Variables d2_bg_k1 and d2_bg_k2 are set in the UFS Weather Model dynamical core namelist and control the strength of second-order damping in top layer (k1) and second-most top layer (k2) of the model. The higher the value, the more damping is applied. However, when d2_bg_k2=0, the code switches to using extra diffusion.
In the UFS Medium-Range Weather public release, the two CompSets use:
- GFSv15p2 CompSet: d2_bg_k1=0.15, d2_bg_k2=0.02
- GFSv16beta CompSet: d2_bg_k1=0.20, d2_bg_k2=0.00
The values used in the GFSv15p2 CompSet may be preferable because setting d2_bg_k2=0.00 has been shown to decrease the intensity of the polar vortex (single forecast test; not necessarily a robust result).
The values used in the GFSv16beta CompSet may be more adequate when running the UFS Weather Model with a higher model top and more vertical levels. The public release configuration uses 64 vertical levels, while the next operational GFS implementation is planned to have 127 levels.
If users of the UFS Medium-Range Weather public release want to run the GFSv16beta CompSet with d2_bg_k1=0.15 and d2_bg_k2=0.02, they can change these namelist values by following the instructions provided in the App User's Guide.