KUSA Boulder Windstorms Effects of Windstorms High winds Rapid temperature shifts Rapid pressure fluctuations Incredible cloud formations Average January Temps By City Climatological Average ID: 816706
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Slide1
What can windstorms do?
Image credit:
KUSA
Slide2Boulder Windstorms
Slide3Effects of Windstorms
High windsRapid temperature shiftsRapid pressure fluctuationsIncredible cloud formations
Slide4Average January Temps By City
(Climatological Average)
Location
Elevation (ft)
Temperature (F)
Greeley
4664
28
Loveland
4982
26
Ft. Collins500328.5Longmont505227Denver528030Boulder534433
Slide5Airmasses
What kind of air mass is typically found over Colorado in the winter?
Photo Courtesy: Texas A&M Dept. of Meteorology Website
Slide6Airmasses
Is Continental Polar (cP) air always present in this area in the winter?
What process is capable of altering local conditions away from
cP
?
Slide7Downslope Winds: Western U.S.
Where do
downslope windstorms occur in western North America? Strong Chinook downslope
windstorms occur every winter in Boulder.
Slide8COLD 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
Slide9A 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
Slide10View from Mount Evans towards Denver on a day with a strong inversion in Denver
Slide11Chinooks require a strong west wind. They are dry and warm due to latent heat release upstream and compression during descent
Slide12When 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.
Mountain Waves
Slide13Different look from the ground
Slide14Flow associated with severe down-slope winds
This figure shows streamlines of flow over a mountain barrier.
Strongest winds occur where streamlines are closest together.
Most notable :
Strong winds upstream & over the mountain
An inversion layer is present above the top of the mountains
A ‘hydraulic jump’ marks eastern boundary of strong surface winds
cloud
cloud
cloud
Slide15Why Do Windstorms Occur In Boulder?
Boulder, CO sits 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.
Slide16COLD AIR MASS
Boulder
WEAKER WINDS
STRONGER WINDS
STRONGEST WINDS
Slide17Examples 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
Slide18Chinooks and rapid temperature fluctuations
Slide19Chinook’s Effects
Slide20Boulder Windstorms 1969-2002 (unofficial)
Windstorm frequency varies from 0 to >15 per year
By far, windstorms are most common in Dec and Jan
Very rarely is there a windstorm from May through September
When do windstorms occur in Boulder?
Slide21How fast can wind speeds get?
January 1982
Slide22Slide23What determines the strength of the winds?
The Pressure Gradient Force (PGF)Bigger pressure difference between two points stronger winds
Winds blow across the gradient: from High pressure to Low pressureThe huge amount of friction associated with flow over the mtns
allows the wind to go almost from high to low pressure
Slide24The January 17, 1982 Boulder Windstorm
Surface weather map:
How does the sea-level pressure change from west to east across Colorado?
It decreases quite a bit, strong PGF!!
How do the temperatures along the Front Range compare to the temperatures further east?
Much warmer!!!
700
mb
map showing height contours for the January 1982 Boulder
downslope
windstorm
So also strong winds at mountain top level!!
Slide25The January 17, 1982 Boulder Windstorm
Temperature soundings from Lander, WY and Grand Junction, CO for the January 1982 Boulder
downslope
windstorm
At what pressure level are inversion layers located in these soundings?
~600mb
Where are the inversion layers located relative to the mountaintop level (700mb)?
Just above mountaintop level!!
This inversion makes the wind accelerate (squeezes the flow) and also acts to direct it downwards towards surface!!!
Slide26Chinook wall cloud, Boulder, 2002
Visual evidence of a Chinook : the wall cloud
Slide27Visual evidence of mountain waves:
Lenticular clouds
Pileus
cloud
Slide28Today’s Procedure
Files have time series informationData in folder on your computersDetermine windstorm conditions & enter into tables
Tables: Use electronic tables available in folder
Slide29Lab 8
Module with associated Canvas quizActivity: 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/kmSubmit your tables to dropbox
(40%)Take Canvas quiz by next week’s meeting time (60%). Follows lab questions.