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Chapter 7: Chapter 7: Atmospheric Atmospheric CirculationsCirculations
By the end of this chapter you should:By the end of this chapter you should:
Understand the scales of motionUnderstand the scales of motion
Be able to give an example of an eddyBe able to give an example of an eddy
Have an understanding of the general circulationHave an understanding of the general circulationof the atmosphereof the atmosphere
Scales of Atmospheric Scales of Atmospheric MotionsMotions
Scales of motionScales of motion – different sizes of circulation – different sizes of circulation MicroscaleMicroscale – smallest scale of motion. 2 meters or – smallest scale of motion. 2 meters or less and last seconds to minutesless and last seconds to minutes MesoscaleMesoscale – circulation around a city (20 km). Can – circulation around a city (20 km). Can last minutes to hourslast minutes to hours Synoptic scaleSynoptic scale – Circulations around highs and lows – Circulations around highs and lows (2,000 km). Can last many days(2,000 km). Can last many days
Planetary scalePlanetary scale – Circulations around the entire – Circulations around the entire Earth.Earth.
Scales of Atmospheric Scales of Atmospheric MotionsMotions
Scales of Atmospheric Scales of Atmospheric MotionsMotions
Eddies - Big and SmallEddies - Big and Small EddyEddy – a circulation formed downwind from an object – a circulation formed downwind from an object
(examples?)(examples?) RotorRotor – rotation formed downwind from a mountain wave – rotation formed downwind from a mountain wave Wind shearWind shear – –
change of windchange of wind
speed orspeed or
direction withdirection with
heightheight
Thermal CirculationsThermal Circulations Thermal circulationsThermal circulations
Circulations broughtCirculations brought
on by changes in airon by changes in air
Temperature.Temperature.
Sea and Land BreezesSea and Land Breezes Types of thermal circulationsTypes of thermal circulations
Sea breezeSea breeze (scale of motion?) (scale of motion?) Land breezeLand breeze
Sea breeze front – leadingSea breeze front – leading
edge of the sea breezeedge of the sea breeze Florida sea breezes can Florida sea breezes can have huge walls of clouds. have huge walls of clouds. Make for great gliding due Make for great gliding due to vertical movementto vertical movement
Sea and Land BreezesSea and Land Breezes
Seasonally Changing Seasonally Changing Winds - the MonsoonWinds - the Monsoon
Monsoon wind system – “monsoon” means seasonalMonsoon wind system – “monsoon” means seasonal Asian monsoon – in winter the land is much colder than Asian monsoon – in winter the land is much colder than
ocean. In summer, opposite is trueocean. In summer, opposite is true
Seasonally Changing Seasonally Changing Winds - the MonsoonWinds - the Monsoon
Mountain and Valley Mountain and Valley BreezesBreezes
Valley breeze – explain thisValley breeze – explain this Mountain breeze – explain thisMountain breeze – explain this
Katabatic WindsKatabatic Winds Katabatic windsKatabatic winds – in general, any wind that flows – in general, any wind that flows
downhilldownhill Perfect scenario is elevated plateau with mountains Perfect scenario is elevated plateau with mountains
aroundaround BoraBora – katabatic – katabatic
wind in the Adriaticwind in the Adriatic
SeaSea
Chinook (Foehn) WindsChinook (Foehn) Winds Chinook windsChinook winds – fierce wind that flows on the eastern – fierce wind that flows on the eastern
slope of the Rockies.slope of the Rockies. Compressional heatingCompressional heating – main source of the heat – main source of the heat
from the Chinook. Air descends very rapidly and from the Chinook. Air descends very rapidly and warmswarms
Chinook wall cloud – a bank of clouds forming over the Chinook wall cloud – a bank of clouds forming over the mountainsmountains
Santa Ana WindsSanta Ana Winds Santa Ana windSanta Ana wind
Warm, dry wind thatWarm, dry wind that
originates from theoriginates from the
Desert. Usually needsDesert. Usually needs
a high in the Greata high in the Great
BasinBasin Compressional heatingCompressional heating Fans huge wildfiresFans huge wildfires
Desert WindsDesert Winds Dust stormsDust storms – formed by surface winds that form from – formed by surface winds that form from
surface heating. surface heating. Wind picks up dustWind picks up dust Dust devilsDust devils – Formed from combination of surface – Formed from combination of surface
heating, unstable atmosphere, and an obstacle in the heating, unstable atmosphere, and an obstacle in the wayway
General Circulation of General Circulation of the Atmospherethe Atmosphere
General circulation only represents the average General circulation only represents the average circulationcirculation Why are the prevailing winds in Hawaii northeasterly Why are the prevailing winds in Hawaii northeasterly
and in san Jose northwesterly?and in san Jose northwesterly?
Cause: Cause: unequal heating of the earth’s surfaceunequal heating of the earth’s surface
Effect: Effect: atmospheric heat transportatmospheric heat transport
Single-cell ModelSingle-cell Model Basic assumptionsBasic assumptions
Covered with waterCovered with water Sun is always over equatorSun is always over equator Earth does not rotateEarth does not rotate
Hadley cellHadley cell
Heat at the equatorHeat at the equatorcreates a low thatcreates a low thatbrings heat to thebrings heat to thepoles aloftpoles aloft
Single-cell ModelSingle-cell Model Why is the single-cell model wrong?Why is the single-cell model wrong?
Earth has the Coriolis force. In this model, all winds Earth has the Coriolis force. In this model, all winds would be easterly. We know that is wrongwould be easterly. We know that is wrong
So let’s spinSo let’s spin
the planet.the planet.
Three-cell ModelThree-cell Model Model for a rotating earth - keep first two assumptionsModel for a rotating earth - keep first two assumptions Hadley cell is still apparentHadley cell is still apparent DoldrumsDoldrums – equatorial region where winds are – equatorial region where winds are light. Warm air rises, condenses and moves light. Warm air rises, condenses and moves laterally towards the poleslaterally towards the poles Subtropical highsSubtropical highs – convergence of air aloft – convergence of air aloft creates highs at 30 degrees. Major deserts and creates highs at 30 degrees. Major deserts and horse latitudes.horse latitudes. Trade windsTrade winds – some of the surface air from the – some of the surface air from the Subtropical highs move back to the equator and Subtropical highs move back to the equator and deflect due to the Coriolis force.deflect due to the Coriolis force. Intertropical convergence zoneIntertropical convergence zone – converge of the – converge of the trade winds from both hemisphere to get rising air trade winds from both hemisphere to get rising air and thunderstormsand thunderstorms
Three-cell ModelThree-cell Model WesterliesWesterlies – some of the air from the Subtropical highs – some of the air from the Subtropical highs
move towards the poles and deflect towards the eastmove towards the poles and deflect towards the east
Polar frontPolar front – the boundary between the mild air towards – the boundary between the mild air towards the equator and the cold air near the pole (Subpolar lows)the equator and the cold air near the pole (Subpolar lows)
Polar easterliesPolar easterlies – air behind the polar front that is – air behind the polar front that is deflected to the west by the Coriolis forcedeflected to the west by the Coriolis force
Fig. 7-21, p. 185
Average Surface Winds Average Surface Winds and Pressure: The Real and Pressure: The Real
WorldWorld Semipermanent highs and lowsSemipermanent highs and lows – two areas of highs and – two areas of highs and
lows per hemispherelows per hemisphere Bermuda high & Pacific highBermuda high & Pacific high Icelandic low & Aleutian lowIcelandic low & Aleutian low Siberian highSiberian high
Fig. 7-22a, p. 188
Fig. 7-22b, p. 189
The General Circulation The General Circulation and Precipitation and Precipitation
PatternsPatterns Where would you expect regions of high and low Where would you expect regions of high and low
precipitation?precipitation?
Heavy precipitationHeavy precipitation Regions close to ITCZ, Regions close to ITCZ,
polar fronts, subpolar polar fronts, subpolar lowslows
Westerly Winds and the Westerly Winds and the Jet StreamJet Stream
Jet streamsJet streams – regions of very fast-moving air in the upper – regions of very fast-moving air in the upper atmosphere due to high pressure at the equator and low atmosphere due to high pressure at the equator and low pressure near the polespressure near the poles
Winds and UpwellingWinds and Upwelling UpwellingUpwelling – rising of cold water from below – rising of cold water from below Wind flow parallel to the coastline is deflected to the right Wind flow parallel to the coastline is deflected to the right
by what force? Cold water replaces thisby what force? Cold water replaces this
El Niño and the Southern El Niño and the Southern OscillationOscillation
El NiñoEl Niño – a natural event that occurs around Christmas. – a natural event that occurs around Christmas. Upwelling weakens along the west coast of South Upwelling weakens along the west coast of South America. Major events can cause great hardshipAmerica. Major events can cause great hardship
Southern OscillationSouthern Oscillation – seasaw pattern of high and low – seasaw pattern of high and low pressures switching on opposite sides of the Pacific pressures switching on opposite sides of the Pacific OceanOcean
La NiñaLa Niña – cold water episodes opposite El Niño – cold water episodes opposite El Niño
El Niño and the Southern El Niño and the Southern OscillationOscillation
Other Atmosphere-Ocean Other Atmosphere-Ocean InteractionsInteractions
North Atlantic OscillationNorth Atlantic Oscillation Arctic OscillationArctic Oscillation Pacific Decadal OscillationPacific Decadal Oscillation