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 poles  Hadley 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 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 F Coriolis = - 2 m (ω × v r ) 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: L Pressure Gradient Force H Coriolis Force 1000 mb 1004 mb Latitude (degree)Speed (m/s) Geostrophic Wind Wind rarely purely geostrophic but approximately Ocean currents also Balance between pressure-gradient force and coriolis force View from above

Centripetal Acceleration Low Pressure gradient Coriolis force Direction of gradient wind Direction of centripetal acceleration High Pressure gradient Coriolis force Direction of gradient wind Direction of centripetal acceleration Flow around low (high) pressure system is cyclonic (anti-cyclonic) F cent = F PG – F cor (low pressure)F PG > F cor F cent = F cor – F PG (high pressure)F PG < F cor wind speed less than v g (subgeostrophic) wind speed higher than v g (supergeostrophic) for same F PG (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 above No friction in middle of troposphere Pressure Gradient Force H Coriolis 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 above Friction Near Surface Pressure Gradient Force H Coriolis 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 Trade winds 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 ContinentalPolar ContinentalTropical Maritime Arctic MaritimePolar Maritime Ret. Polar Maritime Air masses

Air Mass Characteristics TemperatureHumidityVisibilityTypical weather Tropical MaritimeWarmMoistPoor/fogLow clouds, drizzle Tropical continental (summer) HotDryModerateClear, some thunder Tropical continental (winter) AverageMoistPoorClear Polar MaritimeColdMoistGoodVariable, 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