ocnice_prep

Introduction

The ocnice_prep program will downscale a MOM6 or CICE6 1/4deg tripole restart file to a desired lower resolution tripole warm-start file using ESMF regridding.

This document is part of the UFS_UTILS documentation.

The ocnice_prep program is part of the UFS_UTILS project.

Description of MOM6 and CICE6 restarts

A warmstart file can be created from an existing restart file (at higher resolution) by using ESMF Regridding to map fields to a destination grid. For MOM6, the procedure produces a warm-start file (composed of T,S,U,V and Interface Height), where the utility creates the interface heights (eta) using the sea surface height (sfc) and the interface thickness (h) in the MOM6 restart. A full restart file is not generated because MOM6 has resolution dependent parameterizations, so that restart files for different resolutions might contain different fields. For CICE6, a file consisting of all the restart fields is produced. While in the form of a "true" restart, it is also more appropriately considered to be a warm-start file.

Both MOM6 and CICE6 restarts contain fields which include a dimension in addition to the 2 spatial dimensions. For MOM6, this is the vertical dimension (ocean depth) while for CICE6 it is the number of ice thickness categories. In this utility, both are referred to as the nlevs dimension. MOM6 restarts also contain metadata (e.g. units) while CICE6 restarts contain no metadata. Metadata, if any, is written to the new warm start files.

Fields in the restart files for both MOM6 and CICE6 are defined at locations on the Arakawa C-grid. Similar to the cpld_gridgen utility, these are referred to here as the Ct, Cu, Cv and Bu locations. Vectors in MOM6 are natively located at the north and east faces (Cv and Cu) while CICE vectors are located at Bu locations. NOTE: The CICE-C grid is not currently supported.

Pre-generation of a single MOM6 restart file

Because the required fields for MOM6 are located in two separate restart files, a single file containing all the necessary fields must be generated for the ocean case. This is done using NetCDF operators (NCO) with the following commands, assuming that the two required MOM6 restart files are available locally and are named MOM.res.nc and MOM.res_1.nc.

ncks -v Temp,Salt,h,u MOM.res.nc ocean.nc
ncks -v v,sfc -A MOM.res_1.nc ocean.nc

Utilizing a warm-start file for MOM6 or CICE6

For MOM6, the following MOM_input settings can then be used with the warm-start file (e.g. ocean.mx100.nc):

! === module MOM_state_initialization ===
INIT_LAYERS_FROM_Z_FILE = False  !   [Boolean] default = False
                                ! If true, initialize the layer thicknesses, temperatures, and salinities from a
                                ! Z-space file on a latitude-longitude grid.
! WARMSTARTS
THICKNESS_CONFIG = "file"       ! default = "uniform"
                                ! A string that determines how the initial layer thicknesses are specified for a
                                ! new run:
                                !     file - read interface heights from the file specified
                                !       by (THICKNESS_FILE).
THICKNESS_FILE = "ocean.mx100.nc" !
                                ! The name of the thickness file.
ADJUST_THICKNESS = True         !   [Boolean] default = False
                                ! If true, all mass below the bottom removed if the topography is shallower than
                                ! the thickness input file would indicate.
INTERFACE_IC_VAR = "eta"        ! default = "eta"
                                ! The variable name for initial conditions for interface heights relative to
                                ! mean sea level, positive upward unless otherwise rescaled.
TS_CONFIG = "file"              !
                                ! A string that determines how the initial temperatures and salinities are
                                ! specified for a new run:
                                !     file - read velocities from the file specified
TS_FILE = "ocean.mx100.nc"      !
                                ! The initial condition file for temperature.
TEMP_IC_VAR = "Temp"            ! default = "PTEMP"
                                ! The initial condition variable for potential temperature.
SALT_IC_VAR = "Salt"            ! default = "SALT"
                                ! The initial condition variable for salinity.
SALT_FILE = "ocean.mx100.nc"    ! default = "ocean.mx100.nc"
                                ! The initial condition file for salinity.
VELOCITY_CONFIG = "file"        ! default = "zero"
                                ! A string that determines how the initial velocities are specified for a new
                                ! run:
                                !     file - read velocities from the file specified
                                !       by (VELOCITY_FILE).
VELOCITY_FILE = "ocean.mx100.nc" !
                                ! The name of the velocity initial condition file.
U_IC_VAR = "u"                  ! default = "u"
                                ! The initial condition variable for zonal velocity in VELOCITY_FILE.
V_IC_VAR = "v"                  ! default = "v"

For CICE6, the warm-start file (e.g. ice.mx100.nc) can be used directly in the ice_in namelist:

runtype             = 'initial'
runid               = 'unknown'
ice_ic              = 'ice.mx100.nc'

Remapping procedure

Remapping is done using an ESMF remapping via a RouteHandle, where the both the source and destination mask values are 0. This ensures that only water points are mapped between the grids. Extrapolation is used to map destination points which are unmapped because the source grid was not a water point. Since for the ocean, the required mapping varies with depth, the utility makes use of dynamic masking if required. This allows the same RouteHandle to be used for all depths, with masking at each depth done on the fly. This is not required for CICE6, since in that case all nlevs have the same land mask value.

Mapping for MOM6 is done using bilinear mapping, whereas for CICE6 it is done using nearest-source-to-destination mapping. For CICE6, this ensures that the thermodynamic fields in the CICE6 restart, which are for each vertical layer (typically 7) and each thickness category (typically 5), are consistent when mapped.

The general procedure, for either MOM6 or CICE6 is as follows:

1. Fields are retrieved from the source file and placed on the Ct grid location. For vector fields, the vectors are rotated from their orientation along model dimensions (IJ) to eastward-northward (EN) directions.
2. Fields are packed into arrays by mapping type (bilinear or conservative) and dimensionality.
3. Fields are remapped using ESMF. This remapping may use dynamic Masking.
4. Vector fields are re-rotated back to model index direction (EN->IJ) and remapped back to their native stagger locations.
5. Fields are written into a warm-start file.

Note on rotation angles

The rotation angle retrieved from the master grid file is anglet, which is defined in the same sense as used MOM6. This is the opposite sense to the use in CICE6. The rotation formulas implement rotation in the same sense as MOM6. Since -sin(-angle) == sin(angle), no sign change of the angle is required for rotation of the CICE6 vectors. This is in contrast to the ocnpost routine, where the angle used is retrieved from CICE6's own history file and requires a sign change prior to use in the rotation formulas.

Required files

The following files are required.

• The ESMF mesh file for the source and destination grids. These are available as products of the cpld_gridgen utility.
• The remapping weights for mapping fields to and from Ct grid locations on both the souce and destination grids. These are also available as products of the cpld_gridgen utility.
• A text file (csv) listing the fields to be remapped for either the ocean or ice restart file.
• A namelist file defining the type of regridding desired (ocean or ice) and the locations of the needed weights.