Global-scale Winds

Courtesy: U. of Alaska

General Circulation

> Global wind systems General circulation 1-cell, 3-cell models, comparison to real world > Semi-permanent pressure features > Jet streams

02.23.2010

Scales of Motion - Hierarchy

planetary scale synoptic scale mesoscale microscale Small turbulent eddies Thunderstorms Tornadoes Waterspouts Dust devils Land/sea breeze Mtn/Valley breeze Chinook Santa Ana Hurricanes Topical storms Weather Map Highs and Lows Weather fronts Longwaves seconds to minutes minutes to hours hours to days days to weeks

Atmospheric General Circulation

+ Large-scale, hemispheric flow + Average wind patterns across globe + Interrupted by highs and lows moving through + DRIVING FORCE??

COLD

UNEQUAL HEATING OF THE EARTH THERMAL IMBALANCE

WARM COLD

Atmospheric General Circulation

UNEQUAL HEATING OF THE EARTH THERMAL IMBALANCE

COLD Net LOSS WARM Net GAIN Net LOSS COLD

Energy Imbalance Driver of Atmospheric Circulation

Models of Atmospheric Circulation

Single-cell Model

Assume

1) aqua planet 2) sun over equator 3) non-rotating earth

COLD WARM COLD

Models of Atmospheric Circulation

Single-cell Model

Assume

1) aqua planet 2) sun over equator 3) non-rotating earth high pressure

Result

> excessive heating at equator > thermally-driven convection cell > pole-ward flow aloft equator-ward flow sfc > termed,

Hadley cell

low

The Atmosphere, 8th edition

, Lutgens and Tarbuck, 8th edition, 2001

Models of Atmospheric Circulation

Three-cell Model

Assume

1) aqua planet 2) sun over equator 3) non-rotating earth

Result

> excessive heating at equator > thermally-driven convection cells > deflection of winds >

Hadley cell Ferrel cell Polar cell The Atmosphere, 8th edition

, Lutgens and Tarbuck, 8th edition, 2001

Atmospheric Circulation

Models vs Reality

Idealized winds Actual winds

The Atmosphere, 8th edition

, Lutgens and Tarbuck, 8th edition, 2001

Atmospheric Circulation

Models vs Reality

Actual winds

Why the difference?

1) land-water distribution 2) seasonality

unequal heating/cooling rates

Idealized winds

Atmospheric Circulation

Some Features of Note:

Equatorial

•

ITCZ (Intertropical Convergence Zone)

•

Equatorial low

•

Area of low wind speed Doldrums (weak PG)

Atmospheric Circulation

Some Features of Note:

Subtropics

•

Trade winds

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Horse latitudes weak winds

•

Subtropical high warm and dry Desert areas of the world

Atmospheric Circulation

Some Features of Note:

Midlatitudes, Polar

•

Prevailing westerlies

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Polar front and areas of low pressure

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Polar easterlies

Persistent Circulation Patterns NH winter

Sea level pressure and prevailing winds

Persistent Circulation Patterns NH winter

> subtropical highs

Sea level pressure and prevailing winds

Persistent Circulation Patterns NH winter

> 2 major subpolar lows: near polar front, storm track Sea level pressure and prevailing winds

Persistent Circulation Patterns NH winter

> subpolar highs: shallow, thermally induced Sea level pressure and prevailing winds

Persistent Circulation Patterns NH winter

> subpolar trough in SH: high winds and seas, roaring 40s Sea level pressure and prevailing winds

Persistent Circulation Patterns NH winter

> ITCZ displaced south

Sea level pressure and prevailing winds

Persistent Circulation Patterns NH summer

> ITCZ displaced north

Sea level pressure and prevailing winds

Persistent Circulation Patterns NH summer

> Subtropical highs move north

Sea level pressure and prevailing winds

Persistent Circulation Patterns NH summer

> Thermal lows develop over land

Sea level pressure and prevailing winds

Persistent Circulation Patterns NH summer

> Weak Icelandic Low remains, Aleutian Low disappears

Sea level pressure and prevailing winds

Persistent Circulation Patterns NH summer

> Asian Monsoon season

Sea level pressure and prevailing winds

Seasonal Circulation Patterns

winter summer

Strong thermal imbalance Strong pressure gradient Strong winds Strong equator - pole temp gradient Subpolar lows disappear Subtropical highs remain Zone of max heating shifts north Weak equator - pole temp gradient

Thermal Imbalance is the Driver

Sea level pressure and prevailing winds

L H

Seasonal Circulation Animation

Sea level pressure and winds University of Oregon

Thermal Imbalance is the Driver

General Circulation and Precipitation

Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet

seasonally

Dry

General Circulation and Precipitation

Dry Wet

Precipitation (mm) University of Oregon

Jet Streams

•

Fast-moving rivers of air

•

High altitudes (~ 35000 ft) near tropopause

•

Long, shallow, narrow moving west to east

•

First observed during WW II

•

Suspected earlier from ground observations of fast-moving cirrus

Jet Streams

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Mark boundary between surface air masses

•

Shows ridges, troughs, eddies

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Changes in space and time

•

Seasonality

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Note the speed differences

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Surface temperature differences

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Tend to steer storm tracks

Jet Streams

Jet Streams

Jet Streaks

Jet Stream and Jet Travel