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Wind Systems

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Ch. 8: Global Scale Winds

• Size Scales of Weather and Winds

• Circulation Cells—The General Circulation

• Surface Wind Patterns

• Semi-permanent Highs, Lows

• Upper-air Westerlies

• Jet Streams, Rossby Waves

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Ch. 8: Wind Systems

• Monsoon Circulations

• Chinook and Santa Ana Winds

• Sea and Land Breezes

• Mountain and Valley Breezes

• El Niño/La Niña

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Size Scales• Microscale

Less than about 1 km

• Mesoscale

Up to about 100 km

• Synoptic-scale

Up to about 10,000 km

• Global-scale

Planetary-size

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The General Circulation

Non-rotating Earth

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The General Circulation

Rotating Earth

Hadley Cell

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GC Surface Winds90°N

90°S

60°N

30°N

0°

30°S

60°S

Polar Easterlies

Prevailing Westerlies

Trade Winds

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Surface Pressure Distribution

L

HL

H

HH

Aleutian Low

EasternPacific High

Icelandic Low

Bermuda-Azores High

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Upper-Level Westerlies

z

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Upper-Level Westerlies

z

Cold Pole Warm Tropics

Poleward PGF aloft

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North Pole

Equator

Vg

PGF

Fcor

500-mb heights

N

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Equator

South Pole

Vg

Fcor

PGF

500-mb heights

Upper-level winds are generally west-to-east, both hemispheres

N

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Jet Stream

z

To pole 60°N To Equator

Steep pressure gradient in narrow band

Discontinuity at convergence zone

Convergence of warm and cold air near the surface

Jet stream

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Rossby Waves

• “Long waves”

Global scale: large wavelength waves in upper-level westerlies, going around either pole

Propagate very slowly

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Monsoon Circulation

• Synoptic-scale thermally driven circulation

• Large size scale — longer cycle time — flips seasonally instead of daily

Winter Summer

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Chinook Wind

In Europe/Asia: Foehn Wind

Latent heat release warms the rising air

Condensed water precipitates out on windward side

Dry adiabatic compression

Hot, dry!

Compression heat+

Condensation heat

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Santa Ana Wind

So. Cal.

High Deserts

San Gabriel/ Tehachapi

Mts. Hover Great Basin

Adiabatic compression during descent warms the air

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Thermally Driven Winds:Sea and Land Breezes

Ocean

Ocean

Land

Land

Cool Warm

Cool Cold

H L

L H

Thermal High Thermal Low

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Mountain and Valley Breezes

Valley Breeze

Mountain Breeze

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El Niño/Southern Oscillation

“Normal” Tradewind

Upwelling

z

N

E

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El Niño/Southern Oscillationz

N

E

Reversed El Niño wind

Oscillation period: 3–7 years

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La Niña: The Pendulum Swings Back

z

N

E

Upwelling

Strong Easterly Tradewind

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