[MITgcm-devel] NetCDF is cool

[Ed Hill]

On Tue, 2004-09-28 at 06:16, Martin Losch wrote:
> Hi Ed,
> you wanted it that way, I'll keep sending you comments:

Yes, please keep the comments coming!  ;-)


> 1. When I run an executable for the second time (without removing old  
> files), I get this:
> > MNC ERROR: variable 'XC_max' is already defined in file  
> > 'monitor_grid.000001.nc' but using a different grid shape
> > ABNORMAL END: S/R MNC_VAR_INIT_ANY
> Now, it may be a good practice to remove all old output files, before  
> you start a new run, so this may be intentional. But in case it isn't  
> ...

Yes, its a safety feature.  And one elegant way to "fix it" is to use
the "mnc_use_outdir" option described at:

  http://mitgcm.org/pelican/online_documents/node236.html

which will create a new and sequentially named directory containing all
mnc output files each time you run the model.


> 2. I think that it is a bit confusion to have different field sizes for  
> Temp, U, V, but that's only my opinion. Since NetCDF is self  
> explanatory, that's okay (o:

This feature was requested from the very beginning by Alistair and
others.  The idea is that it would help reduce the number of "exchange
type" operations needed during post-processing because you would have
the n+1 velocities for the C-grid on each tile.


> 3. As far as the coordinate variables are concerned: the only solution  
> I see, is that in the case of aligned coordinates-grid (cartesian and  
> spherical coordinates), there is one set of coordinate variables  
> (XC,XG,YC,YG, ZC, ZG, all 1D vectors with dimension XC,YC,XG,YG,ZC,ZG),  
> and in the case of a general orthogonal grid do it the way it's done  
> now. It's probably a lot of work. Otherwise I agree, there is no  
> general solution to this.
> So for cartesian and spherical grids it could look like
> dimension:
> XC=sNx

[...snip...]

> As far as I know the smalles common denominator in terms of conventions  
> is that coorindate variables (1D-vectors) and the corresponding  
> dimensions have the same name.

Yes, I suppose it wouldn't be too hard to collapse the X[CG] and Y[CG]
variables to vectors when the coordinate systems are aligned.  I'll add
this to my to-do list!


> 4. There is a list of attribute conventions in Section 8.1 of the  
> netcdf manual:
> http://my.unidata.ucar.edu/content/software/netcdf/guidef/guidef
> -13.html#HEADING13-12
> Not all of them are used by all tools, but I think, long_name, units,  
> and missing_value are used by most in my experience. At least those are  
> the ones that I always set, when I write netcdf output.

This (the setting of attributes) certainly *does* need work.  During our
next documentation session (mid-October) it would be really helpful if
the group could decide upon (even a preliminary) a set of variable names
and attributes.


> That's all for now.

Thanks again!  Feedback *is* helpful and appreciated!

Ed

-- 
Edward H. Hill III, PhD
office:  MIT Dept. of EAPS;  Rm 54-1424;  77 Massachusetts Ave.
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