Planetary Atmospheres, the Environment and Life (ExCos2Y)

Topic 6: Wind

Chris Parkes

Rm 455 Kelvin Building

5. Atmospheric Convection

Hot air rises, expands circulation cell– Heating at equator, cooling at polesHadley cell

– Coriolis Effect east/west winds disrupts Hadley cell

• Three cell model of

Earth’s atmosphere

– Convection in Sea Breezes

Winds

Horizontal movement of air

Controlled by four main forces:

Pressure-gradient force

Coriolis force

Centripetal acceleration

Friction

The Pressure-Gradient force

If pressure change by Δp over a distance of Δs, then the force is:

where ρ is air density

Bigger pressure change

Lower air density - high altitude Higher wind speed

Isobars = lines of constant pressure

Weather dominated by High & Low pressure systems

s

pF

1

Isobar chart

The Coriolis effect (again)

• Rotation of the earth

• Speed is greater nearer the equator then nearer poles– Further from rotation axis

• Object (not attached to the surface) moving from equator towards poles will appear to deflect eastwards

• Appears as a force the size of which depends on the Coriolis parameter ( f )

N

Equator θ

FCoriolis = - 2 m (ω × vr )

magnitude depends on sin(θ )

The Coriolis effect (again)

The Geostrophic Wind

• As air moves feels perpendicular coriolis force

• wind directions follow isobars• “free atmosphere” above ~500m

– where friction can be neglected• Picture shown for northern

hemisphere – opposite direction for southern hemisphere

• Velocity depends on latitude:

LPressure

Gradient

Force

H

Coriolis

Force

1000 mb

1004 mb

Latitude (degree) Speed (m/s)

43 15

90 10

Geostrophic Wind

•Wind rarely purely geostrophicbut approximately

•Ocean currents also

Balance between pressure-gradient force and coriolis forceView from above

Centripetal Acceleration

Low

Pressuregradient

Coriolisforce

Direction of gradient windDirection of

centripetalacceleration

High

Pressuregradient

Coriolisforce

Direction of gradient wind

Direction of centripetal

acceleration

Flow around low (high) pressure system is cyclonic (anti-cyclonic)

Fcent = FPG – Fcor (low pressure) FPG > Fcor

Fcent = Fcor – FPG (high pressure) FPG < Fcor

wind speed less than vg (subgeostrophic)

wind speed higher than vg (supergeostrophic)for same FPG

(PG usually higher for low pressure systems)

Frictional force

• Friction slows down wind near surface

• Decreases effect of deflective forces (Coriolis & Centripetal)

• Wind direction points more towards pressure gradient

• Direction points across isobars:– 10º - 20º over ocean, – 25º – 30º over land

L

View from aboveNo friction inmiddle of troposphere

Pressure

Gradient

Force

HCoriolis

Force

1000 mb

1004 mb

Geostrophic Wind

Frictional force

• Friction slows down wind near surface

• Decreases effect of deflective forces (Coriolis & Centripetal)

• Wind direction points more towards pressure gradient

• Direction points across isobars:– 10º - 20º over ocean, – 25º – 30º over land

L

View from aboveFriction Near Surface

Pressure

Gradient

Force

HCoriolis

Force

1000 mb

1004 mb

Geostrophic Wind

Frictional force

• Friction slows down wind near surface

• Decreases effect of deflective forces (Coriolis & Centripetal)

• Wind direction points more towards pressure gradient

• Direction points across isobars:– 10º - 20º over ocean, – 25º – 30º over land

Global Wind Patterns• Driven by:– Atmospheric heating– Planetary Rotation

• Equatorial: East to West– Surface wind towards equator

Coriolis effect east to west winds

Global Wind Patterns• Driven by:– Atmospheric heating– Planetary Rotation

• Polar: East to West– Surface wind towards equator (away from poles)

Coriolis effect east to west winds

Global Wind Patterns• Driven by:– Atmospheric heating– Planetary Rotation

• Midlatitude cells: West to East– Surface wind towards poles (away from equator)

Coriolis effect west to east winds

The Three-cell model & Global wind belts

Features

Intertropical Convergence Zone (ITCZ)

Doldrums

Trade Winds

Mid-latitude westerlies

Polar front

Polar easterlies

trade winds

westerlies

easterlies

jet streams

ITCZ, Doldrums

Polar Front

The Intertropical Convergence Zone (ITCZ)

Region of intense rainfall – violent thunderstorms

Position of ITCZ varies with season

Affected by land masses – more land in Nothern hemisphere

Noticeable “spurs” occur at different times

Mean position ~5º north

Convergence zone of winds from North & South Hemispheres

Cloud formation near equator indicating the ITCZ

The Intertropical Convergence Zone (ITCZ)

Westerlies, Trade Winds and the Doldrums

• As explained in three cell mode:• Trade Winds:

– Prevailing pattern of east to west winds in tropics

• Westerlies:– Prevailing pattern of west to east winds in mid-latitudes

East to west

West to East

West to East

Trade winds

Westerlies

Westerlies

Calm region near equator in ITCZ: Doldrums

Air masses

Large parcels of air with almost uniform

temperature; moisture content; lapse rate; stability; visibility

Sources: Stationary for at least a week – from high pressure regions

Modification: Over ocean - moisture increases; over land - dry

Cold air mass over warm region, heating from below – less stable

Warm air mass over water – more stable

Tropical Continental Polar Continental Tropical Maritime

Arctic MaritimePolar Maritime Ret. Polar Maritime

Air masses

Air Mass Characteristics

Temperature Humidity Visibility Typical weather

Tropical Maritime Warm Moist Poor/fog Low clouds, drizzle

Tropical continental (summer)

Hot Dry Moderate Clear, some thunder

Tropical continental (winter)

Average Moist Poor Clear

Polar Maritime Cold Moist Good Variable, showers

Fronts

Formed at the boundary between air masses

Wind movement causes ripples along boundary

Warm front: warm air advances into cold air region

Cold front: cold air advances into warm air region

Fronts

Jet streams

Regions of very high speed upper winds (up to 100m/s)

Polar Front Jet due to Temp difference between tropical and polar air

Subtropical Jet due to Temp gradient in upper troposphere

Jet can influence the track of weather systems

Tropopause high for

tropical air

Induces geostrophic flow (E)

Rising warm air NE

Two components in E direction add

Very high velocities occur (100m/s)

Stronger in winter when Temp.

gradients are greater

Aviation: turbulence

Pollution: mixes in atmosphere

Weather: can influence tracks of depressions

Jet streams

Example exam questions

Q1. List the forces affecting the movement of air current.

Q2. Is anti-cyclone stronger than cyclone? Why?

Q3. What is the Coriolis parameter? How does it vary with latitude?

Q4. Draw a diagram to explain the features of the global wind belts.

Next lecture – effects of water