Toward a standard description of grids
What are grids, and why would we want a standard description?
As stated in the Draft Gridspec,
The comparative analysis of output from multiple models, and against observational data analysis archives, has become a key methodology in reducing uncertainty in climate projections, and in improving forecast skill of medium- and long-term forecasts. There is considerable momentum toward simplifying such analyses by applying comprehensive community-standard metadata to observational and model output data archives.
The principal motivating factor is to be able to examine and analyze gridded data, of which there is an already petabyte-scale and rapidly growing public archive.
What do we mean by a grid? It's the underlying discrete representation of physical space that you'll find in a numerical model code or model output dataset. In the context of Earth system models, it can be further specialized to the representations commonly used in climate and weather models, the atmosphere, ocean and land surface. Observations as well, especially observational analysis datasets, where in-situ and remote observations have been assimilated into a set of gridded fields.
A grid is a finite set of cells in Rn that span Rn in some sense. In [the] particular case [of Earth system models and data], Rn is most likely R4 = R2 x R x R for the 2D horizontal part, the 1D vertical part, and the 1D time part. Usually, the 2D horizontal part is the one that has the largest number of different approaches.
at which point of course, you might well ask, "Well then, what is a cell?"
I'll not attempt here to become any more formal, and instead take the Potter Stewart approach, and claim that we all more or less know what a grid is, and a cell is, or at least, we will when we've looked at a few examples. As you see, there are quite a variety of approaches not only for horizontal grids, as stated in the quote above, but for vertical ones as well.
Assuming you know a grid when you see one, why would you be interested in a standard description of a grid?
First use case
A good starting point to answer this question would be the
IPCC Model Documentation Table. If you click through any of the
documentation links of any model in the table, you will find some
description of the model grids, under item IV. The main point to make
here is that it isn't straightforward even for an initiated user of
the data to discover what the model grid is.
This example, based on Guilyardi (2006) shows how this is done at
present. Some grid may be described by the string
The controlled vocabulary has two parts: keys that state what is being
The idea of the controlled vocabulary is the beginning of standardization. The vocabulary of the keys would belong to a common schema for describing grids. The vocabulary of the values would be part of a convention of agreed-upon standard terms for the description.
Second use case
How much further would we like to take the idea of controlled
vocabularies and standardization? As it turns out, quite a lot. For
many applications, it is necessary to have on hand not only a general
description of the grid (such as
The CF Conventions permit the description of grids to some extent:
certain operations can find necessary information in an
The cell bounds and related information represent the beginnings, in CF, of providing a comprehensive set of grid metrics, wuantities that permit taking the integral, or similar analytic operations on a physical field. A complete set of grid metrics would permit operations not currently possible on anonymous netCDF data. By anonymous data, we mean
In addition, the CF Conventions also provide a
created by v. balaji (balajiprinceton.edu) in emacs using the emacs-muse mode.
last modified: 18 January 2010