!> @file disturb_heatflux.f90
!------------------------------------------------------------------------------!
! This file is part of the PALM model system.
!
! PALM is free software: you can redistribute it and/or modify it under the
! terms of the GNU General Public License as published by the Free Software
! Foundation, either version 3 of the License, or (at your option) any later
! version.
!
! PALM is distributed in the hope that it will be useful, but WITHOUT ANY
! WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
! A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
!
! You should have received a copy of the GNU General Public License along with
! PALM. If not, see <http://www.gnu.org/licenses/>.
!
! Copyright 1997-2021 Leibniz Universitaet Hannover
!------------------------------------------------------------------------------!
!
! Description:
! ------------
!> Generate random, normally distributed heatflux values and store them as the
!> near-surface heatflux.
!------------------------------------------------------------------------------!
 SUBROUTINE disturb_heatflux( surf )
 

    USE arrays_3d,                                                             &
        ONLY:  heatflux_input_conversion
        
    USE control_parameters,                                                    &
        ONLY:  iran, surface_heatflux, random_generator, wall_heatflux
        
    USE kinds
    
    USE indices,                                                               &
        ONLY:  nzb

    USE random_generator_parallel,                                             &
        ONLY:  random_number_parallel, random_seed_parallel, random_dummy,     &
               seq_random_array

    USE surface_mod,                                                           &
        ONLY:  surf_type

    IMPLICIT NONE

    INTEGER(iwp) ::  i  !< grid index, x direction
    INTEGER(iwp) ::  j  !< grid index, y direction
    INTEGER(iwp) ::  k  !< grid index, z direction
    INTEGER(iwp) ::  m  !< loop variables over surface elements
    
    REAL(wp) ::  random_gauss  !<
    REAL(wp) ::  randomnumber  !<

    TYPE(surf_type) ::  surf   !< surface-type variable


!
!-- Generate random disturbances and store them. Note, if 
!-- random_generator /= 'random-parallel' it is not guaranteed to obtain  
!-- the same random distribution if the number of processors is changed. 
    IF ( random_generator /= 'random-parallel' )  THEN

       DO  m = 1, surf%ns

          k = surf%k(m)

          randomnumber = random_gauss( iran, 5.0_wp )      
!
!--       k-1 is topography top index. If this is 0, set surface heatflux. Over
!--       topography surface_heatflux is replaced by wall_heatflux(0).
          IF ( k-1 == 0 )  THEN
             surf%shf(m) = randomnumber * surface_heatflux                     &
                              * heatflux_input_conversion(nzb)
          ELSE
             surf%shf(m) = randomnumber * wall_heatflux(0)                     &
                              * heatflux_input_conversion(k-1)
          ENDIF
       ENDDO
    ELSE

       DO  m = 1, surf%ns

          i = surf%i(m)
          j = surf%j(m)
          k = surf%k(m)

          CALL random_seed_parallel( put=seq_random_array(:, j, i) )
          CALL random_number_parallel( random_dummy )    
!
!--       k-1 is topography top index. If this is 0, set surface heatflux. Over
!--       topography surface_heatflux is replaced by wall_heatflux(0).
          IF ( k-1 == 0 )  THEN
             surf%shf(m) = ( random_dummy - 0.5_wp ) * surface_heatflux        &
                              * heatflux_input_conversion(nzb)
          ELSE
             surf%shf(m) = ( random_dummy - 0.5_wp ) * wall_heatflux(0)        &
                              * heatflux_input_conversion(k-1)
          ENDIF

          CALL random_seed_parallel( get=seq_random_array(:, j, i) )

       ENDDO

    ENDIF


 END SUBROUTINE disturb_heatflux