Dear Joe,
You are correct that most users do not know how to map data from a
complex grid to a cartesian longitude x latitude grid, which is why the
CMIP3 specs were written the way they were. Never-the-less this is not
a desirable situation because whenever data is mapped to a different
grid, information is lost, and certain types of analysis can no longer
be performed accurately (e.g., computing a divergence). CF makes it
possible to describe the most important traits of complex grids, and
should facilitate the development of software that would automatically
regrid data (preserving certain desirable characteristics) to any target
grid. There is at least one reasonably general "regridding" tool with
capability to do this (SCRIP, see
http://public.lanl.gov/pwjones/ ), but
at present I don't think it can harvest the metadata from a CF-compliant
netCDF file and use it to generate the weights needed to map from one
grid to another. Thus, the user must do this himself, which is
considerable work.
By the way for the next CMIP (CMIP5), which will serve the AR5, it is
expected that model output will be accepted on the native grids. Thus,
there is an immediate need to begin developing user-friendly software
that will indeed serve the purposes of users who want data on a
longitude x latitude grid. One proposal is to require modeling groups
to supply the "mapping factors" (i.e., weights) needed by SCRIP to
regrid to a longitude x latitude grid chosen by the contributing
modeling group. A perhaps better solution is for someone to develop a
software tool that allows the user to regrid the data to any target grid.
The bottom line is that it is difficult for most researchers to make use
of data not on longitude x latitude grids, but I don't think this is an
argument for not allowing modeling groups to store their data in a way
that has fundamental advantages (i.e., on the native grid). We simply
need to develop a tool that allows the grid meta-data to be interpreted
by SCRIP, which I think could be done with less than 1 person-year of
effort.
If this isn't done, the data contributed to the CMIP5 archive on
non-cartesian longitude x latitude grids will be of marginal value.
Best regards,
Karl
Joe Sirott wrote:
> Hi all,
>
> I was wondering if anyone involved in the CF process knows of any
> software libraries that are completely CF 1.2 compliant. I know that
> from my point of view -- as a software developer who develops software
> that must be able to read a wide variety of geophysical data formats --
> it has been impossible to keep up with the increasingly complex
> conventions in the specification. It would be very helpful if I had
> access to a library that fully supported the semantics in the CF
> conventions (such as projections from CF allowed coordinate system to
> lat/lon space).
>
> I'm guessing (and hoping that I'm wrong) that this software doesn't
> exist. In fact, it seems that the main focus of the CF conventions
> process has been on the needs of data /providers/ rather than data
> /consumers/. There is an inherent conflict between the two groups -- a
> modeler wants to write data using the coordinate system of his model
> while a data consumer would prefer that the data be in a standard
> lat/lon coordinate system. The CF conventions clearly are biased in
> favor of the provider, and while the full conventions might be suitable
> for data sharing in a small, specialized modeling group they don't work
> well for interdisciplinary data sharing.
>
> So, I'm wondering if it would be possible to come up with a subset of CF
> conventions that would suit the needs of consumers rather than providers
> -- a set of conventions where it would be practical for programmers to
> develop (and share) code to interpret data formatted according to the
> conventions and that would establish guidelines for data providers who
> wanted their data to be widely accessible. A model (no pun intended!)
> for this would be the data submission standards
> <http://www-pcmdi.llnl.gov/ipcc/IPCC_output_requirements.htm>established
> for submission of data to the CMIP3 archive of IPCC AR4 model data. For
> example, that archive required that modelers interpolate their data to a
> lat/lon grid before the data would be accepted.
>
> - Joe S.
>
>
>
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Received on Thu May 22 2008 - 16:56:48 BST
