<p>Your model grid is an Arakawa C grid:</p>

<p><img src="https://i.sstatic.net/hqXKc.gif" alt="enter image description here">
<sub>Image from Skamarock et al. <a href="http://www.mmm.ucar.edu/wrf/users/docs/arw_v3.pdf" rel="nofollow noreferrer"><em>A description of the Advanced Research WRF Version 3</em></a></sub></p>

<p>It appears the geogrid grid generation uses the 25x25 as the furthest extents of the grid (and this is backed up by the grid maximum needing to be a multiple of the grid ratio +1).  As a result, only the <code>u</code> and <code>v</code> variables will see 25 grid points and each only in one direction, east/west and north/south respectively.  All other variables are located horizontally on the $\theta$, or scalar points in the middle of the grid boxes and there will only be 24 of these points in your setup leaving most of your grids having a horizontal dimensions of 24x24.</p>

CC BY-SA 3.0
2015-05-06T21:25:35.440Z
Exactly Right!  If his start value had been 0 (instead of 1) then the end value would be 24.  It is an inclusive extent range.
f.thorpe
CC BY-SA 3.0
2015-05-06T21:39:35.157Z
yes, I believe you are right. Reading the documentation again, I see it says that the e_we and e_sn values do in fact refer to an unstaggered grid. The terminology for the centered coordinates is the mass grid. I think I am doing the right thing with my nested domains, but I might have to rethink how to set up my grids to align with a validation point (originally I was going to centre the grid on such a point, but the mass grid then is not aligned...)
piyushnz
CC BY-SA 3.0
2015-05-06T23:05:08.667Z
@farrenthorpe Thanks. I had to think for a minute as the model I use (CM1) does it the other way around where you specify nx and ny for the scalar/mass points and it uses nx+1, ny+1 for the u,v grids.
casey
CC BY-SA 3.0
2015-05-06T23:56:40.800Z
I can confirm this answer is correct.
milancurcic
CC BY-SA 3.0
2015-05-06T20:53:35.100Z
2015-05-06T20:53:35.100Z
casey
<p>I am running some test forecasts using WRF 3.6.1. and am having some problems with my grids. Specifically, I am specifying a parent and nest domain each with 25 points for longitude and latitude (so that there is a point right at the middle of the inner domain and the parent grid points are coincident with inner grid points with a 3:1 nesting ratio).</p>

<p>I would expect an output of 25 by 25 points for each domain. however, I am only getting 24 by 24 points as output. In fact if I look at the geom_em or met_em netcdf files generated by WPS I already get only grids with 24 by 24 points. </p>

<p>What's going on?</p>

<p>Here is the geogrid section of my namelist.wps file:</p>

<pre><code>&amp;geogrid
 parent_id         =   1,   1,
 parent_grid_ratio =   1,   3,
 i_parent_start    =   1,  9,
 j_parent_start    =   1,  9,
 s_we              =   1,  1,
 e_we              =  25, 25,
 s_sn              =   1,  1,
 e_sn              =  25,  25,
 geog_data_res     = '10m','2m',
 dx = 18000,
 dy = 18000,
 map_proj = 'lambert',
 ref_lat   =  53.526,
 ref_lon   =  -113.529,
 truelat1  =  53.526,
 truelat2  =  53.526,
 stand_lon =  -113.529,
 geog_data_path = '/media/data/WRF3.6.1/data/geog/'
/
</code></pre>

CC BY-SA 3.0
2015-05-06T20:18:41.050Z
Are all of the grids 24x24 or just the `w` and scalar grids?  Is the `u` grid 25x24 and the `v` grid 24x25?  I'm not familiar with WRFs grids but I have a hunch...
casey
CC BY-SA 3.0
2015-05-06T20:46:58.920Z
you are right. The XLONG, XLAT, T2, W variables are 24x24. The U and V are (as you have mentioned) 24x25 and 25x24 respectively. So it has something to do with grid staggering?
piyushnz
<models><wrf>
WRF output grid is smaller than input definition
piyushnz
# WRF output grid is smaller than input definition

(Asked by: username_0 on 2015-05-06)

I am running some test forecasts using WRF 3.6.1. and am having some problems with my grids. Specifically, I am specifying a parent and nest domain each with 25 points for longitude and latitude (so that there is a point right at the middle of the inner domain and the parent grid points are coincident with inner grid points with a 3:1 nesting ratio).

I would expect an output of 25 by 25 points for each domain. however, I am only getting 24 by 24 points as output. In fact if I look at the geom\_em or met\_em netcdf files generated by WPS I already get only grids with 24 by 24 points.

What's going on?

Here is the geogrid section of my namelist.wps file:

```
&geogrid
 parent_id         =   1,   1,
 parent_grid_ratio =   1,   3,
 i_parent_start    =   1,  9,
 j_parent_start    =   1,  9,
 s_we              =   1,  1,
 e_we              =  25, 25,
 s_sn              =   1,  1,
 e_sn              =  25,  25,
 geog_data_res     = '10m','2m',
 dx = 18000,
 dy = 18000,
 map_proj = 'lambert',
 ref_lat   =  53.526,
 ref_lon   =  -113.529,
 truelat1  =  53.526,
 truelat2  =  53.526,
 stand_lon =  -113.529,
 geog_data_path = '/media/data/WRF3.6.1/data/geog/'
/

```

**Tags:** models, wrf

> **Comments:**
> **username_1**: Are all of the grids 24x24 or just the `w` and scalar grids?  Is the `u` grid 25x24 and the `v` grid 24x25?  I'm not familiar with WRFs grids but I have a hunch...  
> **username_0**: you are right. The XLONG, XLAT, T2, W variables are 24x24. The U and V are (as you have mentioned) 24x25 and 25x24 respectively. So it has something to do with grid staggering?  

---

## 1 answers

### Answer by username_1 on 2015-05-06. Score: 6 | Accepted answer

Your model grid is an Arakawa C grid:

![enter image description here](https://i.sstatic.net/hqXKc.gif)
Image from Skamarock et al. [*A description of the Advanced Research WRF Version 3*](http://www.mmm.ucar.edu/wrf/users/docs/arw_v3.pdf)

It appears the geogrid grid generation uses the 25x25 as the furthest extents of the grid (and this is backed up by the grid maximum needing to be a multiple of the grid ratio +1). As a result, only the `u` and `v` variables will see 25 grid points and each only in one direction, east/west and north/south respectively. All other variables are located horizontally on the $\theta$, or scalar points in the middle of the grid boxes and there will only be 24 of these points in your setup leaving most of your grids having a horizontal dimensions of 24x24.

> **Comments:**
> **username_2**: Exactly Right!  If his start value had been 0 (instead of 1) then the end value would be 24.  It is an inclusive extent range.  
> **username_0**: yes, I believe you are right. Reading the documentation again, I see it says that the e_we and e_sn values do in fact refer to an unstaggered grid. The terminology for the centered coordinates is the mass grid. I think I am doing the right thing with my nested domains, but I might have to rethink how to set up my grids to align with a validation point (originally I was going to centre the grid on such a point, but the mass grid then is not aligned...)  
> **username_1**: @farrenthorpe Thanks. I had to think for a minute as the model I use (CM1) does it the other way around where you specify nx and ny for the scalar/mass points and it uses nx+1, ny+1 for the u,v grids.  
> **username_3**: I can confirm this answer is correct.  

---

