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Diffusion
=========

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WRF Dynamics Contents
---------------------

`Dynamics Overview`_ |br|
`Hybrid Vertical Coordinate`_ |br|
Diffusion_ |br|
Damping_ |br|
Advection_ |br|
`Other Dynamics Options`_ |br|
`Lateral Boundary Conditions`_

.. _`Dynamics Overview`: dynamics.html
.. _`Hybrid Vertical Coordinate`: hybrid_vert_coord.html
.. _Diffusion: diffusion.html
.. _Damping: damping.html
.. _Advection: advection.html
.. _`Other Dynamics Options`: other_dynamics.html
.. _`Lateral Boundary Conditions`: lat_bdy_conditions.html 

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WRF diffusion is categorized under two parameters: the "diffusion option," which determines how diffusion derivatives are calculated, and the "K option," which determines how K coefficients are calculated. All diffusion options are set in the &dynamics section of namelist.input. 

    .. note::
       *When a PBL option is used, that PBL scheme processes vertical diffusion, instead of the diffusion scheme. Vertical diffusion is also linked to the surface fluxes.*

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Diffusion Options (diff_opt)
----------------------------
    * **Simple diffusion** : Gradients are simply taken along coordinate surfaces (diff_opt=1)
    * **Full diffusion** : Gradients use full metric terms to more accurately compute horizontal gradients in sloped coordinates. This option can be used with real-data cases (diff_opt=2). 
 
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K Options (km_opt)
------------------
When using a PBL scheme, only the "constant" and "2d deformation" options should be used. The 3d options are designed for 3d diffusion. 

    * **Constant** : K is specified by namelist values for horizontal and vertical diffusion (km_opt=1)
    * **3d TKE** : A prognostic equation for turbulent kinetic energy is used, and K is based on TKE (km_opt=2)
    * **3d Deformation** : K is diagnosed from 3d deformation and stability following a Smagorinsky approach (km_opt=3)
    * **2d Deformation** : K for horizontal diffusion is only diagnosed from horizontal deformation. The vertical diffusion is produced by the PBL scheme (km_opt=4)

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6th Order Horizontal Diffusion (diff_6th_opt)
---------------------------------------------
These options use a 6th-order horizontal hyper diffusion (del^6) on all variables to act as a selective short-wave numerical noise filter. These options can be used in conjunction with "diff_opt." See `Knieval et al`_ to read more about this option. 

    * **diff_6th_opt=0** : Turned off - default
    * **diff_6th_opt=1** : Simple method. Avoid using this method. 
    * **diff_6th_opt=2** : Positive definite. If turning on 6th order horizontal diffusion, this option is recommended
    * **diff_6th_slopeopt=1** : turns off diff_6th_opt over steep terrain (kkw - check this is right)
    * **diff_6th_thresh** : sets the threshold value for terrain slopes above which 6th order diffusion is turned off (m/m)

.. _`Knieval et al`: https://www.researchgate.net/publication/241648605_The_utility_of_6th-order_monotonic_numerical_diffusion_in_the_Advanced_Research_WRF_Model

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Nonlinear Backscatter Anisotropic (NBA) (sfs_opt)
-------------------------------------------------
This is a sub-grid turbulent stress option for momentum in LES applications. 

    * **sfs_opt=0** : NBA turned off (default)
    * **sfs_opt=1** : NBA1 using diagnostic sub-grid stress terms; should be used with diff_opt=2 and km_opt=2 or 3
    * **sfs_opt=2** : NBA2 using TKE-based sub-grid stress terms; should be used with diff_opt=2 and km_opt=2

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