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source: palm/trunk/SOURCE/timestep.f90 @ 1274

Last change on this file since 1274 was 1258, checked in by raasch, 11 years ago
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[1]	1	SUBROUTINE timestep
	2
[1036]	3	!--------------------------------------------------------------------------------!
	4	! This file is part of PALM.
	5	!
	6	! PALM is free software: you can redistribute it and/or modify it under the terms
	7	! of the GNU General Public License as published by the Free Software Foundation,
	8	! either version 3 of the License, or (at your option) any later version.
	9	!
	10	! PALM is distributed in the hope that it will be useful, but WITHOUT ANY
	11	! WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
	12	! A PARTICULAR PURPOSE. See the GNU General Public License for more details.
	13	!
	14	! You should have received a copy of the GNU General Public License along with
	15	! PALM. If not, see <http://www.gnu.org/licenses/>.
	16	!
	17	! Copyright 1997-2012 Leibniz University Hannover
	18	!--------------------------------------------------------------------------------!
	19	!
[258]	20	! Current revisions:
[866]	21	! ------------------
[316]	22	!
[1258]	23	!
[1]	24	! Former revisions:
	25	! -----------------
[3]	26	! $Id: timestep.f90 1258 2013-11-08 16:09:09Z heinze $
[110]	27	!
[1258]	28	! 1257 2013-11-08 15:18:40Z raasch
	29	! openacc porting
	30	! bugfix for calculation of advective timestep in case of vertically stretched
	31	! grids
	32	!
[1093]	33	! 1092 2013-02-02 11:24:22Z raasch
	34	! unused variables removed
	35	!
[1054]	36	! 1053 2012-11-13 17:11:03Z hoffmann
	37	! timestep is reduced in two-moment cloud scheme according to the maximum
	38	! terminal velocity of rain drops
	39	!
[1037]	40	! 1036 2012-10-22 13:43:42Z raasch
	41	! code put under GPL (PALM 3.9)
	42	!
[1002]	43	! 1001 2012-09-13 14:08:46Z raasch
	44	! all actions concerning leapfrog scheme removed
	45	!
[979]	46	! 978 2012-08-09 08:28:32Z fricke
	47	! restriction of the outflow damping layer in the diffusion criterion removed
	48	!
[867]	49	! 866 2012-03-28 06:44:41Z raasch
	50	! bugfix for timestep calculation in case of Galilei transformation,
	51	! special treatment in case of mirror velocity boundary condition removed
	52	!
[708]	53	! 707 2011-03-29 11:39:40Z raasch
	54	! bc_lr/ns replaced by bc_lr/ns_cyc
	55	!
[668]	56	! 667 2010-12-23 12:06:00Z suehring/gryschka
	57	! Exchange of terminate_coupled between ocean and atmosphere via PE0
	58	! Minimum grid spacing dxyz2_min(k) is now calculated using dzw instead of dzu
	59	!
[623]	60	! 622 2010-12-10 08:08:13Z raasch
	61	! optional barriers included in order to speed up collective operations
	62	!
[392]	63	! 343 2009-06-24 12:59:09Z maronga
	64	! Additional timestep criterion in case of simulations with plant canopy
	65	! Output of messages replaced by message handling routine.
	66	!
[226]	67	! 222 2009-01-12 16:04:16Z letzel
	68	! Implementation of a MPI-1 Coupling: replaced myid with target_id
	69	! Bugfix for nonparallel execution
	70	!
[110]	71	! 108 2007-08-24 15:10:38Z letzel
	72	! modifications to terminate coupled runs
	73	!
[3]	74	! RCS Log replace by Id keyword, revision history cleaned up
	75	!
[1]	76	! Revision 1.21 2006/02/23 12:59:44 raasch
	77	! nt_anz renamed current_timestep_number
	78	!
	79	! Revision 1.1 1997/08/11 06:26:19 raasch
	80	! Initial revision
	81	!
	82	!
	83	! Description:
	84	! ------------
	85	! Compute the time step under consideration of the FCL and diffusion criterion.
	86	!------------------------------------------------------------------------------!
	87
	88	USE arrays_3d
[1053]	89	USE cloud_parameters
[1]	90	USE control_parameters
	91	USE cpulog
	92	USE grid_variables
	93	USE indices
	94	USE interfaces
	95	USE pegrid
	96	USE statistics
	97
	98	IMPLICIT NONE
	99
[1257]	100	INTEGER :: i, j, k
[1]	101
[1257]	102	REAL :: div, dt_diff, dt_diff_l, dt_plant_canopy, dt_plant_canopy_l, &
	103	dt_plant_canopy_u, dt_plant_canopy_v, dt_plant_canopy_w, &
	104	dt_u, dt_u_l, dt_v, dt_v_l, dt_w, dt_w_l, u_gtrans_l, u_max_l, &
	105	u_min_l, value, v_gtrans_l, v_max_l, v_min_l, w_max_l, w_min_l
[1]	106
	107	REAL, DIMENSION(2) :: uv_gtrans, uv_gtrans_l
[1257]	108	REAL, DIMENSION(3) :: reduce, reduce_l
[1]	109	REAL, DIMENSION(nzb+1:nzt) :: dxyz2_min
	110
[667]	111
	112
[1]	113	CALL cpu_log( log_point(12), 'calculate_timestep', 'start' )
	114
	115	!
	116	!-- In case of Galilei-transform not using the geostrophic wind as translation
	117	!-- velocity, compute the volume-averaged horizontal velocity components, which
	118	!-- will then be subtracted from the horizontal wind for the time step and
	119	!-- horizontal advection routines.
	120	IF ( galilei_transformation .AND. .NOT. use_ug_for_galilei_tr ) THEN
	121	IF ( flow_statistics_called ) THEN
	122	!
	123	!-- Horizontal averages already existent, just need to average them
	124	!-- vertically.
	125	u_gtrans = 0.0
	126	v_gtrans = 0.0
	127	DO k = nzb+1, nzt
	128	u_gtrans = u_gtrans + hom(k,1,1,0)
	129	v_gtrans = v_gtrans + hom(k,1,2,0)
	130	ENDDO
	131	u_gtrans = u_gtrans / REAL( nzt - nzb )
	132	v_gtrans = v_gtrans / REAL( nzt - nzb )
	133	ELSE
	134	!
	135	!-- Averaging over the entire model domain.
[1257]	136	u_gtrans_l = 0.0
	137	v_gtrans_l = 0.0
	138	!$acc parallel present( u, v )
[1]	139	DO i = nxl, nxr
	140	DO j = nys, nyn
	141	DO k = nzb+1, nzt
[1257]	142	u_gtrans_l = u_gtrans_l + u(k,j,i)
	143	v_gtrans_l = v_gtrans_l + v(k,j,i)
[1]	144	ENDDO
	145	ENDDO
	146	ENDDO
[1257]	147	!$acc end parallel
	148	uv_gtrans_l(1) = u_gtrans_l / REAL( (nxr-nxl+1)(nyn-nys+1)(nzt-nzb) )
	149	uv_gtrans_l(2) = v_gtrans_l / REAL( (nxr-nxl+1)(nyn-nys+1)(nzt-nzb) )
[1]	150	#if defined( __parallel )
[622]	151	IF ( collective_wait ) CALL MPI_BARRIER( comm2d, ierr )
[1]	152	CALL MPI_ALLREDUCE( uv_gtrans_l, uv_gtrans, 2, MPI_REAL, MPI_SUM, &
	153	comm2d, ierr )
	154	u_gtrans = uv_gtrans(1) / REAL( numprocs )
	155	v_gtrans = uv_gtrans(2) / REAL( numprocs )
	156	#else
	157	u_gtrans = uv_gtrans_l(1)
	158	v_gtrans = uv_gtrans_l(2)
	159	#endif
	160	ENDIF
	161	ENDIF
	162
[866]	163	!
[1257]	164	!-- Determine the maxima of the velocity components, including their
	165	!-- grid index positions.
	166	#if defined( __openacc )
	167	IF ( dt_fixed ) THEN ! otherwise do it further below for better cache usage
	168	u_max_l = -999999.9
	169	u_min_l = 999999.9
	170	v_max_l = -999999.9
	171	v_min_l = 999999.9
	172	w_max_l = -999999.9
	173	w_min_l = 999999.9
	174	!$acc parallel present( u, v, w )
	175	DO i = nxl, nxr
	176	DO j = nys, nyn
	177	DO k = nzb+1, nzt
	178	u_max_l = MAX( u_max_l, u(k,j,i) )
	179	u_min_l = MIN( u_min_l, u(k,j,i) )
	180	v_max_l = MAX( v_max_l, v(k,j,i) )
	181	v_min_l = MIN( v_min_l, v(k,j,i) )
	182	w_max_l = MAX( w_max_l, w(k,j,i) )
	183	w_min_l = MIN( w_min_l, w(k,j,i) )
	184	ENDDO
	185	ENDDO
	186	ENDDO
	187	!$acc end parallel
	188	#if defined( __parallel )
	189	reduce_l(1) = u_max_l
	190	reduce_l(2) = v_max_l
	191	reduce_l(3) = w_max_l
	192	IF ( collective_wait ) CALL MPI_BARRIER( comm2d, ierr )
	193	CALL MPI_ALLREDUCE( reduce_l, reduce, 3, MPI_REAL, MPI_MAX, comm2d, ierr )
	194	u_max = reduce(1)
	195	v_max = reduce(2)
	196	w_max = reduce(3)
	197	reduce_l(1) = u_min_l
	198	reduce_l(2) = v_min_l
	199	reduce_l(3) = w_min_l
	200	IF ( collective_wait ) CALL MPI_BARRIER( comm2d, ierr )
	201	CALL MPI_ALLREDUCE( reduce_l, reduce, 3, MPI_REAL, MPI_MIN, comm2d, ierr )
	202	IF ( ABS( reduce(1) ) > u_max ) u_max = reduce(1)
	203	IF ( ABS( reduce(2) ) > v_max ) v_max = reduce(2)
	204	IF ( ABS( reduce(3) ) > w_max ) w_max = reduce(3)
	205	#else
	206	IF ( ABS( u_min_l ) > u_max_l ) THEN
	207	u_max = u_min_l
	208	ELSE
	209	u_max = u_max_l
	210	ENDIF
	211	IF ( ABS( v_min_l ) > v_max_l ) THEN
	212	v_max = v_min_l
	213	ELSE
	214	v_max = v_max_l
	215	ENDIF
	216	IF ( ABS( w_min_l ) > w_max_l ) THEN
	217	w_max = w_min_l
	218	ELSE
	219	w_max = w_max_l
	220	ENDIF
	221	#endif
	222	ENDIF
	223	#else
[866]	224	CALL global_min_max( nzb, nzt+1, nysg, nyng, nxlg, nxrg, u, 'abs', 0.0, &
	225	u_max, u_max_ijk )
	226	CALL global_min_max( nzb, nzt+1, nysg, nyng, nxlg, nxrg, v, 'abs', 0.0, &
	227	v_max, v_max_ijk )
	228	CALL global_min_max( nzb, nzt+1, nysg, nyng, nxlg, nxrg, w, 'abs', 0.0, &
	229	w_max, w_max_ijk )
[1257]	230	#endif
[866]	231
[1257]	232	IF ( .NOT. dt_fixed ) THEN
	233	#if defined( __openacc )
[866]	234	!
[1257]	235	!-- Variable time step:
	236	!-- Calculate the maximum time step according to the CFL-criterion,
	237	!-- individually for each velocity component
	238	dt_u_l = 999999.9
	239	dt_v_l = 999999.9
	240	dt_w_l = 999999.9
	241	u_max_l = -999999.9
	242	u_min_l = 999999.9
	243	v_max_l = -999999.9
	244	v_min_l = 999999.9
	245	w_max_l = -999999.9
	246	w_min_l = 999999.9
	247	!$acc parallel loop collapse(3) present( u, v, w )
	248	DO i = nxl, nxr
	249	DO j = nys, nyn
	250	DO k = nzb+1, nzt
	251	dt_u_l = MIN( dt_u_l, ( dx / ( ABS( u(k,j,i) - u_gtrans ) + 1.0E-10 ) ) )
	252	dt_v_l = MIN( dt_v_l, ( dy / ( ABS( v(k,j,i) - v_gtrans ) + 1.0E-10 ) ) )
	253	dt_w_l = MIN( dt_w_l, ( dzu(k) / ( ABS( w(k,j,i) ) + 1.0E-10 ) ) )
	254	u_max_l = MAX( u_max_l, u(k,j,i) )
	255	u_min_l = MIN( u_min_l, u(k,j,i) )
	256	v_max_l = MAX( v_max_l, v(k,j,i) )
	257	v_min_l = MIN( v_min_l, v(k,j,i) )
	258	w_max_l = MAX( w_max_l, w(k,j,i) )
	259	w_min_l = MIN( w_min_l, w(k,j,i) )
	260	ENDDO
	261	ENDDO
	262	ENDDO
	263	!$acc end parallel
[866]	264
[1257]	265	#if defined( __parallel )
	266	reduce_l(1) = dt_u_l
	267	reduce_l(2) = dt_v_l
	268	reduce_l(3) = dt_w_l
	269	IF ( collective_wait ) CALL MPI_BARRIER( comm2d, ierr )
	270	CALL MPI_ALLREDUCE( reduce_l, reduce, 3, MPI_REAL, MPI_MIN, comm2d, ierr )
	271	dt_u = reduce(1)
	272	dt_v = reduce(2)
	273	dt_w = reduce(3)
[866]	274
[1257]	275	reduce_l(1) = u_max_l
	276	reduce_l(2) = v_max_l
	277	reduce_l(3) = w_max_l
	278	IF ( collective_wait ) CALL MPI_BARRIER( comm2d, ierr )
	279	CALL MPI_ALLREDUCE( reduce_l, reduce, 3, MPI_REAL, MPI_MAX, comm2d, ierr )
	280	u_max = reduce(1)
	281	v_max = reduce(2)
	282	w_max = reduce(3)
	283	reduce_l(1) = u_min_l
	284	reduce_l(2) = v_min_l
	285	reduce_l(3) = w_min_l
	286	IF ( collective_wait ) CALL MPI_BARRIER( comm2d, ierr )
	287	CALL MPI_ALLREDUCE( reduce_l, reduce, 3, MPI_REAL, MPI_MIN, comm2d, ierr )
	288	IF ( ABS( reduce(1) ) > u_max ) u_max = reduce(1)
	289	IF ( ABS( reduce(2) ) > v_max ) v_max = reduce(2)
	290	IF ( ABS( reduce(3) ) > w_max ) w_max = reduce(3)
	291	#else
	292	dt_u = dt_u_l
	293	dt_v = dt_v_l
	294	dt_w = dt_w_l
	295
	296	IF ( ABS( u_min_l ) > u_max_l ) THEN
	297	u_max = u_min_l
	298	ELSE
	299	u_max = u_max_l
	300	ENDIF
	301	IF ( ABS( v_min_l ) > v_max_l ) THEN
	302	v_max = v_min_l
	303	ELSE
	304	v_max = v_max_l
	305	ENDIF
	306	IF ( ABS( w_min_l ) > w_max_l ) THEN
	307	w_max = w_min_l
	308	ELSE
	309	w_max = w_max_l
	310	ENDIF
	311	#endif
	312
	313	#else
[1]	314	!
	315	!-- Variable time step:
[1257]	316	!-- Calculate the maximum time step according to the CFL-criterion,
	317	!-- individually for each velocity component
	318	dt_u_l = 999999.9
	319	dt_v_l = 999999.9
	320	dt_w_l = 999999.9
	321	DO i = nxl, nxr
	322	DO j = nys, nyn
	323	DO k = nzb+1, nzt
	324	dt_u_l = MIN( dt_u_l, ( dx / ( ABS( u(k,j,i) - u_gtrans ) + 1.0E-10 ) ) )
	325	dt_v_l = MIN( dt_v_l, ( dy / ( ABS( v(k,j,i) - v_gtrans ) + 1.0E-10 ) ) )
	326	dt_w_l = MIN( dt_w_l, ( dzu(k) / ( ABS( w(k,j,i) ) + 1.0E-10 ) ) )
	327	ENDDO
	328	ENDDO
	329	ENDDO
[1]	330
[1257]	331	#if defined( __parallel )
	332	reduce_l(1) = dt_u_l
	333	reduce_l(2) = dt_v_l
	334	reduce_l(3) = dt_w_l
	335	IF ( collective_wait ) CALL MPI_BARRIER( comm2d, ierr )
	336	CALL MPI_ALLREDUCE( reduce_l, reduce, 3, MPI_REAL, MPI_MIN, comm2d, ierr )
	337	dt_u = reduce(1)
	338	dt_v = reduce(2)
	339	dt_w = reduce(3)
	340	#else
	341	dt_u = dt_u_l
	342	dt_v = dt_v_l
	343	dt_w = dt_w_l
	344	#endif
	345
	346	#endif
	347
[1]	348	!
	349	!-- Compute time step according to the diffusion criterion.
	350	!-- First calculate minimum grid spacing which only depends on index k
	351	!-- Note: also at k=nzb+1 a full grid length is being assumed, although
	352	!-- in the Prandtl-layer friction term only dz/2 is used.
	353	!-- Experience from the old model seems to justify this.
	354	dt_diff_l = 999999.0
	355
	356	DO k = nzb+1, nzt
[667]	357	dxyz2_min(k) = MIN( dx2, dy2, dzw(k)dzw(k) ) 0.125
[1]	358	ENDDO
	359
	360	!$OMP PARALLEL private(i,j,k,value) reduction(MIN: dt_diff_l)
	361	!$OMP DO
[1257]	362	!$acc parallel loop collapse(3) present( kh, km )
[1]	363	DO i = nxl, nxr
	364	DO j = nys, nyn
	365	DO k = nzb+1, nzt
[1257]	366	dt_diff_l = MIN( dt_diff_l, dxyz2_min(k) / &
	367	( MAX( kh(k,j,i), km(k,j,i) ) + 1E-20 ) )
[1]	368	ENDDO
	369	ENDDO
	370	ENDDO
[1257]	371	!$acc end parallel
[1]	372	!$OMP END PARALLEL
	373	#if defined( __parallel )
[622]	374	IF ( collective_wait ) CALL MPI_BARRIER( comm2d, ierr )
[1]	375	CALL MPI_ALLREDUCE( dt_diff_l, dt_diff, 1, MPI_REAL, MPI_MIN, comm2d, &
	376	ierr )
	377	#else
	378	dt_diff = dt_diff_l
	379	#endif
	380
	381	!
[316]	382	!-- Additional timestep criterion with plant canopies:
	383	!-- it is not allowed to extract more than the available momentum
	384	IF ( plant_canopy ) THEN
[318]	385
	386	dt_plant_canopy_l = 0.0
	387	DO i = nxl, nxr
	388	DO j = nys, nyn
	389	DO k = nzb+1, nzt
	390	dt_plant_canopy_u = cdc(k,j,i) * lad_u(k,j,i) * &
	391	SQRT( u(k,j,i)**2 + &
	392	( ( v(k,j,i-1) + &
	393	v(k,j,i) + &
	394	v(k,j+1,i) + &
	395	v(k,j+1,i-1) ) &
	396	/ 4.0 )**2 + &
	397	( ( w(k-1,j,i-1) + &
	398	w(k-1,j,i) + &
	399	w(k,j,i-1) + &
	400	w(k,j,i) ) &
	401	/ 4.0 )**2 )
	402	IF ( dt_plant_canopy_u > dt_plant_canopy_l ) THEN
	403	dt_plant_canopy_l = dt_plant_canopy_u
	404	ENDIF
	405	dt_plant_canopy_v = cdc(k,j,i) * lad_v(k,j,i) * &
	406	SQRT( ( ( u(k,j-1,i) + &
	407	u(k,j-1,i+1) + &
	408	u(k,j,i) + &
	409	u(k,j,i+1) ) &
	410	/ 4.0 )**2 + &
	411	v(k,j,i)**2 + &
	412	( ( w(k-1,j-1,i) + &
	413	w(k-1,j,i) + &
	414	w(k,j-1,i) + &
	415	w(k,j,i) ) &
	416	/ 4.0 )**2 )
	417	IF ( dt_plant_canopy_v > dt_plant_canopy_l ) THEN
	418	dt_plant_canopy_l = dt_plant_canopy_v
	419	ENDIF
	420	dt_plant_canopy_w = cdc(k,j,i) * lad_w(k,j,i) * &
	421	SQRT( ( ( u(k,j,i) + &
	422	u(k,j,i+1) + &
	423	u(k+1,j,i) + &
	424	u(k+1,j,i+1) ) &
	425	/ 4.0 )**2 + &
	426	( ( v(k,j,i) + &
	427	v(k,j+1,i) + &
	428	v(k+1,j,i) + &
	429	v(k+1,j+1,i) ) &
	430	/ 4.0 )**2 + &
	431	w(k,j,i)**2 )
	432	IF ( dt_plant_canopy_w > dt_plant_canopy_l ) THEN
	433	dt_plant_canopy_l = dt_plant_canopy_w
	434	ENDIF
	435	ENDDO
	436	ENDDO
	437	ENDDO
	438
	439	IF ( dt_plant_canopy_l > 0.0 ) THEN
[320]	440	!
	441	!-- Invert dt_plant_canopy_l and apply a security timestep factor 0.1
[318]	442	dt_plant_canopy_l = 0.1 / dt_plant_canopy_l
[320]	443	ELSE
	444	!
	445	!-- In case of inhomogeneous plant canopy, some processors may have no
	446	!-- canopy at all. Then use dt_max as dummy instead.
	447	dt_plant_canopy_l = dt_max
[318]	448	ENDIF
[320]	449
[316]	450	!
[318]	451	!-- Determine the global minumum
	452	#if defined( __parallel )
[622]	453	IF ( collective_wait ) CALL MPI_BARRIER( comm2d, ierr )
[866]	454	CALL MPI_ALLREDUCE( dt_plant_canopy_l, dt_plant_canopy, 1, MPI_REAL, &
[318]	455	MPI_MIN, comm2d, ierr )
	456	#else
	457	dt_plant_canopy = dt_plant_canopy_l
	458	#endif
[316]	459
	460	ELSE
	461	!
	462	!-- Use dt_diff as dummy value to avoid extra IF branches further below
	463	dt_plant_canopy = dt_diff
	464
	465	ENDIF
	466
	467	!
	468	!-- The time step is the minimum of the 3-4 components and the diffusion time
[1001]	469	!-- step minus a reduction (cfl_factor) to be on the safe side.
[1]	470	!-- The time step must not exceed the maximum allowed value.
[1053]	471	dt_3d = cfl_factor * MIN( dt_diff, dt_plant_canopy, dt_u, dt_v, dt_w, &
	472	dt_precipitation )
[1]	473	dt_3d = MIN( dt_3d, dt_max )
	474
	475	!
	476	!-- Remember the restricting time step criterion for later output.
[316]	477	IF ( MIN( dt_u, dt_v, dt_w ) < MIN( dt_diff, dt_plant_canopy ) ) THEN
[1]	478	timestep_reason = 'A'
[316]	479	ELSEIF ( dt_plant_canopy < dt_diff ) THEN
	480	timestep_reason = 'C'
[1]	481	ELSE
	482	timestep_reason = 'D'
	483	ENDIF
	484
	485	!
	486	!-- Set flag if the time step becomes too small.
	487	IF ( dt_3d < ( 0.00001 * dt_max ) ) THEN
	488	stop_dt = .TRUE.
[108]	489
[320]	490	WRITE( message_string, * ) 'Time step has reached minimum limit.', &
	491	'&dt = ', dt_3d, ' s Simulation is terminated.', &
	492	'&old_dt = ', old_dt, ' s', &
	493	'&dt_u = ', dt_u, ' s', &
	494	'&dt_v = ', dt_v, ' s', &
	495	'&dt_w = ', dt_w, ' s', &
	496	'&dt_diff = ', dt_diff, ' s', &
	497	'&dt_plant_canopy = ', dt_plant_canopy, ' s', &
[1257]	498	'&u_max = ', u_max, ' m/s k=', u_max_ijk(1), &
[320]	499	' j=', u_max_ijk(2), ' i=', u_max_ijk(3), &
[1257]	500	'&v_max = ', v_max, ' m/s k=', v_max_ijk(1), &
[320]	501	' j=', v_max_ijk(2), ' i=', v_max_ijk(3), &
[1257]	502	'&w_max = ', w_max, ' m/s k=', w_max_ijk(1), &
[320]	503	' j=', w_max_ijk(2), ' i=', w_max_ijk(3)
[258]	504	CALL message( 'timestep', 'PA0312', 0, 1, 0, 6, 0 )
[108]	505	!
	506	!-- In case of coupled runs inform the remote model of the termination
	507	!-- and its reason, provided the remote model has not already been
	508	!-- informed of another termination reason (terminate_coupled > 0) before.
[222]	509	#if defined( __parallel )
[108]	510	IF ( coupling_mode /= 'uncoupled' .AND. terminate_coupled == 0 ) THEN
	511	terminate_coupled = 2
[667]	512	IF ( myid == 0 ) THEN
	513	CALL MPI_SENDRECV( &
	514	terminate_coupled, 1, MPI_INTEGER, target_id, 0, &
	515	terminate_coupled_remote, 1, MPI_INTEGER, target_id, 0, &
	516	comm_inter, status, ierr )
	517	ENDIF
	518	CALL MPI_BCAST( terminate_coupled_remote, 1, MPI_INTEGER, 0, comm2d, ierr)
[108]	519	ENDIF
[222]	520	#endif
[1]	521	ENDIF
	522
	523	!
[1001]	524	!-- Ensure a smooth value (two significant digits) of the timestep.
	525	div = 1000.0
	526	DO WHILE ( dt_3d < div )
	527	div = div / 10.0
	528	ENDDO
	529	dt_3d = NINT( dt_3d * 100.0 / div ) * div / 100.0
[1]	530
	531	!
[1001]	532	!-- Adjust the time step
	533	old_dt = dt_3d
[1]	534
[1001]	535	ENDIF
[1]	536
	537	CALL cpu_log( log_point(12), 'calculate_timestep', 'stop' )
	538
	539	END SUBROUTINE timestep

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