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

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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