Following the successful discussion that we had a few year's ago on conventions for storing unstructured grid data in netCDF files which eventually led to the UGRID conventions, I would like to propose a simple convention for documenting staggered data on structured grids that is consistent with the UGRID conventions. My proposal, which I will refer to as SGRID convention, is described below.

The link http://bit.ly/1ELq2hh points to this page.

Introduction

The CF-conventions are widely used for storing and distributing environmental / earth sciences / climate data. The CF-conventions use a data perspective: every data value points to the latitude and longitude at which that value has been defined; the combination of latitude and longitude bounds and cell methods attributes can be used to define spatially averaged rather than point values. This is all great for the distribution of (interpolated) data for general visualization and spatial data processing, but it doesn't capture the relationship of the variables as computed by a numerical model (such as Arakawa staggering). Many models use staggered grids (using finite differences, or finite volume approach) or use a finite element approach of which the correct meaning may not be captured easily by simple cell methods descriptors. This becomes a problem if you don't want to just look at the big picture of the model results, but also at the details at the grid resolution e.g. what is the exact meaning of a flux on the output file in discrete terms? Can we verify the mass balance? Can the data be used for restarting the model? Correctly handling the staggered data has always been a crucial element of the Delft3D post-processing tools. In the UGRID conventions, we have defined the (unstructured) grid as a separate entity on the file which consists of nodes and connections of nodes defining edges, faces, and volumes. For a structured (staggered) grid we are currently lacking a consistent convention. Although one could store structured grid data using UGRID conventions, some fundamental aspects such as distinction between grid directions would be lost. In this context I propose the lightweight SGRID conventions to define the core aspects of a structured staggered grid without trying to capture the details of finite element formulations. This proposal serves merely the purpose of getting the conventions for structured grids on par with those for unstructured grids.

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