Transcript WRFworkshop2012.ppt

Improved Subgrid Drag or Hyper
PBL/Horizontal Resolution: What
Improves the PBL in WRF?
Cliff Mass
University of Washington
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.
The Problem is Obvious in Wind
Histograms
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.
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.
k
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.
Jan1-Feb8 Wind Speed Bias (00 UTC)
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.
10 m wind bias (>=20 kt)
Winter: 00 UTC
With parameterization (>=20 kt)
Summer: All Winds
With parameterization worse inland
where lots of heating and mixing
Summer 00TC
But during morning (12 UTC) we
were helping
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
Results
• We help the high wind situations, but hurt
with the lower wind speeds.
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.
Adding ten more levels below 200
meters (12 UTC 20 January
38 levels
48 levels
Observed
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.