (a dynf${fhr}.nc and a phyf${fhr}.nc file,
# where $fhr is the forecast hour).  Each write group contains
# WRTCMP_write_tasks_per_group tasks. Usually, it is sufficient to
# have just one write group.  This may need to be increased if the
# forecast is proceeding so quickly that a single write group cannot
# complete writing to its set of files before there is a need/request
# to start writing the next set of files at the next output time (this
# can happen, for instance, if the forecast model is trying to write
# output at every time step).

# The number of MPI tasks to allocate to each write group.

# The coordinate system in which the write-component grid is
# specified. See the array valid_vals_WRTCMP_output_grid (defined in
# the script valid_param_vals.sh) for the values this can take on.
# The following example is specifically for the Lambert conformal
# coordinate system.

# The longitude and latitude of the center of the write-component
# grid.

# The first and second standard latitudes needed for the Lambert
# conformal coordinate mapping.

# The number of grid points in the x and y directions of the
# write-component grid. Note that this xy coordinate system is that of
# the write-component grid (which in this case is Lambert conformal).
# Thus, it is in general different than the xy coordinate system of
# the native ESG grid.

# The longitude and latitude of the lower-left corner of the
# write-component grid, in degrees.

# The grid cell sizes along the x and y directions of the
# write-component grid.  Units depend on the coordinate system used by
# the grid (i.e. the value of WRTCMP_output_grid). For a Lambert
# conformal write-component grid, the units are in meters.