Nested Model Options
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A nested domain is a finer-resolution domain embedded in a coarser-resolution domain, and is driven along its lateral boundaries by the parent domain. Using a nest enables running at a higher-resolution without having uniformly high-resolution over a large domain (which can be very expensive), and prevents high resolution for a very small domain, with mismatched time and spatial lateral boundary conditions.

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.. image:: ./../../images/nest_example.png
   :width: 45%

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There are two types of nesting exercises included in this online tutorial. Read below for specifics and then choose the heading link for the desired case.

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`Run the Model for a Two-way Nested Domain`_
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Two-way nesting is a method in which multiple domains at different grid resolutions are run simultaneously and communicate with each other. The coarser domain provides boundary values for the nest, and then the nest feeds its calculations back to the coarser domain. This option is turned on by setting *feedback=1* in the *&domains* namelist.input record.

Click the heading link above to run the two-way nested domain exercise.

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`Run the Model for a One-way Nested Domain Using Ndown`_
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One-way nesting is a method in which the only communication is from the coarser domain, which provides boundary values for the nest. The nest does not feed its calculations back to the coarser domain. This is specified with *feedback=0* in the *&domains* namelist.input record.

One-way nesting can be run in two different ways: 1) multiple domains at different grid resolutions are run simultaneously, and 2) using the *ndown* program in which one-way nesting is used after WRF has already been run for the parent domain. This can be useful if

*   WRF has already been run for a very long (several months/years) simulation, and it is later determined it may be more beneficial to include a nest at a higher-resolution. This can preserve computational cost by only running the new, high-resolution domain.

*   The intent is to use two or more domains, but the size differences (*e_we* and *e_sn*) are so different between domains that it may be difficult to choose an appropriate number of processors to use for both (all) domains at once. `See Choosing an Appropriate Number of Processors`_.

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Click the heading link above to run the one-way ndown nested domain exercise.

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.. seealso::
   For additional specifics on nesting, `see the Nesting presentation from the WRF Tutorial`_ 

.. _`Run the Model for a Two-way Nested Domain`: ./two-way_nested_exercise.html
.. _`Run the Model for a One-way Nested Domain Using Ndown`: ./one-way_nested_ndown_exercise.html
.. _`See Choosing an Appropriate Number of Processors`:   https://forum.mmm.ucar.edu/threads/choosing-an-appropriate-number-of-processors.5082/
.. _`see the Nesting presentation from the WRF Tutorial`: https://www2.mmm.ucar.edu/wrf/site_linked_files/tutorials/nested_domains.pdf