SUBROUTINE lpm

!------------------------------------------------------------------------------!
! Current revisions:
! ------------------
! 
!
! Former revisions:
! -----------------
! $Id: lpm.f90 852 2012-03-15 14:36:44Z hoffmann $
!
! 851 2012-03-15 14:32:58Z raasch
! Bugfix: resetting of particle_mask and tail mask moved from routine
! lpm_exchange_horiz to here (end of sub-timestep loop)
!
! 849 2012-03-15 10:35:09Z raasch
! original routine advec_particles split into several subroutines and renamed
! lpm
!
! 831 2012-02-22 00:29:39Z raasch
! thermal_conductivity_l and diff_coeff_l now depend on temperature and
! pressure
!
! 828 2012-02-21 12:00:36Z raasch
! fast hall/wang kernels with fixed radius/dissipation classes added,
! particle feature color renamed class, routine colker renamed
! recalculate_kernel,
! lower limit for droplet radius changed from 1E-7 to 1E-8
!
! Bugfix: transformation factor for dissipation changed from 1E5 to 1E4
!
! 825 2012-02-19 03:03:44Z raasch
! droplet growth by condensation may include curvature and solution effects,
! initialisation of temporary particle array for resorting removed,
! particle attributes speed_x|y|z_sgs renamed rvar1|2|3,
! module wang_kernel_mod renamed lpm_collision_kernels_mod,
! wang_collision_kernel renamed wang_kernel
!
! 799 2011-12-21 17:48:03Z franke
! Implementation of Wang collision kernel and corresponding new parameter 
! wang_collision_kernel
!
! 792 2011-12-01 raasch
! particle arrays (particles, particles_temp) implemented as pointers in
! order to speed up sorting (see routine sort_particles)
!
! 759 2011-09-15 13:58:31Z raasch
! Splitting of parallel I/O (routine write_particles)
!
! Revision 1.1  1999/11/25 16:16:06  raasch
! Initial revision
!
!
! Description:
! ------------
! Particle advection
!------------------------------------------------------------------------------!

    USE arrays_3d
    USE control_parameters
    USE cpulog
    USE interfaces
    USE particle_attributes
    USE pegrid
    USE statistics

    IMPLICIT NONE

    INTEGER ::  m


    CALL cpu_log( log_point(25), 'lpm', 'start' )

!
!-- Write particle data at current time on file.
!-- This has to be done here, before particles are further processed,
!-- because they may be deleted within this timestep (in case that
!-- dt_write_particle_data = dt_prel = particle_maximum_age).
    time_write_particle_data = time_write_particle_data + dt_3d
    IF ( time_write_particle_data >= dt_write_particle_data )  THEN

       CALL lpm_data_output_particles
!
!--    The MOD function allows for changes in the output interval with restart
!--    runs.
       time_write_particle_data = MOD( time_write_particle_data, &
                                  MAX( dt_write_particle_data, dt_3d ) )
    ENDIF


!
!-- Initialize arrays for marking those particles/tails to be deleted after the
!-- (sub-) timestep
    particle_mask     = .TRUE.
    deleted_particles = 0

    IF ( use_particle_tails )  THEN
       tail_mask = .TRUE.
    ENDIF
    deleted_tails = 0


!
!-- Initialize variables used for accumulating the number of particles
!-- exchanged between the subdomains during all sub-timesteps (if sgs
!-- velocities are included). These data are output further below on the
!-- particle statistics file.
    trlp_count_sum      = 0
    trlp_count_recv_sum = 0
    trrp_count_sum      = 0
    trrp_count_recv_sum = 0
    trsp_count_sum      = 0
    trsp_count_recv_sum = 0
    trnp_count_sum      = 0
    trnp_count_recv_sum = 0


!
!-- Calculate exponential term used in case of particle inertia for each
!-- of the particle groups
    DO  m = 1, number_of_particle_groups
       IF ( particle_groups(m)%density_ratio /= 0.0 )  THEN
          particle_groups(m)%exp_arg  =                                        &
                    4.5 * particle_groups(m)%density_ratio *                   &
                    molecular_viscosity / ( particle_groups(m)%radius )**2
          particle_groups(m)%exp_term = EXP( -particle_groups(m)%exp_arg * &
                                             dt_3d )
       ENDIF
    ENDDO


!
!-- Particle (droplet) growth by condensation/evaporation and collision
    IF ( cloud_droplets )  THEN

!
!--    Reset summation arrays
       ql_c = 0.0;  ql_v = 0.0;  ql_vp = 0.0

!
!--    Droplet growth by condensation / evaporation
       CALL lpm_droplet_condensation

!
!--    Particle growth by collision
       IF ( collision_kernel /= 'none' )  CALL lpm_droplet_collision

    ENDIF


!
!-- If particle advection includes SGS velocity components, calculate the
!-- required SGS quantities (i.e. gradients of the TKE, as well as horizontally
!-- averaged profiles of the SGS TKE and the resolved-scale velocity variances)
    IF ( use_sgs_for_particles )  CALL lpm_init_sgs_tke


!
!-- Initialize the variable storing the total time that a particle has advanced
!-- within the timestep procedure
    particles(1:number_of_particles)%dt_sum = 0.0


!
!-- Timestep loop for particle advection.
!-- This loop has to be repeated until the advection time of every particle
!-- (within the total domain!) has reached the LES timestep (dt_3d).
!-- In case of including the SGS velocities, the particle timestep may be
!-- smaller than the LES timestep (because of the Lagrangian timescale restric-
!-- tion) and particles may require to undergo several particle timesteps,
!-- before the LES timestep is reached. Because the number of these particle
!-- timesteps to be carried out is unknown at first, these steps are carried
!-- out in the following infinite loop with exit condition.
    DO

       CALL cpu_log( log_point_s(44), 'lpm_advec', 'start' )

!
!--    Initialize the switch used for the loop exit condition checked at the
!--    end of this loop.
!--    If at least one particle has failed to reach the LES timestep, this
!--    switch will be set false.
       dt_3d_reached_l = .TRUE.

!
!--    Particle advection
       CALL lpm_advec

!
!--    Particle reflection from walls
       CALL lpm_boundary_conds( 'walls' )

!
!--    User-defined actions after the calculation of the new particle position
       CALL user_lpm_advec

!
!--    Find out, if all particles on every PE have completed the LES timestep
!--    and set the switch corespondingly
#if defined( __parallel )
       IF ( collective_wait )  CALL MPI_BARRIER( comm2d, ierr )
       CALL MPI_ALLREDUCE( dt_3d_reached_l, dt_3d_reached, 1, MPI_LOGICAL, &
                           MPI_LAND, comm2d, ierr )
#else
       dt_3d_reached = dt_3d_reached_l
#endif

       CALL cpu_log( log_point_s(44), 'lpm_advec', 'stop' )

!
!--    Increment time since last release
       IF ( dt_3d_reached )  time_prel = time_prel + dt_3d

!
!--    If necessary, release new set of particles
       IF ( time_prel >= dt_prel  .AND.  end_time_prel > simulated_time  .AND. &
            dt_3d_reached )  THEN

          CALL lpm_release_set

!
!--       The MOD function allows for changes in the output interval with
!--       restart runs.
          time_prel = MOD( time_prel, MAX( dt_prel, dt_3d ) )

       ENDIF

!
!--    Horizontal boundary conditions including exchange between subdmains
       CALL lpm_exchange_horiz

!
!--    Apply boundary conditions to those particles that have crossed the top or
!--    bottom boundary and delete those particles, which are older than allowed
       CALL lpm_boundary_conds( 'bottom/top' )

!
!--    Pack particles (eliminate those marked for deletion),
!--    determine new number of particles
       IF ( number_of_particles > 0  .AND.  deleted_particles > 0 )  THEN
          CALL lpm_pack_arrays
       ENDIF

!
!--    Initialize variables for the next (sub-) timestep, i.e. for marking those
!--    particles to be deleted after the timestep
       particle_mask     = .TRUE.
       deleted_particles = 0

       IF ( use_particle_tails )  THEN
          tail_mask     = .TRUE.
       ENDIF
       deleted_tails = 0


       IF ( dt_3d_reached )  EXIT

    ENDDO   ! timestep loop


!
!-- Sort particles in the sequence the gridboxes are stored in the memory
    time_sort_particles = time_sort_particles + dt_3d
    IF ( time_sort_particles >= dt_sort_particles )  THEN
       CALL lpm_sort_arrays
       time_sort_particles = MOD( time_sort_particles, &
                                  MAX( dt_sort_particles, dt_3d ) )
    ENDIF


!
!-- Calculate the new liquid water content for each grid box
    IF ( cloud_droplets )  CALL lpm_calc_liquid_water_content


!
!-- Set particle attributes.
!-- Feature is not available if collision is activated, because the respective
!-- particle attribute (class) is then used for storing the particle radius
!-- class.
    IF ( collision_kernel == 'none' )  CALL lpm_set_attributes


!
!-- Set particle attributes defined by the user
    CALL user_lpm_set_attributes


!
!-- If required, add the current particle positions to the particle tails
    IF ( use_particle_tails )  CALL lpm_extend_tails


!
!-- Write particle statistics (in particular the number of particles
!-- exchanged between the subdomains) on file
    IF ( write_particle_statistics )  CALL lpm_write_exchange_statistics

    CALL cpu_log( log_point(25), 'lpm', 'stop' )


 END SUBROUTINE lpm