Downslope Windstorm Lab

1. Downslope Windstorm Lab Dr. Louisa Bogar Nance NCAR/RAL/DTC

2. Background Downslope windstorm lab has been part of 2 COMAP courses (Fall 1999 & 2003) Lab accompanied a lecture on mtn waves & downslope windstorms w/ an emphasis on forecasting downslope windstorms Set-up to use tools commonly available in NOAA/NWS forecast offices in the late 1990s

3. Lecture Summary Mountain waves have a natural tendency toward accelerating lee-slope flow Downslope windstorms are the result of a strong low-level amplification of a mtn wave Elevated stable layers and critical layers (mean-state or wave-induced) are key to the low-level amplification process

4. Lab Objectives Reinforce basic mtn wave concepts presented in lecture Atmospheric conditions that lead to downslope windstorms (i.e. stable layers & critical layers) Develop insight into the sensitivities of the mtn wave response to changes in the characteristics of the large-scale flow Exercises employ a nonlinear 2D model to illustrate how the mtn wave structure varies in relation to changes in the large-scale flow

5. Lab Outline Part I: Idealized mtn wave exercise Part II: Forecast exercise / 24 Feb 97 case study Part III: Discussion of forecast exercise Part IV: Modified sounding exercise Part V: Discussion of 24 Feb 97 wind event Part VI: Discussion of modified sounding results

6. 2D Model Description

8. Part I: Idealized Mtn Wave Exercise Instructions: Open a web browser and load /home/workshop/nance/idealized/index.html

9. Part II: Forecast Exercise � Wasatch Front Windstorm 24 February 1997 Use GARP to view data Follow directions on hand-out

10. Part III: Discussion of Forecast Exercise

11. Handout Questions Source region of low-level air mass over N. UT? Influence on stability in the lower part of the atmosphere? Evolution of low-level flow? Evolution of mid-tropospheric flow? Mid-tropospheric vertical motions over N. UT? Influence on stability profile? Why look at conditions over WY when considering potential for high winds in SLC region? Which forecast sounding would you expect to generate the most intense sfc wave response?

21. Part IV: Modified Sounding Exercise Use NSHARP to modify Eta sounding for 97022418 & generate 2D mtn wave simulation using new sounding � see directions on hand-out

22. Part V: Discussion of 24 February 1997 High Wind Event

23. Synoptic Conditions Retrograding upper-level trough from east of the Rockies to Nevada Weak cross-mtn pressure gradient Moderate crest-level flow

25. Mesoscale Conditions Winds > 70 mph observed along northern Wasatch Front � strongest winds in Davis & Weber counties Centerville 66/81 mph Bountiful (bench) 68 mph Ogden (bench) 64 mph Hill A.F.B. 56 mph Strongest terrain-induced high wind event in this region since 14-15 December 1988

26. Damage 3 tractor trailers & a camper overturned near Centerville on I-15 School closures in Weber & Davis counties Downed power lines Power outages in Weber & Davis counties House fire Extensive property damage along Wasatch Front north of Bountiful Downed trees & fences Siding / stucco stripped from houses Roof damage

31. Part VI: Discussion of Modified Sounding Exercise View model results - Open GARP window Click on Model Cross-Section button Select �Group 1�

32. Summary Mountain waves have a natural tendency toward accelerating lee-slope flow Downslope windstorms are the result of a strong low-level amplification of a mtn wave Elevated stable layers & critical layers (mean-state or wave-induced) are key to the low-level amplification process The lee-slope response is sensitive to the vertical distribution of these flow features This sensitivity makes downslope windstorms difficult to forecast!

33. Potential variations or additions Develop a more sophisticated scripting and graphical interface for lab Add simple 3D model to each portion of the lab to explore differences between 2D & 3D behavior Add moisture effects Compile database of modified sounding simulations