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The Ndown Program
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.. container:: row m-0 p-0
.. container:: col-md-12 pl-0 pr-3 py-3 m-0
.. container:: card px-0 h-100
.. rst-class:: card-header-def
.. rubric:: ndown
.. container:: card-body-def
An option for running a nested domain separately after the parent domain, which can be useful for the following circumstances:
#. 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 .
#. 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 `_.
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See the flow-chart for the ndown process:
.. figure:: ./images/ndown.png
:align: center
:width: 80%
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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 <./single_domain.html>`_ exercise to regenerate them.
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.. 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).
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Run WPS for Two Domains
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.. note::
If you saved your *met_em.d01\** and *met_em.d02\** files from the `Two-way Nested Domain Case <./nested.html>`_, skip ahead to :ref:`Run Real for Two Domains`.
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#. Use what you learned from the `Two-way Nested Domain Case <./nested.html>`_ 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.
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#. 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.
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.. _Run Real for Two Domains:
Run Real for Two Domains
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#. If you saved the output and namelist from the `Two-way Nested Domain Case <./nested.html>`_ exercise, copy the following files to your *wrf/test/em_real/* directory, and then skip to step 5 below:
.. code-block::
:class: copyable
cp nested/namelist.input .
cp nested/wrfbdy_d01 .
cp nested/wrfinput_d0* .
cp nested/wrfout_d01* .
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#. 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.).
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#. Link the appropriate WPS files and run real.exe to generate the following files
.. container:: row m-0 p-0
.. container:: col-md-6 pl-0 pr-3 py-3 m-0
.. container:: card-blockquote
wrfinput_d01 |br|
wrfinput_d02 |br|
wrfbdy_d01
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#. 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:
.. code-block::
:class: copyable
mv wrfinput_d02 wrfndi_d02
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Run Ndown
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#. 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_* 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
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#. 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!*
.. code-block::
&time_control
io_form_auxinput2 = 2
&domains
max_dom = 2,
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.. note::
*io_form_auxinput2 = 2* is a mandatory setting for running ndown, and specifies that input stream 2 is in netCDF format
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#. Run ndown.exe, which should output a *wrfinput_d02* and a *wrfbdy_d02* file.
.. code-block::
:class: copyable
qsub runndown.sh
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#. Rename output to: *wrfinput_d01* and *wrfbdy_d01* so that wrf.exe will recognize them as the new "domain 1."
.. code-block::
:class: copyable
mv wrfinput_d02 wrfinput_d01
mv wrfbdy_d02 wrfbdy_d01
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Run WRF for the Fine-resolution Domain
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.. tip::
Copy your *namelist.input* file to something else (eg. *namelist.input.save*) in case you want to repeat this process in the future.
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#. 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:
.. code-block::
&domains
time_step = 60,
max_dom = 1,
e_we = 118,
e_sn = 103,
dx = 10000,
dy = 10000,
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#. Run wrf.exe, as usual. Output: *wrfout_d01_*
.. note::
This is d02 output, even though the file name says d01.
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Return to the Practice Exercise home to page to run another exercise.
.. rst-class:: horizbuttons-primary-m
* `Return to Practice Exercises <./practices.html>`_
.. toctree::
:maxdepth: 4
:hidden:
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