[MITgcm-devel] NetCDF is cool

[Ed Hill]

On Mon, 2004-09-27 at 04:41, Martin Losch wrote:
> 
> I finally found some time to have a look at the MNC package. I am 
> really impressed. Works nice for me. I still have to learn how to add 
> output, but for a dumb user (like me) it's great.

Hi Martin,

Thank you for giving it a try!!!

And please see:

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

for an up-to-date description of the namelist variables for turning
on/off the various kinds of output.


> May I make a suggestion regarding coordinate variables. You have 
> probably thought about this long ago, and I may not put forward 
> anything new. If that's so, tell me and I'll shut up.

Your comments are appreciated!

Please let us know about these things since we're still trying to sort
out the best way to support the various NetCDF "conventions".  Its not
at all clear to me what is the best way of formatting the output for the
various NetCDF plotting packages:

  eg.: INGRID, Ocean Data View, MatLAB, ferret

that people want to use.


> You probably know about this coordinate and record variable stuff so I 
> might be carrying coal to Newcastle, forgive me: A netcdf-variable is 
> called a coordinate variable, when it has the same name as the 
> corresponding dimension, e.g. if you call your netcdf-dimension 'X', 
> you can call a variable X(of length X) and this is then a coordinate 
> variable (I get my infromation from the NetCDF 3.3 or 3.5 user manual, 
> I forget). The cool thing about this is that tools such as ncview ( 
> http://meteora.ucsd.edu:80/~pierce/ncview_home_page.html ), usually 
> recognize these coordinate variables and 1. do not display them and 2. 
> use the information to display other non-coordinate variables. For 
> example:
> netcdf-dimension x,y,time (time being a record dimension)
> coordinate-variables x(x), y(y), time(time)
> record-variable theta(time,y,x)
> will then be displayed automatically with the correct coordinates by 
> ncview, ncbrowse, ferret and whatever tools there are. At the same time 
> a variabe time(time,y,x) would be (and is) treated as a coordinate 
> variable, and not displayed. This happens with variabe T and dimension 
> T in the state.*.nc file. I realize that you cannot use coordinate 
> variables properly because of the general orthogonal horizontal 
> coordinates that should also take into account the cube-sphere case, 
> but giving the record-dimension andthe temperature variable the same 
> name T is a bit unfortunate for the above reasons.
> What do you think?

OK, I just checked in a fix for "T" ==> "Temp".

And I think that the all the names and conventions we use ought to be
reviewed.  Please critically review whats there!  ;-)

Also, virtually all plotting packages expect the grids to be aligned
with the coordinate system and, in the general case, our grids just
aren't.  The only truly general solution that I see to this problem is
to use a tool such as NCO:

  http://nco.sourceforge.net/

to post-process the output so that it fits within the conventions used
by each plotting program.  I know thats not convenient, but is it really
possible to come up with a sufficiently generic/general set of names and
attributes that will satisfy even a majority of the popular plotting
packages for all of our model/grid geometries...?

If you can think of better solutions (or even more small improvements as
with the "T"==>"Temp" case), then that would be very helpful and I'll do
everything I can to implement them.

thanks again,
Ed

-- 
Edward H. Hill III, PhD
office:  MIT Dept. of EAPS;  Rm 54-1424;  77 Massachusetts Ave.
             Cambridge, MA 02139-4307
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