Fixing WRF’s High Speed Wind Bias:  A New Subgrid Scale Drag Parameterization and the Role of Deta...
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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.

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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
Problems with WRF winds Drag Parameterization and the Role of Detailed Verification

  • 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, 00 UTC, 24-h forecast, Jan 1-Feb 8, 2010 Drag Parameterization and the Role of Detailed Verification


10 m wind bias 12 utc 12 h forecast jan 1 feb 8 2010
10-m wind bias, 12 UTC, 12-h forecast, Jan 1-Feb 8, 2010 Drag Parameterization and the Role of Detailed Verification


The problem
The Problem Drag Parameterization and the Role of Detailed Verification


Insufficient contrast between land and water
Insufficient Contrast Between Land and Water Drag Parameterization and the Role of Detailed Verification


So what is the problem
So What is the Problem? Drag Parameterization and the Role of Detailed Verification

  • 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
The 12-km grid versus terrain Drag Parameterization and the Role of Detailed Verification


A new drag surface drag parameterization
A new drag surface drag parameterization Drag Parameterization and the Role of Detailed Verification

  • 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.



Some results for experiment 71
Some Results for Experiment “71” summer

  • Ran the modeling system over a five-week test period (Jan 1- Feb 8, 2010)




Case study
Case Study summer


Old summer

New


An issue
An Issue summer

  • Our method appears to hurt slightly during strong wind speeds and near maximum temperatures in summer.




Summer
Summer summer


Improvement
Improvement? summer

  • 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
The End summer


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