Fixing WRFs High Speed Wind Bias: A New Subgrid Scale Drag Parameterization and the Role of Detailed VerificationCliff Mass and Dave Ovens University of Washington

Problems with WRF windsWRF 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 schemesno magic bullet there. We are using the YSU scheme in most work.Doesnt 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 parameterizationDetermine 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 approachenhancing 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: Multi-month evaluation winter and summer

Some Results for Experiment 71Ran 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

OldNew

An IssueOur 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 stepcould have the parameterizaton fade out for higher winds speeds and lower stability, possibility by depending on Richardson number.Actually, this makes some sensesometimes the atmosphere is well-mixed, and at these times variations in sub-grid roughness would be less important.

The End