GENIE Gridspec

Ian Henderson ([mailto:I.R.Henderson@reading.ac.uk I.R.Henderson@reading.ac.uk])

Gridspec introduction
The gridspec is a way of describing the grids used in Earth system models. It is intended to provide a standard format (or 'schema') for describing grids. The gridspec will have several advantages:


 * The gridspec is a formalised description of grid topography, making it suitable for use by computer programs.


 * Tools used to analyse model output data can leverage the information supplied in a gridspec to provide more advanced visualisation and analysis functionality to scientists, without requiring model-specific customisation.


 * Frameworks of Earth system models must transfer coupling fields between model grids. Increasingly, standard tools are being developed for model framework construction, running and coupling.  The gridspec makes it easier for couplers (for instance the PRISM coupler, OASIS) to transform fields from one grid to another by providing a standard description to which many models can be expected to adhere.


 * Regridding of model input or output data will be more straightforward, as it will be standards-based rather than bespoke.


 * The gridspec is designed to be extensible so that it can be used to describe many different types of grid, both extant and future.

The gridspec should be able to describe grids at different spatial resolutions, and of different shapes and structures. It should be able to cover, or be easily extended to cover, as wide a range of grids as possible.

Gridspec proposal
A draft proposal is currently being worked on by V. Balaji (GFDL). The proposal currently consists of a PDF document that describes the proposed standard, implemented as an extension to the NetCDF Climate and Forecast (CF) conventions, and a couple of UML class diagrams that provide a diagrammatic overview of the proposed grid schema.

As part of the work to create a version of the GENIE ESM framework using the PRISM coupler (OASIS) using the Bespoke Framework Generator (BFG) tool, a working implementation of Balaji's grid schema proposal is being developed. This gridspec implementation will be used to describe the grids used by individual GENIE models, allowing BFG to generate automatically the PRISM grid initialisation code for each model required for OASIS coupling. A draft of this gridspec is presented below.

The gridspec has been implemented as an XML schema in preference to NetCDF to fit in with the XML metadata implementation used by BFG; eventually the gridspec should be available in both NetCDF/CF and XML formats, making it accessible to a wide range of Earth system modelling tools and programs.

Draft gridspec
The gridspec is based very closely on Balaji's gridspec proposal, UML diagrams for which can be found here (gridspec model) and here (gridspec details). For more detailed information please refer to Balaji's gridspec PDF document.

The following information about the XML gridspec implementation is available:


 * Top-down view of the gridspec with diagrams and descriptions for the following types:
 * Gridspec - top-level element in a gridspec
 * Mosaic - groups tiles into grids, or "child" grids into "parent" grids
 * Mosaic grid descriptor - sample grid mosaic types
 * Tile - contiguous and distinct area of grid cells
 * Tile discretisation - sample grid tile discretisations
 * Horizontal coordinate system - description of horizontal coordinate system and axes for tile
 * Vertical coordinate system - description of vertical coordinate system and axes for tile
 * Unstructured cell - possible cell type for unstructured discretisations
 * Contact region - describes where and how grid tiles are connected
 * Overlap - describes grid tiles connected by an overlap
 * Boundary - describes grid tiles that meet but do not overlap
 * Gridspec schema file (gridspec.xsd)
 * Example gridspec instance (igcm_gridspec.xml)

Known issues
The following issues have been identified so far:


 * Splitting up gridspec into different files for each mosaic/tile, more modular, better for point/cell IDs which are constrained to be unique across a gridspec instance file (therefore only have to be unique for the tile, rather than across tiles). Balaji says there should be separate files for each mosaic, tile and contact region, e.g. "grid_mosaic.xsd", "grid_tile.xsd", "contact_region.xsd".


 * Cell vertex data is potentially duplicated - many cells may share vertices. A list of vertices should be associated with a tile rather than a cell, then each cell contains references to these vertices rather than the vertices themselves, as is done currently for cell edges (defined by refs to two vertices, rather than duplicating the vertices themselves).


 * How and where should halo regions be described for grids that are split up into "tiles" for parallel processing? (Is this something the gridspec should cover?)


 * How should exchange grids be specified for grid tiles where coordinate axes are specified instead of individual cells (e.g. for regular grids like those used by IGCM and GOLDSTEIN)? Currently, exchange grid cells consist of references to individual cells in the overlapping grids.  A similar problem may exist for boundary anchor points.


 * Gridspec needs to be extended - e.g. missing cell types for different discretisations. The use of base types/base classes should make the gridspec easy to extend.  This should be the job of people who are experts on the different grids in question.