WRF ARW User's Guide - MMM - UCAR

MODEL
Spectral Nudging is a new upper-air nudging option in Version 3.1. This selectively
nudges the coarser scales only, but is otherwise set up the same way as grid-nudging.
This option also nudges geopotential height. The wave numbers defined here are the
number of waves contained in the domain, and the number is the maximum one that is
nudged.
grid_fdda = 2
xwavenum = 3
ywavenum = 3
h. Observation Nudging Run
In addition to the usual input data preparation using WPS, station observation files are
required. See http://www.mmm.ucar.edu/wrf/users/wrfv2/How_to_run_obs_fdda.html for
instructions. The observation file names expected by WRF are OBS_DOMAIN101 for
domain 1, and OBS_DOMAIN201 for domain 2, etc.
Observation nudging is activated in the model by the following namelists:
obs_nudge_opt = 1
fdda_start = 0 (obs nudging start time in minutes)
fdda_end = 360 (obs nudging end time in minutes)
Look for example to set other obs nudging namelist variables in namelist template
namelist.input.obs_fdda in test/em_real/ directory. See
http://www.mmm.ucar.edu/wrf/users/wrfv2/How_to_run_obs_fdda.html and
README.obs_fdda in WRFV3/test/em_real/ for more information.
i. Global Run
WRFV3 begins to support global capability. To make a global run, run WPS starting with
namelist template namelist.wps.gloabl. Set map_proj = ‘lat-lon’, and
grid dimensions e_we and e_sn without setting dx and dy in
namelist.wps. The geogrid program will calculate grid distances and their values
can be found in the global attribute section of geo_em.d01.nc file. Type
ncdump –h geo_em.d01.nc to find out the grid distances, which will be needed in
filling out WRF’s namelist.input file. Grid distances in x and y directions may be
different, but it is best they are set similarly or the same. WRF and WPS assume earth is a
sphere, and its radius is 6370 km. There is no restrictions on what to use for grid
dimensions, but for effective use of the polar filter in WRF, the east-west dimension
should be set to 2 P *3 Q *5 R +1 (where P, Q, and R are any integers, including 0).
Run the rest of WPS programs as usual but only for one time period. This is because the
domain covers the entire globe, lateral boundary conditions are no longer needed.
WRF-ARW V3: User’s Guide 5-19