Diffusion


WRF Dynamics Contents

Dynamics Overview
Hybrid Vertical Coordinate
Diffusion
Damping
Advection
Other Dynamics Options
Lateral Boundary Conditions


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.


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).


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)


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)


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