1. FLIGHT HAZARDS OF MOUNTAIN WAVES AND WIND EVENTS Stan Rose National Weather Service, Pueblo, Colorado

2. Roughly one-half of the air accidents in vicinity of the Rocky Mountains are Weather Related.

3. Mountain Turbulence: Convection Stan Rose 2008

4. Convection: Thermally Generated flows

5. Gap Winds Strong winds flowing through gaps or passes. Strongest winds found in the exit of the gap. Pressure difference from one side to another is an indicator. A reverse in expected pressure difference can indicate reversed flow, a possible hazard.

6. Mountain turbulence: mechanical

7. What determines how wind behaves when it encounters terrain? The Nature of the obstacle (height, width) The direction and strength of the wind (kinetic energy) The Stability of the atmosphere (potential energy)

8. Stability and turbulence: UNSTABLE Only moderate cross-barrier flow needed. Unstable air : if unstable air crosses a mountain barrier, turbulence is almost certain. Cumulus clouds are good indicators. Leeside downdrafts can be strong, but generally don’t extend far beyond the range.

9. Stability and turbulence: STABLE Strong cross-barrier flow needed. Cloud rows and lenticular clouds are indicators. Rotor clouds can form in the lee, downstream, below ridge-top. Turbulence can be found well downstream.

10. A common meteorological index: The Froude Number A Ratio, Similar to KE/PE or Velocity/Stability If F > 1, Flow moves over the obstacle (strong flow, unstable conditions) If F < 1, Flow is blocked. (weak flow, stratified atmosphere). F = 1, a resonance (favorable for windstorms)

11. Mountain turbulence: mechanical 1) vertically propagating wave

12. Mountain turbulence: mechanical 2) trapped lee wave

13. ROTOR CLOUD

14. Rotor Clouds are indicative of Wakes and/or eddies

15. Lenticular clouds Indicative of high winds aloft (jet stream) Moist air Dry air

17. Kelvin-Helmholz (K-H) Wave cloud Stan Rose 2008

18. Severe Down-slope Winds  Most common in Winter Months.  Can occur night or day: Often night  Downdrafts of 50+ fps

19. Severe Down-slope Winds: Significant Indicators 1.Winds at mountain top generally > 30 Kts. 2.Jet stream in the vicinity. 3.Frontal passage can precede SDW.

21. WINDSTORMS Most often occur in the presence of a CRITICAL LAYER.

22. CRITICAL LAYER: Reverse Shear Winds decrease with height (Shear is zero)

23. CRITICAL LAYER: Change in Stability A Mountain top Stable Layer. Decreasing stability with height.

24. Vertical X-Section

25. Model Forecast Data Sea Level Pressure or 850mb Charts NAM 12km 27h forecast; 10/10/2008 18Z valid 21Z 10/11/2008

26. Site Sounding

27. Observational Data (adds.aviationweather.gov)

28. Observational Data Satellite imagery (wave clouds/cloud streets, lee drying)

29. Observational Data Satellite imagery (wave clouds/cloud streets, lee drying)

30. Observational Data Satellite imagery (wave clouds/cloud streets, lee drying)

31. Crossing Mountains Climb to 3,000+ feet above ridge-tops. Begin ascent 100+ miles from the mountains. Be prepared to avoid the obstacle entirely. Approach ridges/passes at 45 degree angle. Don’t fly when there are widespread lenticulars!

32. THE END