Version 1 (modified by snguyen, 2 years ago) (diff)

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# How the code for the Schwarz method implementation in NEMO is organized

## Implementation of the Schwarz method

### The time stepping loop

The essential part to understand is how the original time stepping loop of NEMO in nemogcm.F90 is modified to allow rewinding of the state of NEMO after a coupling window has passed.

Initially the code has one loop over the command CALL stp( istp ) for each time step.

With the Schwarz algorithm, the total amount of time steps is divided in coupling windows (called Schwarz loops in the code), each coupling window is repeated the number of schwarz iterations given as a parameter (mswr), and for each schwarz iteration a number of time steps (the length of a coupling window) is done.

For a computation of 5 days with 15 time steps per day, a coupling window of 1 day and 5 schwarz iterations (mswr=5) there will be 3 loops.

• One global loop over the coupling windows : 5 to iterate. iswloop
• One inner loop over the number of schwarz iteration for each coupling window : 5 (mswr=5). kswr
• One most inner loop over the time steps done in one Schwarz coupling window : 15 time steps. istp

The counter given to stp is rewind when the Schwarz coupling window is repeated.

This is coded like this in nemogcm.F90:

```         iswloop = 1
IF (lwp) WRITE(numout,*) 'init iswloop =',iswloop

DO WHILE ( iswloop <= nsloops .AND. nstop == 0 ) ! iterate on n schwarz loops

kswr = 1
IF (lwp) THEN
WRITE(numout,*) '*** schwarz loops ***'
WRITE(numout,*) 'iswloop =',iswloop
WRITE(numout,*) 'kswr = ',kswr
ENDIF

CALL swz_store  ! store Ocean state at first schwarz iteration before first time step
CALL swz_store_lim3 ! store lim3 variables state
CALL swz_store_limdiahsb ! store limdiahsb variables
IF(lwp) WRITE(numout,*) 'store schwarz initial state '

DO WHILE ( kswr <= mswr .AND. nstop == 0 )

IF (lwp) WRITE(numout,*) '  kswr = ',kswr
istp = nit000 + (iswloop-1)*ntsinswr ! set time step to first value for iswloop (current schwarz loop)

IF ( kswr > 1 ) CALL swz_reinit(istp) ! reset Ocean state to first time step for new schwarz iteration

IF (lwp) WRITE(numout,*) 'maxistp  = ',nit000+iswloop*ntsinswr-1
DO WHILE ( istp <= nit000+iswloop*ntsinswr-1 .AND. nstop == 0 )

IF (lwp) WRITE(numout,*) '    istp = ',istp,';    istpswz = ',istpswz
#if defined key_agrif
CALL stp                         ! AGRIF: time stepping
#else
CALL stp( istp )                 ! standard time stepping
#endif
istp = istp + 1
istpswz = istpswz + 1
IF( lk_mpp )   CALL mpp_max( nstop )
END DO ! istp

kswr = kswr + 1

END DO ! kswr

iswloop = iswloop +1

END DO ! iswloop
```

The variable iswloop is the current Schwarz loop and nsloops is the number of Schwarz loops for the given time of simulation and Schwarz window size. For 5 days with a window of 1 day nsloops=5.

Inside the Schwarz repeating loop DO WHILE ( kswr <= mswr .AND. nstop == 0 ), in the time stepping loop, istp starts at the value nit000 + (iswloop-1)*ntsinswr and finishes at nit000+iswloop*ntsinswr-1.

This allows istp to take the value it would have for the current Schwarz window if no sub-iteration was done: setting mswr=1 gives you the original time stepping.

The Schwarz parameters are initialized (or computed) before entering the time loops, before and after the call to nemo_init depending on what is needed:

```      ntsinswr = 1 !! set to allow fldread in nemo_init before computation of schwarz indices
kswr = 0     !! set here to allow day_init to set nitrst properly

WRITE(numout,*) "ntsinswr = ",ntsinswr

!                            !-----------------------!
CALL nemo_init               !==  Initialisations  ==!
!                            !-----------------------!

!! set integer constants for computation
!      ncplfrq  = 86400 !cpl_freq( 'O_QnsMix' ) read in namrun (nemoinit)
ntsinswr = ncplfrq / INT(rdt)
nsloops  = (nitend-nit000+1)/ntsinswr

!! outputs to check values
IF(lwp) THEN                  ! control print
WRITE(numout,*)
WRITE(numout,*) ' computation of loop indices for schwarz computation '
WRITE(numout,*) '   nit000   = ',nit000
WRITE(numout,*) '   nitend   = ',nitend
WRITE(numout,*) '   rdt      = ',rdt
WRITE(numout,*) '   ncplfrq  = ',ncplfrq
WRITE(numout,*) '   ntsinswr = ',ntsinswr
WRITE(numout,*) '   nsloops  = ',nsloops
WRITE(numout,*) '   mswr     = ',mswr
WRITE(numout,*) '   nitrst   = ',nitrst
ENDIF

```

There is also an "absolute" time step counter : istpswz which is incremented at each time step including those repeated for the Schwarz algorithm. It is needed for the coupling with OASIS.

### The Schwarz algorithm: storing and restoring

The Schwarz algorithm is implemented by storing and restoring the state of the ocean and ice at the proper times.

The routines used to store and restore a state are all in schwarz.F90 which is a new subroutine.

The storing is done just before the schwarz looping is done: inside iswloop loop, before kswr loop. Like this we are sure to store the initial condition of a schwarz loop.