39. Appendix H Unstructured Grids

39.1. Introduction

METv12.0.0+ includes limited support for using datasets on unstructured grids (UGRIDs) in the PointStat and GridStat tools. This support includes verifying UGRID forecasts against point observations (PointStat), and verifying UGRID forecasts regridded to a structured grid against gridded observations on a structured grid (GridStat). The implementation of UGRID support in METv12.0.0+ provides a mechanism for a user to describe the topology of their unstructured grid. Thus far, development to support datasets on unstructured grids has been driven by the LFRic (“elfrick”) NWP model (Adams et al. 2019), and the Model for Prediction Across Scales (MPAS) NWP model (Skamarock et al., 2012). However, the support for unstructured grids was implemented in such a way that additional unstructured grids can be utilized, with the appropriate mapping of the user’s topology to the elements that MET requires.

39.2. Unstructured Grid Files

Support for UGRID files in NetCDF format is provided. MET will attempt to auto-detect UGRID files. If the file is CF-compliant and the “Conventions” global attribute is “UGRID” or “MPAS”, or if the global attribute “mesh_spec” is present, MET recognizes the file as a UGRID file. If the user is not using a supported UGRID, or a UGRID file that does not contain the required global attributes for autodetection, the user should set the file_type configuration entry to NETCDF_UGRID. In some cases, the UGRID topology is provided in an ancillary file separate from forecast variables. This is controlled via the ugrid_coordinates_file configuration entry. This file is checked first, and then the UGRID data file. If the same elements are found in both files, the information from the data file takes precedence, and thus it is recommended to leave ugrid_coordinates_file unset if all required elements are found in the data file. If both a data and coordinates file are provided, the shape of the latitude and longitude dimensions are cross-referenced as a limited enforcement measure that the coordinates file and data file belong to the same UGRID.

39.3. Required Unstructured Grid Metadata

To correctly parse the UGRID topology, MET needs the following information that must be contained within the UGRID coordinates file or the UGRID data file. It is often the case that the variable names for these elements differ between UGRIDs. If the user is not using one of the supported UGRIDs, they will need to define the ugrid_metadata_map in a custom configuration file that they provide on the command line using the -ugrid_config command line argument, which tells MET the variable names that correspond to the following elements:

Table 39.1 Required UGRID Metadata
Metadata Map Key	Description	Required
dim_face	Dimension name for UGRID cells	Required
lat_face	Coordinate variable name for the latitude of each UGRID cell	Required
lon_face	Coordinate variable name for the longitude of each UGRID cell	Required
dim_time	Dimension name for the time coordinate	Required
time	Coordinate variable for time	Required
dim_vert	Dimension name for the vertical coordinate, if present	Optional
vert_face	Coordinate variable name for the vertical coordinate	Optional

39.4. Unstructured Grid Configuration Entries

In the PointStat and GridStat config files, a user can control aspects of the UGRID capability. Since the UGRID support in MET is optional, these items are not included in the default MET config files. If the user requires modification for any of the following, they must add them to their MET config file.

39.4.1. ugrid_dataset

If the UGRID dataset is supported by MET, then this item is set to the string identifying the UGRID. Currently supported ugrid_dataset options include “lfric”, and “mpas”. If this item is not set, the user must provide a separate UGRID configuration file on the command line using the -ugrid_config command line argument.

39.4.2. ugrid_max_distance_km

For PointStat, this is the distance from each UGRID cell center that PointStat will search outward to collect observations to use for verification. The default is 0 km, which by default will use the closest observation to the UGRID cell for verification. For GridStat, this is the distance from each forecast grid point to search for observation grid cells to include when interpolating to the forecast grid point locations. Currently, only the nearest point observation or gridded observation cell is used. See Unstructured Grid Limitations for additional details about interpolation when using GridStat.

39.4.3. ugrid_coordinates_file

The absolute path to the ancillary NetCDF file that describes the UGRID topology. This file is checked for the required UGRID metadata first, and the data file is checked second and takes precedence over this file if the same elements are found in both files.

39.4.4. ugrid_metadata_map

The mapping dictionary which allows a user to specify the variable names of the required UGRID topology elements that are required by MET. For “lfric” and “mpas”, ugrid_metadata_map comes pre-configured with MET and a user can simply set ugrid_dataset. Otherwise, the user must create a separate UGRID configuration file containing the ugrid_metadata_map containing the elements from Table 39.1 and provide it on the command line using the -ugrid_config command line argument.

39.5. Unstructured Grid Limitations

We anticipate expanding the UGRID capabilities in MET in the near future. Until then, users are encouraged to be mindful of the following limitations:

In GridStat, there is no support for verifying directly on a UGRID. The UGRID (either forecast, obs, or both) must be regridded to a structured verification grid prior to verification being performed. In most cases, gridded observations are on a structured grid and so the users are encouraged to regrid their UGRID forecast to the structured observation grid. If the user wishes to verify a UGRID observation against a UGRID forecast (for example, a UGRID model analysis), the user must define a custom verification grid tailored to their UGRID characteristics, being mindful that the regridding can be slow for large verification grids. The user is referred to Appendix B Map Projections, Grids, and Polylines for guidance on defining a structured verification grid.
Data at cell edges are currently not supported, only those variables which have data at the cell centers are supported. Users should note in particular that wind components that are typically derived using data at cell edges are currently unsupported.
No aggregation methods of point observations within the ugrid_max_distance_km are supported except NEAREST, and no aggregation methods of gridded observations within the ugrid_max_distance_km are supported except NEAREST.