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The GRIB files from an existing experiment can also be used to create initial files for the shallow-water model. In the following example, the initial files from the T21 test case distributed with the OpenIFS tarfile are used.

#  Extract a single level (level 1) for the shallow water model
#  and change the experiment id in the file.

expid="epc8"
newid="sw01"

grib_copy -w level=1,shortName=vo ICMSHepc8INITICMSH${expid}INIT tmp
grib_set -s experimentVersionNumber=epc9"$newid" -w experimentVersionNumber=epc8"$expid" tmp vo.grb

grib_copy -w level=1,shortName=d  ICMSHepc8INITICMSH${expid}INIT tmp
grib_set -s experimentVersionNumber=epc9"$newid" -w experimentVersionNumber=epc8"$expid" tmp d.grb

grib_copy -w shortName=z          ICMSHepc8INIT ICMSH${expid}INIT z.grb

#  Combine the 3 files to form the spectral initial file
cat vo.grb d.grb z.grb  > ICMSHepc9INIT ICMSH${newid}INIT

#  Create gridpoint file (stl1 is used in this example)
grib_copy -w shortName=stl1 ICMGGepc8INITICMGG${expid}INIT gg.grb
grib_set -s experimentVersionNumber=epc9${newid} -w experimentVersionNumber=epc8${expid} gg.grb ICMGGepc9INITICMGG${newid}INIT

For idealized configurations the field in the initial data file is used correctly set the grid. The actual values are overwritten by the model code to set the idealized start. Hence stl1 is used in this example to for the initial gridpoint file because its values will be overwritten and not used to initialize the model fields.

If using the Euler method, make sure a smaller suitable choice of timestep compared to the semi-Lagrangian setup is used. Also, ensure the Asselin time-filter is on.

!  Shallow-water model with semi-Lagrangian advection.
!  The variables below must be set in order to correctly configure
!  the shallow-water model.
!  Other variables (if present) can be left as-is.

For namelist : NAMDYN
 HDIRVOR=1.e15,        ! horizontal diffusion parameters
 HDIRDIV=1.e15,        ! see code for further description
 HDIRSP=1.e15,
 LHDIFFM=true,
 NDIFFACT=6,
 TSTEP=1800.0,         ! model timestep (resolution dependent)
 LSETTLS=true,         ! ensure SL scheme is enabled
 LSETTLST=true,
 SITR=350.,            ! reference temperature
 REFGEO=78452.0,       ! reference geopotential for SW model

For namelist: NAMCT0
 LRFRIC=false,         ! turn of Rayleigh friction
 LFPOS=false,          ! disable fullpos post-processing
 N2DINI=1,             ! 2D initialisation switch
 NCONF=201,            ! select shallow-water configuration
 LTWOTL=true,          ! enable two-time level SL scheme
 LSLAG=true,           ! enable SL scheme
 LSLPHY=false,         ! turn off Sl physics option
 LSPRT=false,          ! turn off 'virtual temperature' as spectral variable
 LFRHISNFRHIS=10,            ! frequency of results output

For namelist: NAEPHY
 LEPHYS=false,           ! turn off ECMWF Physics package (master switch)
 LERADI=false,           ! turn off radiation scheme

For namelist: NAMDYNA
 LGRADSP=.false.,        ! disable de-aliasing the pressure gradient term

For namelist: NAMDIM
  NUNDEFLD=0,            ! make sure uninitialised variables are all set to zero

!  Shallow-water model with Euler advection.
!  The variables below must be set in order to correctly configure
!  the shallow-water model.
!  Other variables (if present) can be left as-is.

For namelist: NAMDYN
 TSTEP=300.0,               ! reduce timestep for Euler stepping
 REPS1=0.01,                ! turn on Asselin time-filtering coefficient
 REPS2=0.01,
 LHDIFFM=.false.,           !  horizontal diffusion on / off
 LSETTLS=.false.,           ! extrapolations in SL scheme
 LSETTLST=.false.,          ! ditto

For namelist: NAMCT0
 NCONF=201,                 ! model configuration: 201 = shallow-water (see yomct0.F90)
 LSLAG=.false.,             ! turn off semi-lagrangian scheme
 LTWOTL=.false.,            ! disable two-time-level SL scheme.
 LRFRIC=.false.,            ! turn off Rayleigh friction
 LSLPHY=.false.,            ! turn off split time-step physics
 LVERTFE=.false.,           ! turn off vertical finite element scheme
 N2DINI=1,                  ! initialise  1 = Haurwitz wave, 2 = real fields
 LSPRT=.false.,             ! if T temperature is 'virtual temperature'
 LFPOS=false,               ! turn off fullpos diagnostics, does not work with SW
 N3DINI=0,                  ! no 3D initialisation
 NSTOP=600,                 ! no. of steps to run
 NFRHIS=10,                 ! frequency of results output

For namelist: NAEPHY
 LEPHYS=false,           ! turn off ECMWF Physics package (master switch)
 LERADI=false,           ! turn off radiation scheme

For namelist: NAMDYNA
 LGRADSP=.false.,        ! disable de-aliasing the pressure gradient term

For namelist: NAMDIM
  NUNDEFLD=0,            ! make sure uninitialised variables are all set to zero

with thanks to Nils Wedi and Sylvie Malardel, ECMWF.