The Santa Ana Winds

Ryan KittellAS 240A

2004

Well Known

Fire wind oh desert windShe was born in a desert breezeAnd wind her wayThrough canyon wayFrom the desert to the silvery sea

- The Beach Boys

What Are They

• North to Easterly Wind• Warm and Dry Wind on Leeside of Local

Mtns• Originates from the Great Basin• Impacts:

– Wind damage below passes and canyons– Favorable wildfire environment– Aircraft hazards– Beach busy-ness

Wildfires

Where Did the Name Come From?

• Named after Mexican president and general Antonio Lopez de Santa Anna

• First to occur on Saint Anne’s Day - July 26th

• Derived from the Spanish word Satana

• Derived from the Native American word Santana – “Big Bad Wind”

• Named by early Orange County settlers

Reporter and casual historian Terry E. Stephenson researched the origin of the name in 1943. He concluded that early Spanish settlers living near the Santa Ana Canyon named the wind after the canyon.

Climatology

Santa Ana winds impact the long term climatology.

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Climatology

Strong events impact more than just the local environment.

Climatology

• Have occurred between September – June– Most occur between October – February

• Last 1 ½ days on average• Average winds 35kts, Gust 50kts canyon

winds• ~20 events per year• Gusts over 100kts possible

– Mt Wilson, early 1900’s, Cat 2 Hurricane Strength

Climatology

Climatology work by Raphael, 2003

Peak in November and December

Take a Ride…Typical MSLP

pattern

• High pressure in eastern deserts and Great Basin– From passing

upper-level troughs.

H

LL

Take a Ride…

H

LFlow tries to maintain geostrophic/cyclostrophic

balance… but complex terrain causes air to flow

across isobars

Take a Ride…

4 major entry points into coastal plain

Why So Hot and Dry?

• Great Basin: elevation 4000- 6000 feet– Tonopah: 5426 ft.

• Air compresses along the way.– Warming ~ 5.4oF per 1000ft– ~30oF warming potential– Beaches often warmest

• Relative Humidity ~ inverse temperature

Why So Windy?

• Strongest winds often seen below passes and canyons.– Conservation of Mass & The Bernoulli

Effect

– Area decreases velocity increases

Why So Windy?

• During strong Santa Ana events, intense winds felt throughout the leeside of the Southern California mountains– Strong event large airmass– Flow spills over mountains– Downslope wind amplification

• Often surface winds are stronger than aloft• Energy source needed to accelerate wind

Three Theories

1. Hydraulic Jump Theory- Long, 1954; Durran, 1986

2. Gravity Wave Reflection- Klemp and Lilly, 1975

3. Breaking Wave Trapping- Clark and Peltier, 1977; Durran and Klemp, 1987

Hydraulic Jump

What Panu said…

SupercriticalFr > 1

SubcriticalFr < 1

Hydraulic JumpFluid Transition

Fr = U / (gH)1/2

Gravity Wave Reflection

Vertically propagating GWs formed by downslope flow

…regions of strong stability (Scorer Number) gradients reflect them back toward the surface

…mountain wave amplification by superposition.

Breaking Waves

Vertically propagating GWs formed by downslope flow

…critical layers formed by intense GWs that break

…critical layer regions trap and reflect them back toward the surface

Forecasting

• All 3 models rely on the presence of an inversion layer or strong static stability.

• Strong events that lead to downsloping winds usually has strong winds aloft.…often called “upper level support”

Warnings…

Constantly improving observational network…in strategic locations.

Merry Christmas