NDOWN (Nest-Down) Program

Ndown is an option to run a nested domain after running the parent domain, which can be useful for two circumstances:

1. If you have run the model for coarser grid(s) over a long period of time (e.g., years), and would like to run the case again with a finer nested grid, without having to re-run the coarse grid.


2. When using nested domains where the sizes (e_we and e_sn) are greatly different than each other. To process small domains, you will not be able to use as many processors as you may need to use for the larger domains. See this explanation for additional details.

Before running ndown, make sure you have output files from a previous coarse domain run (you can get these if you saved them in a 'single_domain' directory inside your test/em_real/ directory).

Below is a flow chart for a one-way nested run.

Notes When using the ndown option, prepare the runs as if you were doing a normal (basic) nested simulation. When running WRF for the coarser domain, make sure to write model output in adequate intervals (e.g., 1-3 hourly, especially if the finer domain has resolutions of 1-3 km or less).

**Click on the links to jump to the desired section.**

Run WPS for Two Domains
Run Real for Two Domains
Run Ndown
Run WRF for the Fine-resolution Domains

Run WPS for Two Domains

Note If you still have met_em* files for both domains 01 and 02 from the basic two-way-nest case, then you can skip this entire section and move on to step 2 .

1. Use what you learned from the two-way-nest case to modify the namelist appropriately for 2 domains.
   
   
   • Set up a domain over your area of interest
   
   
   • Keep your domain 2 size around 118 x 103 to save space and time.
   
   
   • HINT: Use plotgrids_new.ncl to position your domain before you run geogrid.exe.
   
   



2. Once the namelist is set-up, run the three WPS executables (Note: if you still have intermediate files from your original single or nested case, you can skip ungrib. These are not domain–dependent).

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Run Real for Two Domains

1. Edit namelist.input to make sure columns 1 and 2 are set up to run 2 domains.
   
   
   • If you previously ran the nested case and saved the namelist.input file from that case, start with that namelist. If not, make sure there are no leftover settings from other practice exercises in there (e.g., any aux* settings, sst_update, restart, etc.).
   
   


2. Link–in the appropriate WPS files and run real.exe.
   
   
   • This will generate the files wrfinput_d01, wrfinput_d02, and wrfbdy_d01. The only file we need for the next step (ndown.exe) is wrfinput_d02, which is used with previously-generated wrfout_d01* files.
   
   
   • The ndown program was written to recognize a specific naming convention for this file, so rename the file using the following command:

     mv wrfinput_d02 wrfndi_d02

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Run Ndown

1. ndown.exe uses the following 2 files as input when running. Make sure they are all in the em_real directory:
   
   
   • wrfout_d01_<date> from the single domain case, which serves as the boundary condition file
   
   
   • wrfndi_d02 from step 2, which was previously wrfinput_d02, and serves as the initial condition file
      
     
   
   
2. Add the following to the &time_control namelist record:
   

   io_form_auxinput2 = 2

   • This is mandatory for running ndown, and specifies that input stream 2 is in netCDF format
   
   
   • 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!
   
   
   • Make sure max_dom =2
   
   
   • Make sure "interval_seconds" reflects the history output interval from the coarse domain model run.
   
   


3. Run ndown.exe
   

   qsub runndown.sh

   Output from this run: wrfinput_d02 and wrfbdy_d02




4. Rename output to: wrfinput_d01 and wrfbdy_d01

   mv wrfinput_d02 wrfinput_d01
   
   mv wrfbdy_d02 wrfbdy_d01

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Run WRF for the Fine-resolution Domain

Hint: 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:

   • e_we
   • e_sn
   • dx
   • dy
   • Set max_dom = 1
   • time_step – make sure this is set appropriately for the fine-resolution domain (i.e. 6 x the grid distance in km of this domain). 50 is an okay setting for a resolution of 9 km

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

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