[MITgcm-support] Offline Model Help

[Stephanie Dutkiewicz]

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/
>
>
>