Advection


WRF Dynamics Contents

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


Several advection options are available for use during WRF model simulations. These options are set in the &dynamics section of namelist.input. See section 6 of this WRF Tutorial presentation on dynamics for additional information.


Horizontal Advection Order

These can be set to 2nd - 6th order, but 5th order is the default and recommended value.

  • h_mom_adv_order : horizontal advection order for momentum

  • h_sca_adv_order : horizontal advection order for scalar


Vertical Advection Order

These can be set to 2nd - 6th order, but 3rd order is the default and recommended value.

  • v_mom_adv_order : vertical advection order for momentum

  • v_sca_adv_order : vertical advection order for scalar


Monotonic Transport and Positive-definite Advection

Positive-definite and monotonic options are available for moisture, scalars, chemical scalars, and TKE in the ARW solver. Both the monotonic and positive-definite transport options conserve scalar mass locally and globally and are consistent with the ARW mass conservation equation. It is recommended to use the positive-definite option for moisture variables on all real-data simulations. The monotonic option may be beneficial in chemistry applications and for moisture and scalars in some instances.

When using these options there are certain aspects of the ARW integration scheme that should be considered in the simulation configuration:

  1. The integration sequence changes when the positive-definite or monotonic options are used.

    • When the options are not activated, timestep tendencies from the physics (excluding microphysics) are used to update the scalar mixing ratio at the same time as the transport (advection). Microphysics is computed, and moisture is updated, based on the transport+physics update.

    • When monotonic or positive definite options are activated, the scalar mixing ratio is first updated with the physics tendency, and the new updated values are used as starting values for the transport scheme. Then the microphysics update occurs, using these latest values as its starting point. For any scalars, the local and global conservation properties, positive definiteness and monotonicity depend upon each update possessing these properties.

  2. Some model filters may not be positive definite.

    • “diff_6th_opt=1” is not positive definite, nor monotonic. Use diff_6th_opt=2, which is both monotonic and positive-definite, should be used if you need this diffusion option. Cases have been encountered where departures from monotonicity and positive-definiteness are very noticeable.

    • “diff_opt=1” and “km_opt=4” (a commonly-used real-data case mixing option) is not guaranteed to be positive-definite nor monotonic due to the variable eddy diffusivity, K. Significant departures from positive-definiteness or monotonicity have not been observed when this filter is used with these transport options.

    • The diffusion option that uses a user-specified constant eddy viscosity is positive definite and monotonic.

    • Other filter options that use variable eddy viscosity are not positive definite or monotonic.

  3. Most model physics are not monotonic, nor should they be - they represent sources and sinks in the system. All should be positive definite, although we have not examined and tested all options for this property.

  4. The monotonic option adds significant smoothing to transport in regions where it is active. You may consider turning off other model filters for variables using monotonic transport (filters such as the second and sixth order horizontal filters). It is not possible to turn off filters for the scalars but not for the dynamics using the namelist - one must manually comment out the calls in the solver in the code, and then recompile the model.

The following namelist.input options are set in the &dynamics section. Set each to =1 for positive-definite and =2 for monotonic transport for each domain.

  • moist_adv_opt : for moisture

  • scalar_adv_opt : for scalars

  • chem_adv_opt : for chemistry variables

  • tracer_adv_opt : for tracer variables (must have WRF-Chem activated)

  • tke_adv_opt : for TKE


Weighted Essentially Non-oscillatory Options (WENO)

The following are set in the &dynamics section of namelist.input. Set each to =3 for 5th-order WENO and =4 for 5th-order WENO with a positive-definite limiter for each domain.

  • moist_adv_opt : for moisture

  • scalar_adv_opt : for scalars

  • chem_adv_opt : for chemistry variables

  • tracer_adv_opt : for tracer variables (must have WRF-Chem activated)

  • tke_adv_opt : for TKE

  • momentum_adv_opt : for momentum


Implicit Explicit Vertical Advection (IEVA)

For grids with large aspect ratios (dx/dz >> 1) that permit explicit convection, the large time step is limited by the strongest updraft that occurs during integration. This often results in time steps 20-30% smaller, or requires the use of w-filtering, such as latent-heat tendency limiting. Regions of large vertical velocities are often very small relative to the domain. The IEVA scheme permits a larger time step by partitioning the vertical transport into an explicit piece, which uses the normal vertical schemes present in WRF, and an implicit piece which uses implicit transport (and is unconditionally stable). The combined scheme permits a larger time step than previously used, as well as reduced w-filtering. (Wicker and Skamarock, 2020, MWR)

  • zadvect_implicit : set to 1 to turn this option on (default is 0=off)

  • w_crit_cfl : the default vertical courant number (1.2) where vertical velocity damping begins; however, when “zadvect_implicit” is turned on, this value can be increased to ~2.0