[MITgcm-support] Offline Model Help

[Ryan Abernathey]

Many thanks for this explanation. I understand much better now.
-Ryan

On Aug 15, 2007, at 5:43 PM, Stephanie Dutkiewicz wrote:

>
>
> Ryan -
> Just realised that your offlineForcingPeriod
> is wrong - it has to be the frequency of your
> fields. In your case that would be 1200*360
> (if your timestep is 1200 and you have the
> U,V etc fields averaged over 360 timesteps).
>
> steph
>
> On Wed, 15 Aug 2007, Ryan Abernathey wrote:
>
>> Hello Wise GCC Users,
>>
>> I'm hoping someone can help me understand the inner workings of  
>> the offline package... This is a slightly complicated question, so  
>> please disregard this message if you don't care to wade through it.
>>
>> I have a 2 year long set of velocity fields that I want to use to  
>> advect a tracer. I want to be sure I understand how the offline  
>> fields are imported and interpolated in the model.
>>
>> I don't want any cycling of these fields, just a simple 2 year  
>> run. Therefore, I made all these parameters the same:
>> data
>> periodicExternalForcing =.TRUE.,
>> externForcingPeriod = 62208000.,
>> externForcingCycle = 62208000.,
>> data.off
>> offlineForcingPeriod=62208000.,
>> offlineForcingCycle=62208000.,
>>
>> It is less clear to me how the input files are mapped to actual  
>> times inside the model. From reading the documentation (http:// 
>> mitgcm.org/r2_web_testing/latest/online_documents/node172.html) I  
>> concluded that the input files need to be number with suffixes  
>> that correspond to the offline model timestep. But the  
>> documentation doesn't mention interpolation, even though I am  
>> fairly sure it is happening.
>>
>> I am using a different deltat in my advection run (1200) than was  
>> used to generate the offline files (900), so I have a script to re- 
>> number the offline files accordingly. (i.e. .0000000480 becomes . 
>> 0000000360 etc.) But I do not have a file for every timestep. I  
>> just have one for every 360 timesteps. If I change the value of  
>> offlineForcingPeriod etc., the gcm sometimes crashes with the  
>> following sort of error:
>> (PID.TID 0000.0001)  MDS_READ_FIELD: filename: uVeltave. 
>> 0000000518.data
>> (PID.TID 0000.0001)  MDS_READ_FIELD: File does not exist
>> ...with the actual timestep depending on the value. I don't  
>> understand why. Why does it require certain offline timesteps but  
>> not others? What logic is used when searching for offline data  
>> files? This makes me worried that I don't understand how the  
>> timesteps are being mapped, and my results also suggest that maybe  
>> it is not using the full two years but rather doing something  
>> strange and mysterious.
>>
>> In general, I am not confident that the offline model is doing  
>> what I want and would be very grateful (i.e. buy you a beer  
>> grateful) for someone to explain it to me thoroughly.
>>
>> Cheers,
>> __________________
>> Ryan Abernathey
>> rpa at mit.edu
>> MIT Ph.D. Student
>> Program in Oceans Atmospheres & Climate
>> http://web.mit.edu/rpa/
>>
>>
>>
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