Grid Spacing vs. Resolution

 If you spend enough time with meteorologist, you'll inevitably hear criticism against using the non-interchangeable terms "grid spacing" and "resolution" to describe the spacing between grid points in a numerical weather model. Just last week I reviewed a paper where one of the other reviewers blasted the authors for incorrectly using the terms interchangeably. I cringed at the comment. What drives me nuts about this is that the reviewer didn't even explain why the terms are not interchangeable. The supposed incorrect use of the term model "resolution" in place of "grid spacing" will go on forever because those who are deeply offended by those who interchange the term rarely explain how the two terms are different.  

What is the difference?

In a March 2000 BAMS commentary, Lewis D. Grasso explains: 

The two terms  grid  spacing and resolution refer to two different length scales that characterize a grid configuration. Because of this, they are not interchangeable in linear and nonlinear numerical  models. It is not possible to resolve a wave on the scale of one grid spacing in any spatial direction.  Since 2Δx and 3Δx waves are removed to prevent nonlinear instability, waves on the scale of at least 4Δx may be resolved. Other terms that may be used in place of grid spacing are grid interval, grid length, and grid increment. 

There you have it; resolution is not the correct term to describe a model's grid spacing because resolution refers to the length scale that waves are resolved in the model.

More resources on the topic

• https://weather.mailasail.com/Franks-Weather/Grid-Length-Resolution
• https://www.meted.ucar.edu/nwp/model_structure/navmenu.php?tab=1&page=3-0-0

My personal thoughts on the subject:

1. The confusion in the term "resolution" to describe NWP models likely stems from the ubiquitous use of the term in digital imagery. Resolution in digital images is measured in pixels per inch (ppi), megapixels, etc. When describing a model's grid configuration, I don't interpret the term "resolution" as referring to the length scale waves are resolved; instead, I think about how the terrain and land features of the model are depicted. Similar to how digital images are described in terms of the images "resolution," I think about the model grid in the same way--the number of "pixels" in a given area (i.e., 1 "pixel" or grid point every 3 km). This is perfectly clear to me and is likely understood that way by anyone else reading a description of how a model was configured.
2. Please stop being critical of the terms "grid spacing" and "resolution" used interchangeably. If you insist on being correct, take a moment to teach the person why "grid spacing" is more appropriate--also please consider if the correction is really needed. Ask yourself, "in this context, is there confusion if a person is talking about grid spacing or wave resolution?" When a person says "model resolution" when describing how a model was set up, it is likely clear that they are referring to the number of grid points in an area. 
3. Meanings of words do change over time, and because of the ubiquitous use of the term "resolution" in digital images in our day, it would be more clear to non-meteorologists if we used the term "resolution" to describe a model's grid spacing. Someone please tell me, how often do we talk about the length scale waves are resolved when describing a model's setup?
4. The term "grid resolution" should be acceptable to describe the spacing between grid points.