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Vortex-Following Exercise
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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:: Vortex-following Option

         .. container:: card-body-def

            A WRF configuration that tracks the center of low pressure in a tropical cyclone, and allows the nested domain to move inside the parent to follow the cyclone as it moves. 
            
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Tropical cyclones often move across a large area, and when simulating them, a large, high-resolution domain can be computationally expensive. The `Vortex Following option <https://www2.mmm.ucar.edu/wrf/users/wrf_users_guide/build/html/running_wrf.html#automatic-moving-nest-vortex-following>`_ uses a large static coarse parent domain, with a nest that moves with the center of low pressure in a tropical cyclone. To use this option, WRF must be compiled with the "vortex-following" nest option, but it uses standard WPS. 

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.. note::
   The Vortex-following capability only works for tracking cyclones.

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Set-up WPS
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.. important::
   To avoid overwriting files from other exercises, create a dedicated directory to store the *geo_em\**, *met_em\**, ungrib output, and namelist.wps.

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#.   Move to the **/glade/derecho/scratch/$USER/practice_exercises/wps/** directory.

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#.   Use the following information to modify *namelist.wps*:

     *   Case : `Hurricane Matthew <https://en.wikipedia.org/wiki/Hurricane_Matthew>`_, which made landfall as a category 4 hurricane over Haiti, and then weakened before hitting the U.S. coast of South Carolina.
     *   Dates: 2016-10-06_00:00:00 to 2016-10-08_00:00:00
     *   Although you will ultimately process two domains (one parent and one nest), for the WPS process, only configure the parent domain - the only static domain.
     *   Domain size : 91 x 100 grid points
     *   Grid resolution : 27 km
     *   Map projection: *mercator* - the recommended projection when closer to the equator
     *   To place the domain in the right location on the Earth, make the following additional namelist changes:
     
         .. code-block::

            ref_lat = 28.00
            ref_lon = -75.00
            truelat1 = 28.0,
            stand_lon = -75.00

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#.   The following pertains to the meteorological input data:
      
     .. csv-table::
        :escape: \

        **Type**, Global `NCEP GFS <https://rda.ucar.edu/datasets/ds084.1/>`_
        **Interval**, Every 3 hours
        **Initial Time**, 2016-10-06_00:00:00
        **End Time**, 2016-10-08_00:00:00
        **Data Directory**, */glade/campaign/mmm/wmr/wrf_tutorial/input_data/climate/*
        **File Root**, *gfs.0p25.2016100600.f0*
        **Vtable**, Vtable.GFS

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#.   Check the domain's location, which should be similar to the below image:

     .. figure:: ./images/matthew_domain.png
        :align: center
        :width: 65%

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#.   Run the WPS executables to create input files for wrf.

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     .. note::
        Because this case uses a lot of data, NCAR's HPC system **MAY** give a segmentation fault when running it with the standard command (``./ungrib.exe``) if the login nodes become saturated. To avoid this, use the following (where *UMMM0017* is this class's computing project):

        .. code-block::
           :class: copyable

           qcmd -A UMMM0013 -- ./ungrib.exe

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Set-up and Run WRF
==================

#.   Move to the **/glade/derecho/scratch/$USER/practice_exercises/wrf_vortex/test/em_real/** directory.

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#.   Modify *namelist.input* to include settings for the static parent domain, and the moving nest (see below), as well as the appropriate dates/times. Domain 01 values should match those from WPS for d01. 

     .. code-block::

        &time_control
        input_from_file = .true.,.false.,
        interval_seconds = 21600

        &domains
        time_step = 150,
        max_dom = 2,
        e_we = 91, 88,
        e_sn = 100, 79,
        num_metgrid_levels = 32
        dx = 27000
        dy = 27000
        i_parent_start = 1, 25,
        j_parent_start = 1, 14,
        parent_grid_ratio = 1, 3,
        parent_time_step_ratio = 1, 3,

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     .. note::
        *   No *wrfinput_d02* file will be created since *input_from_file* is set to *.false.* for domain 2. This is standard for vortex-following runs.
        *   The d01 \**_parent_start* values represent the initial nest location. 

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#.   Run real and wrf.

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#.   Check your output.

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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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