WRF output grid is smaller than input definition

Question

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/'
/

Answer 1

Your model grid is an Arakawa C grid:

_{Image from Skamarock et al. A description of the Advanced Research WRF Version 3}

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.