SUBROUTINE disturb_field( nzb_uv_inner, dist1, field )

!------------------------------------------------------------------------------!
! Current revisions:
! -----------------
!
! Former revisions:
! -----------------
! $Id: disturb_field.f90 668 2010-12-23 13:22:58Z helmke $
!
! 667 2010-12-23 12:06:00Z suehring/gryschka
! nxl-1, nxr+1, nys-1, nyn+1 replaced by nxlg, nxrg, nysg, nyng
! Calls of exchange_horiz are modified.
!
! 420 2010-01-13 15:10:53Z franke
! Loop was split to make runs reproducible when using ifort compiler
!
! 75 2007-03-22 09:54:05Z raasch
! xrp, ynp eliminated, 2nd+3rd argument removed from exchange horiz
!
! RCS Log replace by Id keyword, revision history cleaned up
!
! Revision 1.11  2006/08/04 14:31:59  raasch
! izuf renamed iran
!
! Revision 1.1  1998/02/04 15:40:45  raasch
! Initial revision
!
!
! Description:
! ------------
! Imposing a random perturbation on a 3D-array.
! On parallel computers, the random number generator is as well called for all
! gridpoints of the total domain to ensure, regardless of the number of PEs
! used, that the elements of the array have the same values in the same
! order in every case. The perturbation range is steered by dist_range.
!------------------------------------------------------------------------------!

    USE control_parameters
    USE cpulog
    USE grid_variables
    USE indices
    USE interfaces
    USE random_function_mod

    IMPLICIT NONE

    INTEGER ::  i, j, k
    INTEGER ::  nzb_uv_inner(nysg:nyng,nxlg:nxrg)

    REAL    ::  randomnumber,                             &
                dist1(nzb:nzt+1,nysg:nyng,nxlg:nxrg), &
                field(nzb:nzt+1,nysg:nyng,nxlg:nxrg)
    REAL, DIMENSION(:,:,:), ALLOCATABLE ::  dist2


    CALL cpu_log( log_point(20), 'disturb_field', 'start' )

!
!-- Create an additional temporary array and initialize the arrays needed
!-- to store the disturbance
    ALLOCATE( dist2(nzb:nzt+1,nysg:nyng,nxlg:nxrg) )
    dist1 = 0.0
    dist2 = 0.0

!
!-- Create the random perturbation and store it on temporary array
    IF ( random_generator == 'numerical-recipes' )  THEN
       DO  i = dist_nxl(dist_range), dist_nxr(dist_range)
          DO  j = dist_nys(dist_range), dist_nyn(dist_range)
             DO  k = disturbance_level_ind_b, disturbance_level_ind_t
                randomnumber = 3.0 * disturbance_amplitude * &
                               ( random_function( iran ) - 0.5 )
                IF ( nxl <= i  .AND.  nxr >= i  .AND.  nys <= j  .AND.  &
                     nyn >= j ) &
                THEN
                   dist1(k,j,i) = randomnumber
                ENDIF
             ENDDO
          ENDDO
       ENDDO
    ELSEIF ( random_generator == 'system-specific' )  THEN
       DO  i = dist_nxl(dist_range), dist_nxr(dist_range)
          DO  j = dist_nys(dist_range), dist_nyn(dist_range)
             DO  k = disturbance_level_ind_b, disturbance_level_ind_t
#if defined( __nec )
                randomnumber = 3.0 * disturbance_amplitude * &
                               ( RANDOM( 0 ) - 0.5 )
#else
                CALL RANDOM_NUMBER( randomnumber )
                randomnumber = 3.0 * disturbance_amplitude * &
                                ( randomnumber - 0.5 )
#endif
                IF ( nxl <= i .AND. nxr >= i .AND. nys <= j .AND. nyn >= j ) &
                THEN
                   dist1(k,j,i) = randomnumber
                ENDIF
             ENDDO
          ENDDO
       ENDDO

    ENDIF

!
!-- Exchange of ghost points for the random perturbation

    CALL exchange_horiz( dist1, nbgp )
!
!-- Applying the Shuman filter in order to smooth the perturbations.
!-- Neighboured grid points in all three directions are used for the
!-- filter operation.
!-- Loop has been splitted to make runs reproducible on HLRN systems using
!-- compiler option -O3
     DO  i = nxl, nxr
        DO  j = nys, nyn
          DO  k = disturbance_level_ind_b-1, disturbance_level_ind_t+1
             dist2(k,j,i) = ( dist1(k,j,i-1) + dist1(k,j,i+1) &
                            + dist1(k,j+1,i) + dist1(k+1,j,i) &
                            ) / 12.0
          ENDDO
          DO  k = disturbance_level_ind_b-1, disturbance_level_ind_t+1
              dist2(k,j,i) = dist2(k,j,i) + ( dist1(k,j-1,i) + dist1(k-1,j,i)  &
                            + 6.0 * dist1(k,j,i)                               &
                            ) / 12.0
          ENDDO
        ENDDO
     ENDDO

!
!-- Exchange of ghost points for the filtered perturbation.
!-- Afterwards, filter operation and exchange of ghost points are repeated.
    CALL exchange_horiz( dist2, nbgp )

    DO  i = nxl, nxr
       DO  j = nys, nyn
          DO  k = disturbance_level_ind_b-2, disturbance_level_ind_t+2
             dist1(k,j,i) = ( dist2(k,j,i-1) + dist2(k,j,i+1) + dist2(k,j-1,i) &
                            + dist2(k,j+1,i) + dist2(k+1,j,i) + dist2(k-1,j,i) &
                            + 6.0 * dist2(k,j,i)                               &
                            ) / 12.0
          ENDDO
       ENDDO
    ENDDO

    CALL exchange_horiz( dist1, nbgp )

!
!-- Remove perturbations below topography (including one gridpoint above it
!-- in order to allow for larger timesteps at the beginning of the simulation
!-- (diffusion criterion))
    IF ( TRIM( topography ) /= 'flat' )  THEN
       DO  i = nxlg, nxrg
          DO  j = nysg, nyng
             dist1(nzb:nzb_uv_inner(j,i)+1,j,i) = 0.0
          ENDDO
       ENDDO
    ENDIF

!
!-- Random perturbation is added to the array to be disturbed.
    DO  i = nxlg, nxrg
       DO  j = nysg, nyng
          DO  k = disturbance_level_ind_b-2, disturbance_level_ind_t+2
             field(k,j,i) = field(k,j,i) + dist1(k,j,i)
          ENDDO
       ENDDO
    ENDDO

!
!-- Deallocate the temporary array
    DEALLOCATE( dist2 )

!
!-- Set a flag, which indicates that a random perturbation is imposed
    disturbance_created = .TRUE.


    CALL cpu_log( log_point(20), 'disturb_field', 'stop' )


 END SUBROUTINE disturb_field