Inconsistent halo values across restarts in SETTE tests

[acc]

Context

Attempts to derive a SETTE test for SAS configurations by comparing restarts in restart ability tests has revealed a minor issue with sbc_tsc_b values differing between continuous runs and restarted runs covering the same period. Differences are found in halo regions which have no impact on results but invalidate the proposed new test.

Analysis

The cause has been traced to the initialisation carried out in trasbc.F90. Currently the code is (in shorter, pre-OMP form):

      IF( kt == nit000 ) THEN             !* 1st time-step
         IF( ln_rstart .AND.    &               ! Restart: read in restart file
              & iom_varid( numror, 'sbc_hc_b', ldstop = .FALSE. ) > 0 ) THEN
            IF(lwp) WRITE(numout,*) '          nit000-1 sbc tracer content field read in the restart file'
            zfact = 0.5_wp
            CALL iom_get( numror, jpdom_autoglo, 'sbc_hc_b', sbc_tsc_b(:,:,jp_tem) )   ! before heat content sbc trend
            CALL iom_get( numror, jpdom_autoglo, 'sbc_sc_b', sbc_tsc_b(:,:,jp_sal) )   ! before salt content sbc trend
         ELSE                                   ! No restart or restart not found: Euler forward time stepping
            zfact = 1._wp
            sbc_tsc(:,:,:) = 0._wp
            sbc_tsc_b(:,:,:) = 0._wp
         ENDIF
      ELSE                                !* other time-steps: swap of forcing fields
         zfact = 0.5_wp
         sbc_tsc_b(:,:,:) = sbc_tsc(:,:,:)
      ENDIF
      !                             !==  Now sbc tracer content fields  ==!
      DO jj = 2, jpj
         DO ji = fs_2, fs_jpim1   ! vector opt.
            sbc_tsc(ji,jj,jp_tem) = r1_rau0_rcp * qns(ji,jj)   ! non solar heat flux
            sbc_tsc(ji,jj,jp_sal) = r1_rau0     * sfx(ji,jj)   ! salt flux due to freezing/melting
         END DO
      END DO

Note that on a restart, sbc_tsc is not initialised and only inner points are subsequently set before the:

         sbc_tsc_b(:,:,:) = sbc_tsc(:,:,:)

instruction on the next time step. Thus the unused haloes can (and do) contain arbitrary values unless unset values are automatically set to zero by the compiler.

Recommendation

The simplest solution is to introduce a:

            sbc_tsc(:,:,:) = 0._wp

immediately before the first call to iom_get.

Of course for OMP operability what actually needs to be inserted is:

            DO jn = 1, jpts
!$OMP PARALLEL DO schedule(static) private(jj, ji)
               DO jj = 1, jpj
                  DO ji = 1, jpi
                     sbc_tsc(ji,jj,jn) = 0._wp  ! needed just to ensure haloes are consistent across restarts
                  END DO
               END DO
            END DO

[acc]

Fix submitted to trunk at Changeset: [7710]