WRF ARW User's Guide - MMM - UCAR

MODEL
b. Vertical velocity damping (w_damping): For operational robustness, vertical motion
can be damped to prevent the model from becoming unstable with locally large
vertical velocities. This only affects strong updraft cores, so has very little impact on
results otherwise.
c. Divergence Damping (sm_div): Controls horizontally propagating sound waves.
d. External Mode Damping (em_div): Controls upper-surface (external) waves.
e. Time Off-centering (epssm): Controls vertically propagating sound waves.
Advection Options
a. Horizontal advection orders for momentum (h_mom_adv_order) and scalar
(h_sca_adv_order) can be 2 nd to 6 th , with 5 th order being the recommended one.
b. Vertical advection orders for momentum (v_mom_adv_order) and scalar
(v_sca_adv_order) can be 2 nd and 6th, with 3 rd order being the recommended one.
c. Monotonic transport (option 2, new in Version 3.1) and positive-definite advection
option (option 1) can be applied to moisture (moist_adv_opt), scalar (scalar_adv_opt),
chemistry variables (chem_adv_opt) and tke (tke_adv_opt). Option 1 replaces
pd_moist = .true. etc. in previous versions.
Some notes about using monotonic and positive-definite advection options:
The positive-definite and monotonic options are available for moisture, scalars,
chemical scalers and TKE in the ARW solver. Both the monotonic and positivedefinite
transport options conserve scalar mass locally and globally and are consistent
with the ARW mass conservation equation. We recommend using the positivedefinite
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 in ARW changes when the positive-definite or
monotonic options are used. When the options are not activated, the timestep
tendencies from the physics (excluding microphysics) are used to update the scalar
mixing ratio at the same time as the transport (advection), and the microphysics is
computed and moisture is updated based on the transport+physics update. When the
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 the
starting values for the transport scheme. The microphysics update occurs after the
transport update using these latest values as its starting point. It is important to
remember that for any scalars, the local and global conservation properties, positive
definiteness and monotonicity depend upon each update possessing these properties.
WRF-ARW V3: User’s Guide 5-29