2.2 Results from the implementation of an implicit-explicit vertical advection scheme in WRF-ARW for CAM systems.
Wicker, Lou, NOAA/NSSL
Over the last two
decades computational power has increased sufficiently to enable prediction of
high-impact weather using Convection Allowing Models (CAMs) at nearly every
major numerical weather prediction center. Due to their high cost, CAM models
almost always use horizontal and vertical grid resolutions having large aspect
ratios (△x 1). Consequently the CAM’s △z
time step is often constrained by the vertical Courant-Levy-Friedrichs (CFL)
number associated with strong convective drafts which typically occur only in a
few grid columns. To increase operational efficiency, CAMs often employ extra
filters to reduce maximum updraft speeds that then permits the use of a larger
time step. In this case, integration efficiency is increased at the
expense of solution accuracy. Alternatively, implicit advection methods often have
no time step restrictions, but introduce much larger phase and amplitude
errors as CFL exceeds 1. A hybrid vertical advection method currently used in
a three-dimensional ocean model is implemented in NCAR’s Weather
Research and Forecasting model to improve the efficiency of the model. The method
combines explicit and implicit advection methods to a create a scheme
enabling stable integrations even when the vertical advection CFL number
exceeds 2. Tests show that the scheme permits a ∼ 20 percent larger time step
even with the removal of the currently used vertical velocity filters. Storm
up-draft cores then retain most of their unfiltered intensities while still
achieving
a ∼ 15 percent reduction in wallclock time for a full-physics CAM integration when
compared to the filtered operational run. Several examples where the
IEVA demonstrates improved numerical efficiency will be shown.