(RESOLVED) ref_lat problem; domain not as expected

[mfe_et]

Hi,
I am trying to run WPS for a certain area of Europe and North of Africa but I have a problem. As I understood in the tutorial (http://www2.mmm.ucar.edu/wrf/users/tutorial/201601/wps_general.pdf) and the WPS documentation (http://homepages.see.leeds.ac.uk/~lecrrb/wrf/aRWUsersGuide.pdf) the variables ref_lat and ref_lon define the center of the domain, so I set the variables ref_lat and ref_lon in the center of my area, 44.25N 11.25E. However the output file is not centered on that point: the longitude is perfect (it starts on 27.2W and finishes on 49.7E, centered on 11.25E) but the latitude variable goes from 10.8N to 65.7N, so the center is 38.25N instead of 44.25N.
This is my namelist.wps file:
&share
wrf_core = 'ARW',
max_dom = 1,
start_date = '2014-10-06_03:00:00',
end_date = '2014-10-06_06:00:00',
interval_seconds = 10800
io_form_geogrid = 2,
/
&geogrid
parent_id = 0,
parent_grid_ratio = 1,
i_parent_start = 1,
j_parent_start = 1,
e_we = 1024,
e_sn = 1024,
geog_data_res = '2m',
dx = 6000.0,
dy = 6000.0,
map_proj = 'mercator',
ref_lat = 44.25,
ref_lon = 11.25,
truelat1 = 44.25,
truelat2 = 60.0,
stand_lon = 0,
geog_data_path = '/home/Build_WRF/WPS_GEOG'
opt_geogrid_tbl_path = '/home/Build_WRF/WPS/geogrid/'
/
&ungrib
out_format = 'WPS',
prefix = 'FILE',
/
&metgrid
fg_name = 'FILE'
io_form_metgrid = 2,
opt_metgrid_tbl_path = '/home/Build_WRF/WPS/metgrid/'
/
And the geogrid.log file is attached.

I also have tryed setting stand_lon=11.25 (stand_lon=ref_lon) and the variables (ref_x,ref_y) = (512,512) which is the (i,j) location of the center of my domain (domain resolution= 1024x1024 points), but the result still have latitudes centered in a latitude different from ref_lat.

I can't understand what's wrong, I would like to know which is the problem and what do I have to modify.

Thanks

 

[mgduda]

The ref_lat value describes the latitude of the center of your WRF domain in grid-point space (i.e., in projection space). Since the latitude varies non-linearly with y-coordinate in the Mercator projection, averaging the minimum and maximum latitudes in the domain will in general not give the latitude at the center of your WRF domain (except, e.g., if your domain is symmetric about the equator).

With your namelist, we have 1024 velocity points in the y-direction, which means we have 1023 mass points. So, the center mass point in your domain is (1 + 1023) / 2 = 512 as a 1-based index. If you check the latitude of any mass point with y-coordinate 512, you should see that its latitude is the same as the ref_lat value specified in your namelist (i.e., 44.25 N).

As a side note, because all meridians (longitude lines) are parallel in the Mercator projection, I think the value of stand_lon may not matter in this case.

 

[mgduda]

It's also worth pointing out that with the Mercator projection, the map scale factors in this WRF domain will go from about 0.73 to 1.74, which is not ideal. The importance of the map scale factors is explained briefly in slide 44 of this presentation: http://www2.mmm.ucar.edu/wrf/users/tutorial/201801/wps_general.pdf . Essentially, the nominally 6-km grid cells in this domain will actually be about 8.2 km at the southern edge of the domain and about 3.4 km at the northern edge.

For domains that span a large range in latitude, it may be worth considering the use of the Lambert conformal conic projection rather than the Mercator projection.