The Ndown Program

ndown

An option for running a nested domain separately after the parent domain, which can be useful for the following circumstances:

1. WRF was previously run for a coarse grid over a long period (e.g., years), and a finer nested grid is now needed, but without having to re-run the coarse grid .

2. The domains vary greatly in size (e_we and e_sn), so the number of processors required for larger domains may be too large for smaller domains. See details.

See the flow-chart for the ndown process:

This exercise covers how to run ndown, using output files from the Single Domain Run to add a nest. If those output files are no longer available, return to the Single Domain Run exercise to regenerate them.

Note

• When using the ndown option, set up the runs as you would for a standard (basic) nested simulation.

• When running WRF for the coarser domain, ensure model output is written at adequate intervals (e.g., 1-3 hourly), especially if the finer domain has a resolution of 1-3 km or less).

Run WPS for Two Domains

Note

If you saved your met_em.d01* and met_em.d02* files from the Two-way Nested Domain Case, skip ahead to Run Real for Two Domains.

1. Use what you learned from the Two-way Nested Domain Case to modify the namelist for 2 domains.

   • Set up a domain over your area of interest

   • Keep domain 2’s size around 118 x 103 to save on computation and time.

   • HINT: Use plotgrids_new.ncl to position your domain before running geogrid.exe.

2. Once the namelist is set-up, run the three WPS executables - if you saved your intermediate files from the Single Domain or Nested case, you can skip ungrib since intermediate files only contain atmospheric data and are not domain-dependent.

Run Real for Two Domains

1. If you saved the output and namelist from the Two-way Nested Domain Case exercise, copy the following files to your wrf/test/em_real/ directory, and then skip to step 5 below:

   cp nested/namelist.input .
   
   cp nested/wrfbdy_d01 .
   
   cp nested/wrfinput_d0* .
   
   cp nested/wrfout_d01* .
   

2. If you do not have the files mentioned in step 1, edit columns 1 and 2 in namelist.input for a 2-domain run. Remove any leftover settings from other practice exercises (e.g., any aux* settings, sst_update, restart set to .true., etc.).

3. Link the appropriate WPS files and run real.exe to generate the following files

   wrfinput_d01
   wrfinput_d02
   wrfbdy_d01

4. The only file needed for the next step (ndown.exe) is wrfinput_d02, which is used with previously-generated wrfout_d01* files. The ndown program is written to recognize a specific naming convention for this file, so rename the file using the following command:

   mv wrfinput_d02 wrfndi_d02
   

Run Ndown

1. ndown.exe uses the following 2 files as input when running. Make sure they are all in the wrf/test/em_real/ directory:

   • wrfout_d01_<date> from the single domain case, which serves as the boundary condition file

   • wrfndi_d02 from the above section, which was previously wrfinput_d02, and serves as the initial condition file

2. Set-up the namelist.

   • Edit the namelist so that it now has the following settings - make sure the start and end times for d01 and d02 are set-up for the same period as for your initial run, or you will receive an error!

     &time_control
     io_form_auxinput2 = 2
     
      &domains
      max_dom    = 2,
     

   Note

   io_form_auxinput2 = 2 is a mandatory setting for running ndown, and specifies that input stream 2 is in netCDF format

3. Run ndown.exe, which should output a wrfinput_d02 and a wrfbdy_d02 file.

   qsub runndown.sh
   

4. Rename output to: wrfinput_d01 and wrfbdy_d01 so that wrf.exe will recognize them as the new “domain 1.”

   mv wrfinput_d02 wrfinput_d01
   
   mv wrfbdy_d02 wrfbdy_d01
   

Run WRF for the Fine-resolution Domain

Tip

Copy your namelist.input file to something else (eg. namelist.input.save) in case you want to repeat this process in the future.

1. Edit namelist.input for the below parameters, copying the following fine-grid domain data from column 2 to column 1 so that this run will be for the fine-grid domain only:

   &domains
   time_step  = 60,
   max_dom    = 1,
   e_we       = 118,
   e_sn       = 103,
   dx         = 10000,
   dy         = 10000,
   

2. Run wrf.exe, as usual. Output: wrfout_d01_<date>

   Note

   This is d02 output, even though the file name says d01.

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