[MITgcm-support] Offline tracer advection

[Oliver Jahn]

Hi Neill,

the fields are actually assumed to correspond to the middle of the 
forcing period (which makes sense if they are time averages).  It will 
then interpolate between the December and January fields to get a value 
for January 1st.

If the fields you provide actually are at times 0, 1 month, etc., you 
can set

  offlineOffsetIter=-30

to tell the model to look for the January 1st field in a file with 
0000000060 in the name, etc.

I am assuming that your time step is 43200 seconds (from what you 
describe) and

  offlineForcingPeriod=2592000.

However, I do not see why what you did should lead to large negative 
tracer values if the December files contain valid velocity fields.  Are 
you sure the files have the correct precision/endianness?

Cheers,
Oliver


On 06/20/2011 05:04 AM, Martin Losch wrote:
> Neill,
>
> I don't know too much about this package, but there was a thread on a similar subject just a few days/weeks ago, maybe looking at the support archives will help. I assume that you can set a phase difference by using parameters like "offlineIter0" (have a look at offline_readparams.F)
>
> Martin
>
> On Jun 16, 2011, at 8:02 PM, Neill Mackay wrote:
>
>> Hi all,
>>
>> I'm attempting to modify the tutorial_offline experiment to advect a tracer using a velocity field I am defining. I've been able to get the model to accept the time-varying velocity field I've provided by setting the 'useOFFLINE=.TRUE.,' flag in the data.pkg file (as prompted by an e-mail on this list yesterday from someone grappling with the same problem), but I'm having difficulty with the filename format. On my first attempt, I used the following format, as recommended in the manual section 3.20.3.2:
>>
>> uVeltave.0000000001.data
>>
>> With 12 files each for u,v and w, and with sequence identifiers 1-12 (one for each month of the year-long model run). However, when running the model (compilation works ok), the output ended with the following lines:
>>
>> (PID.TID 0000.0001)  MDS_READ_FIELD: filename: input_off/uVeltave.0000000720 , input_off/uVeltave.0000000720.001.001.data
>> (PID.TID 0000.0001)  MDS_READ_FIELD: Files DO not exist
>>
>>
>> So I modified the input files to have indexes starting at 060 for the first file, and ending at 720 for the last, and this allowed the model to pick up and use the velocity fields. However what it appears to do, looking at the output, is to take the '720' files first, and interpolate between that and the '060' file, and then move forward from there. This is not what I want, since actually the '060' file (originally named '001' in my first attempt) has data from the start of January, with the next file the start of Feb and so on, with '720' being the start of December. Using the velocity fields the way they are currently being treated gives some funny results (large negative values of tracer concentration after the first timestep).
>>
>> Since the model seems to expect the files to correspond to the end of each period and not the start, is there a way I can get around this? The main problem seems to be that the model is using the last file to wrap-around for the beginning, so is there a way I can tell it to use a different file to start?
>>
>> Thanks,
>>
>> Neill.
>>
>>
>>
>> _______________________________________________
>> MITgcm-support mailing list
>> MITgcm-support at mitgcm.org
>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>
>
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support