Mountain windstorms

Downslope windstorms:general term of a windstorm in which air flows down the side of a mountain.

Local names include:• Foehn: Alps (warm)• Bora: Adriatic Sea (cold)• Chinook: Rockies (warm or cold)• Santa Ana: Southern California (warm)• Katabatic: Antarctica (cold)

Locations of downslope windstorms in western North America

Very strong Chinook downslope windstorms occur every winter in Boulder., Colorado

The Dynamics of Downslope Winds

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 are a series of waves that form

downstream of mountains.

Lenticular cloud

In the above diagram, the lines are streamlines, representing flow over the mountain. Winds are very strong where streamlines are close together.

The combination of an inversion upstream and the mountain created strong winds accelerating down the mountain.

This may result in a hydraulic jump, an abrupt change in velocity in fluid flow. East of the jump, areas of rotation called rotors develop.

The intensity and position of the jump – and therefore the strength of the windstorm – is tied to the strength and location of the upstream inversion.

Cold versus Warm Downslope Winds

three factors determine the temperature of the downstream wind

A Chinook wall: Clouds have formed on the windward side of the Rockies, precipitating and releasing latent heat.

On the leeward side, the clouds evaporate.

1. At what rate does air warm as it descends?

2. How much does air warm as it descends from the crest of the Rockies to Boulder (dz = ~1.5 to 2 km)?

3. How would this warming affect the relative humidity?

4. And how would the relative humidity of descending air affect snow cover in Boulder?

Chinook wall

Forecasting Chinooks

What are the three requirements for a Chinook to form?

Forecasting Chinooks

What are the three requirements for a Chinook to form?

1. Stable layer upstream of the mountain 1-2 km above mountaion top

2. Strong surface pressure gradient across the mountain

3. Background flow at mountaintop (~700 mb) is strong and westerly

Chinook winds can produce rapid temperature fluctuations, depending on the position of hydraulic jump and cold air that may be located at the surface.

A B C D

World records: 27˚C (49˚F) increase in temperature in 2 minutes, 1943, Spearfish, SD.

15 to 20˚C (27 to 36˚F) increase in temperature not uncommon in Boulder.

Surface temperature and SLP from the Jan 17, 1982 Chinook

in Boulder.

Lee trough: area of low pressure on the lee side of the

mountains.

700 mb height field with shaded isotachs for January 17, 1982.

Soundings from Grand Junction, CO and Lander, WY (just upstream of Front Range) from Jan 17, 1982.

Where are the inversion levels located relative to the crest of the

mountains?

Santa Ana Windstorms of California

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 two conditions are required for Santa Ana winds to form?

Santa Ana winds are not typically as strong as Chinook winds, but wind speed can be enhanced when flow is funned through canyons east of Los Angeles.

Why would we care about Santa Ana winds – particularly in California’s dry season in late fall and winter?

Figure 16.11

Figure 16.13

Figure 16.14

Katabatic Winds

Katabatic wind – a cold, downslope wind that is the result of cold air flowing downslope under the force of gravity

How do katabatic winds form?

How do katabatic winds differ from Chinook winds and Santa Ana winds?

Where would katabatic winds typically occur?

Figure 16 A