1. The 10th annual Northeast Regional Operational Workshop, Albany, NY Northeast Regional Ensemble Simulations of the 10 February 2008 Lake Effect Snow Event: WRF Wind Biases and their Implications on Forecasted Snow Band Position Justin Arnott and Michael Evans NOAA / NWS Binghamton, NY

2. The 10th annual Northeast Regional Operational Workshop, Albany, NY Introduction Zaff and Ballentine (COMET partners project) showed a southward bias for forecasted Erie/Ontario LES bands for a collection of 2006-2007 cases Zaff and Ballentine (COMET partners project) showed a southward bias for forecasted Erie/Ontario LES bands for a collection of 2006-2007 cases Has this bias continued into 2007-2008? Has this bias continued into 2007-2008? If so, is this bias adversely impacting the Northeast Regional Ensemble? If so, is this bias adversely impacting the Northeast Regional Ensemble? What causes the bias/how do we fix it? What causes the bias/how do we fix it? Both WRF cores exhibited a southward bias. The ’s south bias was greater than the The ARW’s south bias was greater than the NMM ARW (mean -18.7 km) NMM (mean -6.3 km).

3. The 10th annual Northeast Regional Operational Workshop, Albany, NY The Northeast Regional Ensemble Six 12 km WRF-EMS runs Six 12 km WRF-EMS runs Goal: Probabilistic LES guidance that adds to what is provided by operational 12 km NAM (NMM) Goal: Probabilistic LES guidance that adds to what is provided by operational 12 km NAM (NMM) MemberWRF Core IC/BCPhysicsPBLCPS BGMARWNAMLin et al.YSUKF CTP*NMMNAMFerrierMYJ TKEBMJ CLEARWGFSLin et al.YSUKF ALBARWNAMLin et al.YSUKF BTVNMMGFSFerrierMYJ TKEBMJ BUFNMMGFSFerrierMYJ TKEKF Experimental*ARWNAMFerrierMYJ TKEBMJ

4. The 10th annual Northeast Regional Operational Workshop, Albany, NY The Event – 10Feb2008 Persistent deep, cold westerly flow Persistent deep, cold westerly flow “Easy” case for models “Easy” case for models LAPS Sounding 0611FEB2008 RUC 850 hPa Analysis 0611FEB2008

5. The 10th annual Northeast Regional Operational Workshop, Albany, NY The Event – 10Feb2008

6. The 10th annual Northeast Regional Operational Workshop, Albany, NY The Event – 10Feb2008

7. The 10th annual Northeast Regional Operational Workshop, Albany, NY A good forecast? Ensemble does a good job pointing out band location Ensemble does a good job pointing out band location Some question about whether band sinks into Onondaga/Madison counties Some question about whether band sinks into Onondaga/Madison counties Do any ARW/NMM biases exist here? Do any ARW/NMM biases exist here?

8. The 10th annual Northeast Regional Operational Workshop, Albany, NY ----- NMM Members ----- ARW Members The Event – 10Feb2008

9. The 10th annual Northeast Regional Operational Workshop, Albany, NY ARW vs. NMM Bias is apparent – ARW further south Bias is apparent – ARW further south ARW “upwind band development point” near or even on shore! ARW “upwind band development point” near or even on shore! ARW members alone in bringing band into northern half of Onondaga/Madison counties ARW members alone in bringing band into northern half of Onondaga/Madison counties Current BGM run is ARW…uh oh! Current BGM run is ARW…uh oh! Can we change the settings in this run to improve its performance? Can we change the settings in this run to improve its performance?

10. The 10th annual Northeast Regional Operational Workshop, Albany, NY Sensitivity Simulation MemberWRF Core IC/BCPhysicsPBLCPS BGMARWNAMLin et al.YSUKF CTP*NMMNAMFerrierMYJ TKEBMJ CLEARWGFSLin et al.YSUKF ALBARWNAMLin et al.YSUKF BTVNMMGFSFerrierMYJ TKEBMJ BUFNMMGFSFerrierMYJ TKEKF Experimental*ARWNAMFerrierMYJ TKEBMJ mm QPF, for the one hour ending 06Z11FEB008 (mm).

11. The 10th annual Northeast Regional Operational Workshop, Albany, NY Sensitivity Results Sensitivity simulation shows no improvement Sensitivity simulation shows no improvement This run was identical to CTP-NMM run except model core This run was identical to CTP-NMM run except model core Numerous other sensitivity simulations showed nearly identical results Numerous other sensitivity simulations showed nearly identical results Implications for ensemble Implications for ensemble ARW members biased (this bias was seen during other 2007-2008 events) ARW members biased (this bias was seen during other 2007-2008 events) Bias causes unnecessary spread in the ensemble Bias causes unnecessary spread in the ensemble Usefulness of the ensemble suffers Usefulness of the ensemble suffers

12. The 10th annual Northeast Regional Operational Workshop, Albany, NY Reason for Bias 10 m wind speed, averaged through a 24 hour simulation (m/s). m/s

13. The 10th annual Northeast Regional Operational Workshop, Albany, NY The Solution Still working with NMM/ARW developers on the reason for this low-level wind difference Still working with NMM/ARW developers on the reason for this low-level wind difference Surface roughness computation changes? Surface roughness computation changes? WRF version 3 (WRF EMS uses version 2.1.2) was released in 2008. Can a newer WRF version help? WRF version 3 (WRF EMS uses version 2.1.2) was released in 2008. Can a newer WRF version help? Let’s try an identically configured run using version 3 Let’s try an identically configured run using version 3

14. The 10th annual Northeast Regional Operational Workshop, Albany, NY The Solution…for now! mm m/s 10 m wind speed, averaged through a 24 hour simulation (m/s). QPF, for the one hour ending 06Z11FEB008 (mm).

15. The 10th annual Northeast Regional Operational Workshop, Albany, NY The Solution…continued WRF ARW version 3 outperforms version 2 even with identical settings WRF ARW version 3 outperforms version 2 even with identical settings Smaller (although not removed) wind bias Smaller (although not removed) wind bias Smaller LES Band position error Smaller LES Band position error Suggestion: Suggestion: If using ARW, consider version 3 for 2008-2009 LES season If using ARW, consider version 3 for 2008-2009 LES season

16. The 10th annual Northeast Regional Operational Workshop, Albany, NY Conclusions LES band position bias continues for 2007-2008 LES band position bias continues for 2007-2008 Significantly worse for ARW Significantly worse for ARW Bias degrading Bias degrading Bias must be removed before ensemble can reach full potential Bias must be removed before ensemble can reach full potential Bias appears linked to overly strong low level wind fields in ARW Bias appears linked to overly strong low level wind fields in ARW Numerous sensitivities show that this bias may be core (and not physics) dependent Numerous sensitivities show that this bias may be core (and not physics) dependent Version 3 of WRF ARW improves simulations Version 3 of WRF ARW improves simulations

17. The 10th annual Northeast Regional Operational Workshop, Albany, NY Future Work Expand study to additional cases Expand study to additional cases Similar results so far for 25JAN2008 Similar results so far for 25JAN2008 Migrate local BGM WRF run to version 3 Migrate local BGM WRF run to version 3 WRF-EMS version 3 expected this fall WRF-EMS version 3 expected this fall Determine optimal local settings for LES Determine optimal local settings for LES Hone in on reason for version 2  3 change in ARW performance Hone in on reason for version 2  3 change in ARW performance Perform 2008-2009 case studies Perform 2008-2009 case studies

18. The 10th annual Northeast Regional Operational Workshop, Albany, NY Acknowledgements Stephen Jascourt (UCAR/COMET) Stephen Jascourt (UCAR/COMET) Jimy Dudhia (UCAR) Jimy Dudhia (UCAR) Robert Rozumalski (NWS/OCWWS) Robert Rozumalski (NWS/OCWWS) …for guidance on model formulations, model changes and suggestions for sensitivity simulations