On Tue, Jan 2, 2018 at 12:52 PM, Charlie Zender <zen...@uci.edu> wrote:
> > By the way, the UGRID standard maps pretty well to how unstructured grid > > models generally store their grids and read and write them anyway, so it > > shouldn't be a heavy lift to support it. > > Chris, I think you underestimate the work required. > Many models on unstructured grids (e.g., CAM-SE, MPAS) > use CF conventions for their native grid output, yet > do not include much if any mesh connectivity information. > most of the mesh connectivity information is optional in UGRID: http://ugrid-conventions.github.io/ugrid-conventions/#2d-triangular-mesh-topology all that is required is: cf_role mesh_topology topology_dimension 2 node_coordinates face_node_connectivity You clearly can't do without node coordinates, and face_node_connectivity is how you map the nodes to the faces -- i.e. each face is defined by a particular set of nodes, without that, you don't have a grid. You may have data only nodes without any grid info, but that would be a bit too lossy for my taste, and we wouldn't be having the discussion anyway -- that's clear how to represent. So I'm not sure what the problem is here ? that only have lat, lon, and gridcell vertices. > IIUC, then you have all the required elements -- gridcell vertices Is face_node_connectivity, I think. Groups that are already running CMIP6 experiments might > prefer that new output requirements like UGRID be deferred > until CMIP7 :) This isn't my baby, I'm simply suggesting: If there is information about the grid that can't be represented only with current CF standards (and CF does not include one clear way to represent an unstructured grid) Then you have three choices: 1) don't specify a standard way to do it, which seems like a really bad idea. 2) make up a new standard for CMIP 3) use UGRID If CMIP is well on the way with a standard already, then I guess (2) it is, but even then, if there are still pieces to clarify, then you might as well borrow from UGRID. Now I'll editorialize a bit: I got involved with UGRID because I am primarily a user of model data, rather than a creator, and the number of unstructured grid models has greatly increased in the last 10 yrs or so. CF is great for structured grids -- I can write my code to read CF-compliant results, and then I can work with any old model out there. But there was no such standard for unstructured grids, so everyone did their own thing, and this was a big pain in the neck. Even worse a number of folks write their model output without preserving the grid info at all, and while that is OK if I want to, say, plot arrows indicating velocity at the nodes, it is not so great if I need to properly interpolate, etc. So if CMIP is "allowing" folks to put out their model results without properly specifying the grid, then important information is being lost, and if folks are providing the grid specification, then wouldn't you want it in an appropriate efficient and standardised format? -CHB -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception chris.bar...@noaa.gov
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