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What can windstorms do?

What can windstorms do?. Image credit: KUSA. Boulder Windstorms. Effects of Windstorms. High winds Rapid temperature shifts Rapid pressure fluctuations Incredible cloud formations. Downslope Winds: Western U.S. Where do downslope windstorms occur in western North America?

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What can windstorms do?

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  1. What can windstorms do? Image credit: KUSA

  2. Boulder Windstorms

  3. Effects of Windstorms • High winds • Rapid temperature shifts • Rapid pressure fluctuations • Incredible cloud formations

  4. Downslope Winds: Western U.S. Where do downslope windstorms occur in western North America? Strong Chinook downslope windstorms occur every winter in Boulder.

  5. Why Do Windstorms Occur In Boulder? • Near the boundary separating the colder, denser air from the warmer winds that flow over/down the mountains • The strength of the winds will determine the extent to which the cold air is displaced • When the winds are strong off the mountains (from the west!!!), Boulder sits in the warm air, and when they’re weak, Boulder remains cold.

  6. Average January Temps By City Places closer to the base of the mountains spend more time in the warm downsloping winds

  7. COLD AIRMASS Cold air is denser than warm air, and hugs the surface at it moves southward A shallow cold air mass can get trapped against the Rockies  cold air damming

  8. A class winter set-up with a shallow cold air mass pushed against the Rockies from the East, with strong and warmer west wind aloft Wind speeds increase with height

  9. View from Mount Evans towards Denver on a day with a strong inversion in Denver

  10. Examples of this sloshing effect: When west winds are weak: Cold air climbs hill, and Boulder is cold, more moist, & winds are calm When west winds are strong: Cold air pushed east, and Boulder is dry, windy, & warm

  11. STRONGEST WINDS STRONGER WINDS WEAKER WINDS COLD AIR MASS Boulder

  12. Chinook’s Effects

  13. Why are Chinooks warm and dry? Chinooks require a strong west wind. They are dry and warm due to latent heat release & loss of moisture during ascent, as well as due to compression during descent

  14. Mountain Waves When large-scale weather patterns produce a strong, deep flow of air across the Rockies, the peaks along the Continental Divide act like rocks in a streambed. Forced up by a bump (island or mtns), air seeks to return to its original level but oscillates through several up-and-down cycles before settling back into a horizontal flow over the plains.

  15. Flow associated with severe down-slope winds If conditions are just right, the wind storm gets trapped on the mtn and hits Boulder Strongest winds occur where streamlines are closest together. cloud cloud cloud • Strong winds upstream & over the mountain • An inversion above mtns accelerates the flow (like thumb on hose) • A ‘hydraulic jump’ marks eastern boundary of strong surfacewinds • Winds ‘jump’ up off ground at hydraulic jump. Less windy to east.

  16. The base of the hydraulic jump separates warm and cold airmasses

  17. When do windstorms occur in Boulder? • Windstorm frequency varies from 0 to >15 per year • By far, windstorms are most common in Dec and Jan Boulder Windstorms 1969-2002

  18. How fast can wind speeds get? January 1982

  19. What determines the strength of the winds? • The Pressure Gradient Force (PGF) • Bigger pressure difference between two points  stronger winds • The huge amount of friction associated with flow over the mtns allows the wind to go almost directly from high to low pressure • i.e., Coriolis force is less important than normal

  20. The January 17, 1982 Boulder Windstorm BIG PGF WARM There were also strong winds at mountaintop level

  21. The January 17, 1982 Boulder Windstorm Inversions above mountains caused the air to be squeezed between mtn top and inversion

  22. Visual evidence of a Chinook : the wall cloud Chinook wall cloud, Boulder, 2002

  23. Visual evidence of mountain waves: Lenticular clouds Pileus cloud

  24. Today’s Procedure • Files have time series information • Data in folder on your computers • Determine windstorm conditions & enter into tables • Tables: Use electronic tables available in folder

  25. Lab 8 • Module with associated Canvas quiz • Activity: Tables A & B (don’t print meteograms). Electronic tables in module (folder). • 3 questions • Careful in Q3, as you will have to convert C to F in here, or more simply use the dry lapse rate in ºF/km = 18ºF/km • Submit your tables to dropbox (40%) • Take Canvas quiz by next week’s meeting time (60%). Follows lab questions.

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