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Downslope Winds and Associated Phenomena: Foehn, Chinook, Santa Ana, and Katabatic Winds, Lecture notes of Earth, Atmospheric, and Planetary Sciences

Various types of downslope winds, including Foehn, Chinook, Santa Ana, and Katabatic winds. It explains their occurrence, dynamics, and associated phenomena such as lee waves, hydraulic jumps, and lenticular clouds. The document also covers the factors determining the temperature of downslope winds and the impact of these winds on weather conditions.

Typology: Lecture notes

2021/2022

Uploaded on 09/27/2022

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Chapter 17:
Mountain Windstorms
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Download Downslope Winds and Associated Phenomena: Foehn, Chinook, Santa Ana, and Katabatic Winds and more Lecture notes Earth, Atmospheric, and Planetary Sciences in PDF only on Docsity!

Chapter 17:

Mountain Windstorms

Downslope winds

 Foehn – Alps ( warm )

 Bora – Adriatic Sea ( cold )

 Chinook – Rocky Mountains ( warm )

 Santa Ana – Southern California ( warm )

 Katabatic – Antarctica and Greenland ( cold )

Dynamics of downslope winds

 Recall:

Windward side – upwind side of a mountain range  Leeward (or lee) side – downwind side of a mountain range  As air encounters a mountain range it must rise on the windward side and then descend on the leeward side. Lee waves – a series of waves in the flow downstream from a mountain What type of clouds form in lee waves?

Altocumulus lenticularis

 “Lenticular” clouds,

lens shaped clouds

Hydraulic Jump

 The hydraulic jump marks the transition from

strong winds upstream of the jump to weaker winds downstream of the jump.

 Strong vertical winds and turbulence occur at the

hydraulic jump, creating dangerous conditions for aircraft. Hydraulic Jump — Where the flow abruptly rises and becomes turbulent as it adjusts within the deeper layer over the plains downstream of the mountain

Breaking waves

 Kelvin-Helmholtz wave

clouds Waves break similar to those in the ocean

Demo

 http://nimbus.colorado.edu/1050/PAGES/

Contents/Experiments/channel.htm

Clicker Question

 You would expect air to _______ on the windward

side of a mountain range resulting in ________ skies. A. Rise, mostly sunny B. rise, mostly cloudy C. Sink, mostly sunny D. sink, mostly cloudy

Clicker Question

 A hydraulic jump may form downstream of a

mountain range if the winds are _________ upstream of the mountain and an inversion layer is present _________ the top of the mountains. A. weak, above B. strong, above C. weak, below D. strong, below

Cold vs Warm downslope winds

 3 factors that determine the temperature of the air

in downslope winds “Chinook wall”

3 weather features for a Chinook

 Stable layer upstream of mountains

(inversion just above mountaintop level)

 Strong surface pressure gradient across

the mountain  Lee trough – an area of low pressure on the lee side of the mountains

 Mountaintop air flow (~700mb) is strong

and westerly How does the sea-level pressure change from west to east across Colorado? How do the temperatures along the Front Range compare to those further east? Inversions ~600 mb Mountaintop ~700 mb

Santa Ana winds

Santa Ana winds – the name given to downslope windstorms in Southern California  Santa Ana winds are easterly winds that blow from the desert of the Great Basin across the San Bernadino and San Gabriel Mountains and into Southern California.  What weather conditions are required for Santa Ana winds to form?

  1. Strong high pressure over the Great Basin
  2. Strong north-south pressure gradient over southern California
  3. An inversion layer above mountaintop level may also be needed

Katabatic winds

 Katabatic wind – a cold,

downslope wind that is the result of cold air flowing downslope under the force of gravity  As opposed to Chinook and Santa Ana winds that are driven by strong pressure gradient forces  They occur in cold, high elevation regions of the world with vast ice sheets (Antarctica, Greenland coast)  Severe winds result (>100 kts)

Clicker Question

 Which of the following is a cold downslope wind?

A. Foehn B. Chinook C. Katabatic D. Santa Ana