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.