subroutine write_field( fname, u )

  include 'sam.h'
  include 'mpif.h'

  complex u(Nz_min,Ny_min,nxp,Lu)
  character(*) fname
  character(6) ext
  integer(kind=mpi_offset_kind) :: offset, irecl_out
  integer amode, status(mpi_status_size)

  n_words_out = Nz_min*Ny_min
  irecl_out   = n_words_out*16

  amode = ior( mpi_mode_create, mpi_mode_wronly )

  call mpi_file_open( mpi_comm_world, fname, amode,
 &                    mpi_info_null, fh(3), ierr )

  do n=1, Lu
     do i=1, nxp
        ii = ixs + i-1
        offset = int(((n-1)*Nx_min + (ii-1)),8)*irecl_out
        call mpi_file_write_at( fh(3), offset, u(1,1,i,n),
 &           n_words_out, mpi_double_complex, status, ierr )
     end do
  end do

  call mpi_file_close( fh(3), ierr )

  return
  end