## Horizontal resolution

Stencil
Posts: 10
Joined: Tue Jun 09, 2015 7:44 pm

### Horizontal resolution

Hello,

I wanted to know what is real output data resolution. For example, if I am doing nesting up to 3 km grid spacing and then extracted data and then placed to arcgis, length between points are not 3 km.

For Latitude
http://imgur.com/sa6fEkw

For Longitude
http://imgur.com/sa6fEkw

What is the real horizontal resolution?
Stencil
Posts: 10
Joined: Tue Jun 09, 2015 7:44 pm

### Re: Horizontal resolution

So no ideas? or is it just a unwritten rule where you should write in the scientific article "approximately 3 km resolution" ?
Posts: 1604
Joined: Wed Aug 19, 2009 10:05 am

### Re: Horizontal resolution

It depends on what grid you chose.

DX and DY values from namelist.wps are, quoting WRF ARW user's guide:
10. DX : A real value specifying the grid distance in the x-direction where the map scale factor is 1. For ARW, the grid distance is in meters for the 'polar', 'lambert', and 'mercator' projection, and in degrees longitude for the 'lat-lon' projection; for NMM, the grid distance is in degrees longitude. Grid distances for nests are determined recursively based on values specified for parent_grid_ratio and parent_id. No default value.

11. DY : A real value specifying the nominal grid distance in the y-direction where the map scale factor is 1. For ARW, the grid distance is in meters for the 'polar', 'lambert', and 'mercator' projection, and in degrees latitude for the 'lat-lon' projection; for NMM, the grid distance is in degrees latitude. Grid distances for nests are determined recursively based on values specified for parent_grid_ratio and parent_id. No default value.
For NMM, "resolution in km" is given by diagonal distance between mass points; which then has to be calculated from DX and DY, like this (quoting NMM user's guide):
Assuming 111 km/degree (a reasonable assumption for the rotated latitude-longitude grid) the grid spacing in km is approximately equal to:
111.0 * (SQRT (DX**2 + DY**2))
So, for NMM core, "grid resolution" does not correspond to lenght between two next grid cell corners neither in N-S, neither in E-W direction.

I hope this helps.
Stencil
Posts: 10
Joined: Tue Jun 09, 2015 7:44 pm

### Re: Horizontal resolution

I used Lambert Conformal projection (ARW core).

What I understand that my grids real values goes along x-direction.
10. DX : A real value specifying the grid distance in the x-direction where the map scale factor is 1.
But I am not sure where to find the values of longitude.. :/

Is there some kind of equation or other method to get the real values?
Posts: 1604
Joined: Wed Aug 19, 2009 10:05 am

### Re: Horizontal resolution

What is your contents of namelist.wps inside static directory? dx and dy values?
Stencil
Posts: 10
Joined: Tue Jun 09, 2015 7:44 pm

### Re: Horizontal resolution

Here you go: Dx and Dy = 27000. I guess these values you need

Code: Select all

``````&share
wrf_core                   = 'ARW'
max_dom                    = 3
start_date                 = '2010-08-07_00:00:00', '2010-08-07_00:00:00', '2010-08-07_00:00:00'
end_date                   = '2010-08-09_00:00:00', '2010-08-07_00:00:00', '2010-08-07_00:00:00'
interval_seconds           = 21600
io_form_geogrid            = 2
opt_output_from_geogrid_path = '/home/virmantas/wrfems/runs/hurricane/static/'
debug_level                = 0
/

&geogrid
parent_id                  = 1, 1, 2
parent_grid_ratio          = 1, 3, 3
i_parent_start             = 1, 86, 93
j_parent_start             = 1, 69, 73
e_we                       = 250, 238, 226
e_sn                       = 200, 190, 178
geog_data_res              = 'modis_lakes+modis_30s+10m', 'modis_lakes+modis_30s+5m', 'modis_lakes+modis_30s+30s'
dx                         = 27000
dy                         = 27000
map_proj                   = 'lambert'
ref_lat                    = 55.159
ref_lon                    = 20.691
truelat1                   = 55.159
truelat2                   = 55.159
stand_lon                  = 20.691
ref_x                      = 125
ref_y                      = 100
geog_data_path             = '/home/virmantas/wrfems/data/geog'
opt_geogrid_tbl_path       = '/home/virmantas/wrfems/runs/hurricane/static/'
/

&ungrib
out_format                 = 'WPS'
prefix                     = 'FILE'
/

&metgrid
fg_name                    = CFSR, CFSRPTILE
io_form_metgrid            = 2
constants_name             = TAVGSFC, ICE5MIN:2010-08-07_00:00, SSTHR:2010-08-07_00:00
opt_output_from_metgrid_path = '/home/virmantas/wrfems/runs/hurricane/wpsprd'
opt_metgrid_tbl_path       = '/home/virmantas/wrfems/runs/hurricane'
process_only_bdy           =
/

&mod_levs
press_pa                   = 200100, 201300, 100000, 97500, 95000, 92500, 90000, 87500, 85000, 82500, 80000, 77500, 75000, 70000, 65000, 60000, 55000, 50000, 45000, 40000, 35000, 30000, 25000, 22500, 20000, 17500, 15000, 12500, 10000, 7000, 5000, 3000, 2000, 1000
/

&domain_wizard
grib_data_path             = 'null'
grib_vtable                = 'Vtable.GFS'
dwiz_center_over_gmt       = true
dwiz_desc                  = hurricane
dwiz_gridpt_dist_km        = 27.0
dwiz_latlon_linecolor      = -8355712
dwiz_latlon_space_in_deg   = 10
dwiz_map_horiz_scrollbar_pos = 802
dwiz_map_scale_pct         = 12.5
dwiz_map_vert_scrollbar_pos = 0
dwiz_mpi_command           = null
dwiz_name                  = hurricane
dwiz_show_political        = true
dwiz_user_rect_x1          = 941
dwiz_user_rect_x2          = 1318
dwiz_user_rect_y1          = 69
dwiz_user_rect_y2          = 323
dwiz_modis                 = true
dwiz_tcvitals              = null
dwiz_bigmap                = Y
dwiz_lakes                 = true
/
``````
Posts: 1604
Joined: Wed Aug 19, 2009 10:05 am

### Re: Horizontal resolution

That looks good. Now please give small chunk of "wgrib2 -V yourgribfile" output, like this one:

Code: Select all

``````73:4449316:vt=2015100206:300 mb:anl:VGRD V-Component of Wind [m/s]:
ndata=205781:undef=0:mean=-2.68005:min=-43.1:max=33.2
grid_template=30:winds(grid):
Lambert Conformal: (529 x 389) input WE:SN output WE:SN res 56
Lat1 33.783000 Lon1 349.745000 LoV 10.200000
LatD 48.700000 Latin1 48.700000 Latin2 48.700000
LatSP 0.000000 LonSP 0.000000
North Pole (529 x 389) Dx 7300.000000 m Dy 7300.000000 m mode 56
``````
Stencil
Posts: 10
Joined: Tue Jun 09, 2015 7:44 pm

### Re: Horizontal resolution

This one will be alright?

Code: Select all

``````1106:30976417:vt=2010080706:975 mb:6 hour fcst:VGRD V-Component Of Wind [m/s]:
ndata=39825:undef=0:mean=3.96969:min=-13.29:max=12.84
grid_template=30:winds(grid):
Lambert Conformal: (225 x 177) input WE:SN output WE:SN res 56
Lat1 53.297000 Lon1 17.125000 LoV 20.691000
LatD 55.159000 Latin1 55.159000 Latin2 55.159000
LatSP 0.000000 LonSP 0.000000
North Pole (225 x 177) Dx 3000.000000 m Dy 3000.000000 m mode 56
``````
Posts: 1604
Joined: Wed Aug 19, 2009 10:05 am

### Re: Horizontal resolution

That all looks good. Your grid distance appears to really be 3000m in both directions in grib file.

I'm not familiar with your method of checking, but I did quick try in Qgis over my ARW grib file with dx=dy=9800m:

Code: Select all

``````1276:71264433:vt=2015050215:surface:3 hour fcst:WTMP Water Temperature [K]:
ndata=117806:undef=0:mean=290.821:min=269.1:max=323.6
grid_template=30:winds(grid):
Lambert Conformal: (394 x 299) input WE:SN output WE:SN res 56
Lat1 33.417000 Lon1 349.882000 LoV 10.200000
LatD 48.700000 Latin1 48.700000 Latin2 48.700000
LatSP 0.000000 LonSP 0.000000
North Pole (394 x 299) Dx 9800.000000 m Dy 9800.000000 m mode 56
``````
Those are results (first segments lengths - upper numbers):

It is clearly 9.8km in both directions, as it should be.
Stencil
Posts: 10
Joined: Tue Jun 09, 2015 7:44 pm

### Re: Horizontal resolution

well... I did convert .ctl file to .nc (with GRADS) to get a few variables and then imported to arcgis. Could convertion with Grads distort the resolution?

By the way, these links to the screenshots of distance (which I posted earlier). The points defines the middle of the GRID not the corners or something.