1. Cliff Mass University of Washington
Improved Subgrid Drag or Hyper PBL/Horizontal Resolution: What Improves the PBL in WRF?
Cliff Mass
University of Washington

2. The Problem
Many of us have found several related problems with WRF during stable conditions:
Inability to maintain shallow cold air layers near the surface
Overmixing during stable conditions.
Winds too strong and geostrophic at the surface
Over the Northwest we have tested the available PBL schemes in WRF and none solved this problem.

3. The Problem is Obvious in Wind Histograms

4. A new drag surface drag parameterization
Last meeting I reported on a promising approach---increasing the surface roughness dependent on the variability of the subgrid scale terrain.
Plausible that current PBL scheme are missing the drag of features that are not resolved.
In our testing this summer we found that although it worked during the winter, we lost runs during the summer. Switched to the closely related u*, but still had problems.
By changing, parameters in surface layer, messing up surface fluxes under less stable conditions.

5. A Partial Fix: A New Low Level Drag Parameterization
Consulting with Jimy Dudhia of NCAR came up with an approach—enhancing u* in the YSU boundary layer scheme, with the enhancement proportional to the subgrid terrain variance.
The idea was that the model was missing the drag from subgrid terrain elements.
No changes over water.

6. k

7. The Initial Results Looked Quite Favorable
Good enough that we went operational with it in late 2010 for the UW WRF 36, 12, and 4 km domains.
Not in the UW 1.3 km domain.

8. Jan1-Feb8 Wind Speed Bias (00 UTC)

11. But there were issues…
Although overall the impact was highly positive, there we were hurting the results in some situations
Our WRF runs were underplaying high wind situations. The new drag made it worse.
There seemed to be too much drag added during the summer days.
This all made some sense since if the atmosphere has a lot of mixing, surface drag elements will be less important.

12. 10 m wind bias (>=20 kt)
Winter: 00 UTC

13. With parameterization (>=20 kt)

14. Summer: All Winds

15. With parameterization worse inland where lots of heating and mixing
Summer 00TC

16. But during morning (12 UTC) we were helping

18. So why not make an alteration to the parameterization
So why not make an alteration to the parameterization? Have it back off when the winds are strong or the vertical sounding indicates things are well mixed? Tried two approaches: pull back with strong wind or pull back with either strong wind or well-mixed sounding

19. Results
We help the high wind situations, but hurt with the lower wind speeds.

28. Dealing with the stable PBL problem
Would hyper-resolution help?
Version 3.3 of WRF and later allows adding more levels in PBL without it going unstable
Inspired by overmixing last December and January.
Tried an extra 10 levels below 200 m and 1000m.
Tried a few December 2011 dates and several in January 2012.

29. Adding ten more levels below 200 meters (12 UTC 20 January

33. Observed

41. Conclusions
Sub-grid scale drag parameterization is highly beneficial, but does hurt when atmosphere is well mixed.
Can shut it down when mixing is strong, with modest benefits, mainly at high wind speeds.
Hyperresolution in PBL helps model surface based inversions and fog, but otherwise little real impact.