[MITgcm-support] Offline Model Help

[Ryan Abernathey]

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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