Contents of /MITgcm/pkg/longstep/README

Package longstep
================

This package allows the passive tracer time step to be longer than that for
dynamical fields: the ptracers are updated only every LS_nIter time step. 
Dynamical fields are averaged over LS_nIter time steps and are available as
fields LS_* (declared in LONGSTEP.h):

original fld.  averaged fld.
------------------------------               
UVEL           LS_uVel
VVEL           LS_vVel
WVEL           LS_wVel
THETA          LS_theta
SALT           LS_salt
IVDConvCount   LS_IVDConvCount
Qsw            LS_Qsw
               
Kwx            LS_Kwx
Kwy            LS_Kwy
Kwz            LS_Kwz
               
KPPdiffKzS     LS_KPPdiffKzS
KPPghat        LS_KPPghat

The T and S time step remains the same as that for u,v,...


Packages that use ptracers (like DIC) need to be adapted:

1. replace dtTracerLev by PTRACERS_dTLev
2. replace THETA, SALT, etc. by their longstep averages from the table above.
   This should be made between #ifdef ALLOW_LONGSTEP ... #endif.
   (You need to #include "LONGSTEP.h")

The package is activated by including "longstep" in packages.conf.  There is
no "use_LONGSTEP", the package is always on when compiled.


The run-time parameters are set in data.longstep.  The default is:

 &LONGSTEP_PARM01
 LS_nIter=1,
 LS_whenToSample=0,
 &

LS_nIter        :: number of dynamical time steps between ptracer time steps.
LS_whenToSample :: when to sample dynamical fields for the longstep average
                   0 - at beginning of timestep (reproduces offline results)
                   1 - after first THERMODYNAMICS but before DYNAMICS
                       (use use old U,V,W for advection, but new T,S for GCHEM if
                       staggerTimeStep=.FALSE.; reproduces online with
                       staggerTimeStep=.FALSE. for LS_nIter=1)
                   2 - after DYNAMICS and second THERMODYNAMICS
                       (use new U,V,W and T,S; reproduces online with
                       staggerTimeStep=.TRUE. for LS_nIter=1)

Default is to sample dynamical fields at the beginning of the time step.  This
reproduces results from offline runs with unshifted time averages used for the
dynamical fields.

To have a time stepping closer to the online model (and reproduce results
for LS_nIter=1), use

LS_whenToSample=1 if staggerTimeStep=.FALSE.
LS_whenToSample=2 if staggerTimeStep=.TRUE.
1	Package longstep
2	================
3
4	This package allows the passive tracer time step to be longer than that for
5	dynamical fields: the ptracers are updated only every LS_nIter time step.
6	Dynamical fields are averaged over LS_nIter time steps and are available as
7	fields LS_* (declared in LONGSTEP.h):
8
9	original fld. averaged fld.
10	------------------------------
11	UVEL LS_uVel
12	VVEL LS_vVel
13	WVEL LS_wVel
14	THETA LS_theta
15	SALT LS_salt
16	IVDConvCount LS_IVDConvCount
17	Qsw LS_Qsw
18
19	Kwx LS_Kwx
20	Kwy LS_Kwy
21	Kwz LS_Kwz
22
23	KPPdiffKzS LS_KPPdiffKzS
24	KPPghat LS_KPPghat
25
26	The T and S time step remains the same as that for u,v,...
27
28
29	Packages that use ptracers (like DIC) need to be adapted:
30
31	1. replace dtTracerLev by PTRACERS_dTLev
32	2. replace THETA, SALT, etc. by their longstep averages from the table above.
33	This should be made between #ifdef ALLOW_LONGSTEP ... #endif.
34	(You need to #include "LONGSTEP.h")
35
36	The package is activated by including "longstep" in packages.conf. There is
37	no "use_LONGSTEP", the package is always on when compiled.
38
39
40	The run-time parameters are set in data.longstep. The default is:
41
42	&LONGSTEP_PARM01
43	LS_nIter=1,
44	LS_whenToSample=0,
45	&
46
47	LS_nIter :: number of dynamical time steps between ptracer time steps.
48	LS_whenToSample :: when to sample dynamical fields for the longstep average
49	0 - at beginning of timestep (reproduces offline results)
50	1 - after first THERMODYNAMICS but before DYNAMICS
51	(use use old U,V,W for advection, but new T,S for GCHEM if
52	staggerTimeStep=.FALSE.; reproduces online with
53	staggerTimeStep=.FALSE. for LS_nIter=1)
54	2 - after DYNAMICS and second THERMODYNAMICS
55	(use new U,V,W and T,S; reproduces online with
56	staggerTimeStep=.TRUE. for LS_nIter=1)
57
58	Default is to sample dynamical fields at the beginning of the time step. This
59	reproduces results from offline runs with unshifted time averages used for the
60	dynamical fields.
61
62	To have a time stepping closer to the online model (and reproduce results
63	for LS_nIter=1), use
64
65	LS_whenToSample=1 if staggerTimeStep=.FALSE.
66	LS_whenToSample=2 if staggerTimeStep=.TRUE.