[MITgcm-support] timing of snapshots and time averages in pkg/timeave and pkg/diagnostics

[Jean-Michel Campin]

Hi Dimitris,

Regarding timeave pkg, state variables are using this half value
at the beginning and at the end of the averaging period.
I would say that it's an "historical" reason, hard to track
with cvs log (I remember beginning of 2001 working on putting back 
the time-ave utility as a standard pkg, which had been 
disable after the merging with the atmos-branch, and it was just
inherited from the older convention).
But it was found to be not so practical, and later on (Oct 07, 2002)
was added the option to weight differently the first and last time-step 
with the run-time parameter "tave_lastIter" (parameter file "data", 
namelist "PARM03", default value = 0.5):
 first time step of time-averaging interval: field*(1.-tave_lastIter)
 usual time step within time-averaging interval: field*1.
 last time step of the time-averaging interval: field*tave_lastIter
Documented in PARAMS.h:
681 C tave_lastIter :: (for state variable only) fraction of the last time
682 C                  step (of each taveFreq period) put in the time average.
683 C                  (fraction for 1rst iter = 1 - tave_lastIter) 
And at the end, this feature (partial fraction) was not exported 
to pkg/diagnostics which only uses full time-step averaging.

Regarding state-variables snap-shot output from pkg/diagnostics, 
I found an old email I wrote which try to justified the way it's
done now:
http://mitgcm.org/pipermail/mitgcm-devel/2005-January/001265.html
Note that iteration number contained in the file-name suffix of 
plain binary output is consistent between:
a) main model snap-shot output (dumpFreq)
   T,S,U,V.{10-digits-iter-number}.*data/meta
b) pkg/diagnostics snap-shot output (using negative frequency & timePhase=0)
   THETA,SALT,UVEL,VVEL.{10-digits-iter-number}.*data/meta
But the 1 time-step time averaged output from pkg/diagnostics 
(using positive frequency equal to the time-step) corresponds to
the iteration suffix iter-number + 1 
since it is assumed that pkg/diagnostics time-averaged output have 
the iteration number that corresponds to the end of the averaging period.
For instance, with deltaT=86400:
 diff instUVEL.0000036011.data U.0000036010.data        <- no difference
 diff instUVEL.0000036010.data averUVEL.0000036011.data <- no difference
 diff instUVEL.0000036010.meta averUVEL.0000036011.meta
9,10c9,10
<  timeStepNumber = [      36010 ];
<  timeInterval = [  3.111264000000E+09 ];
---
>  timeStepNumber = [      36011 ];
>  timeInterval = [  3.111264000000E+09  3.111350400000E+09 ];

Regarding snap-shot output from pkg/diagnostics of non state-variables
fields (i.e., for intermediate fields, which are not store in pickup-files 
and/or not available before the 1rst model time-step, but instead computed
within one model time-step such as flux, tendencies, etc ...)
there is the same 1.iter shift between time-averaged and snap-shot
(when both output from pkg/diagnostics) as there is with state variables;
but the correspondence with direct output (i.e., not from pkg/diagnostics)
is less clear since it depends from where in the code the output
comes from (before or after updating the iteration number) and
from where the diagnostics has been filled.
For instance, with staggerTimeStep=T, 
1.time-step averaged diag: WVELMASS{iter-number} <--> W.{iter-number}.*data
and for a snap-shot diag:  WVELMASS{iter-number} <--> W.{iter-number + 1}.*data

I am not sure this will contribute to clarify the issue (I was hoping but ...).

Cheers,
Jean-Michel

On Sat, Jan 28, 2012 at 01:40:59PM -0800, Menemenlis, Dimitris (3248) wrote:
> Hello MITgcm support,
> is a discussion of the exact timing of diagnostic snapshots and time averages available somewhere?
> 
> I notice that pkg/diagnostics is not consistent with the old definitions in pkg/timeave.
> Which time-averaging definition is correct, that of pkg/timeave or that of pkg/diagnostics?
> 
> Specifically, in pkg/timeave, except for PHL where
> one-time-step-average of PHL(t) = snapshot of PHL(t)
> all other variables have
> one-time-step-average of VARIABLE(t) = ( snapshot of VARIABLE(t-1) + snapshot of VARIABLE(t) ) / 2
> 
> For pkg/diagnostics the correspondence between snapshots and time averages is (from diagnostics_write.F):
>        myItM1 = myIter - 1
>          IF ( freqSec.LT.0. ) THEN
> C--     write snap-shot with suffix = myIter-1 to be consistent with
> C       state-variable time-step:
>            wrIter = myItM1
>            wrTime = myTime - deltaTclock
>          ELSE
>            wrIter = myIter
>            wrTime = myTime
>          ENDIF
> that is,
> one-tine-step-average of VARIABLE(t) = snapshot of VARIABLE(t-1)
> 
> Thank you,
> 
> Dimitris Menemenlis
> 
> 
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