Transcript AnnualSubGrid.ppt

Fixing WRF’s High Speed Wind Bias:
A New Subgrid Scale Drag
Parameterization and the Role of
Detailed Verification
Cliff Mass and Dave Ovens
University of Washington
Problems with WRF winds
• WRF generally has a substantial
overprediction bias for all but the lightest
winds.
• Not enough light winds.
• Winds are generally too geostrophic over land.
• Not enough contrast between winds over land
and water.
• This problem is evident virtually everywhere
and appears to occur in all PBL schemes
available with WRF.
10-m wind bias, 00 UTC, 24-h forecast, Jan 1-Feb 8, 2010
10-m wind bias, 12 UTC, 12-h forecast, Jan 1-Feb 8, 2010
The Problem
Insufficient Contrast Between Land and Water
So What is the Problem?
• As noted earlier, tried all available WRF PBL
schemes…no magic bullet there. We are using
the YSU scheme in most work.
• Doesn’t improve going from 36 to 12 km
resolution, 1.3 km somewhat better.
• Inherent problem with all PBL schemes?
• What about the roughness of subgrid terrain
that we are not resolving?
The 12-km grid versus terrain
A new drag surface drag
parameterization
• Determine the subgrid terrain variance and
make surface drag or roughness used in model
dependent on it.
• Consulting with Jimy Dudhia of NCAR came up
with an approach—enhancing u* and only in
the boundary layer scheme (YSU).
• For our 12-km and 36-km runs used the
variance of 1-km grid spacing terrain.
38 Different Experiments: Multimonth evaluation winter and summer
Some Results for Experiment “71”
• Ran the modeling system over a five-week test
period (Jan 1- Feb 8, 2010)
10-m wind speed bias: Winter
MAE 10m wind speed
Case Study
Old
New
An Issue
• Our method appears to hurt slightly during
strong wind speeds and near maximum
temperatures in summer.
Summer-0000 TC-Original
With Sub-grid drag
Summer
Improvement?
• Next step—could have the parameterizaton
fade out for higher winds speeds and lower
stability, possibility by depending on
Richardson number.
• Actually, this makes some sense…sometimes
the atmosphere is well-mixed, and at these
times variations in sub-grid roughness would
be less important.
The End