Initialization WRF model with various meteorological data sources happens at the WRF prepossessing system, before start the actual model run and after geogrid.exe which uses for define model domain and terrain info. ungrib.exe serves for the purpose of initialization.

Initialization with NARR

A. Downloading NARR data

 1. Go to http://rda.ucar.edu/#home 
 2. Need to register to access data.
 3. Download NARR Fixed Fields (rr-fixed.grb file located at http://rda.ucar.edu/datasets/ds608.0/index.html#sfol-wl-/dsszone/ds608.0?g=1 )
 4. Select and download all 3-hourly Data Files during period of the simulation (including Narr Fixed fields, 3D, clm, flx, pbl, sfc files).
    The data is located at: http://rda.ucar.edu/datasets/ds608.0/index.html#sfol-wl-/dsszone/ds608.0?g=3 (Named "3-HRLY Regrouped NARR files").
 5. Will have to wait for email with links to data to arrive. Extract data at certain directory.

B. Link Vtable.NARR to WPS directory

 1. enter WPS directory
 2. type: ln -s ./ungrib/Variable_Tables/Vtable.NARR Vtable

C. Change WPS namelist.wps & ungrib.exe

 1. create fixed narr file
    a. ./link_grib.csh ../DATA/NARR/rr-fixed.grb 
    b. -Edit namelist.wps
       start_date = '1979-11-08_00:00:00',
       end_date = '1979-11-08_00:00:00',
       interval_seconds = 10800,
       -Edit ungrib section to add correct prefix
       &ungrib
       out_format = 'WPS',
       prefix = 'NARRFIX',
       /
    c. Run Ungrib: ./ungrib.exe >& ungrib_data.log 
    d. rename the file generated which start with 'NARRFIX' to 'NARR_FIXED'
  
 2. ungrib most other narr data
    a. link data files form the data directory to WPS directory
       type: ./link_grib.csh data_directory/data_files
    b. go to WPS directory, edit ungrib and metgrid sections in namelist.wps , make it like the follows:
       &ungrib
       out_format = 'WPS',
       prefix = 'NARR',
       /
       &metgrid
       constants_name = 'NARR_FIXED',
       fg_name = 'NARR',
       io_form_metgrid = 2,
       /
        also change the start and end time to the period of simulation
     c. type: ./ungrib.exe
        should see files starting with 'NARR:' generated if successful.
     d. type: ./metgrid.exe

D. before running wrf, change namelist.input

 In &domains, add:
 p_top_requested                     = 10000,
 because the top of NARR data is 100mb.