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_afterTS=.FALSE.,
 LS_staggerTimeStep=.FALSE.,
 &

LS_nIter   :: number of dynamical time steps per ptracer time step.
LS_afterTS :: sample dynamical fields after updating T and S (after thermodynamics),
              i.e. at the same time as gchem_forcing_sep is called.
              This reproduces runs without longstep when LS_nIter == 1
LS_staggerTimeStep :: same as LS_afterTS for staggerTimeStep == .TRUE.

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