general circulation of atmosphere

7/30/2019 General Circulation of atmosphere

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PHYC 40050 Environmental Physics

Climate, Climate ChangeNuclear Power and the

Alternatives


PHYC 40050

Peter Lynch

Meteorology & Climate Centre

School of Mathematical Sciences

University College Dublin

Climate, Climate ChangeNuclear Power and the

Alternatives


The General CirculationThe General Circulation

Lecture 4Lecture 4

Atmospheric Forces and WindAtmospheric Forces and Wind


FORCES THAT MOVE THE AIRFORCES THAT MOVE THE AIR

Gravitational force:Gravitational force:

Is directed downward , normal to theIs directed downward , normal to the

earthearth

Equal to the mass of the air times theEqual to the mass of the air times the

gravitational acceleration: mggravitational acceleration: mg

Pressure gradient forcePressure gradient force


PressurePressure

gradientgradient

forceforce


PRESSURE GRADIENT FORCEPRESSURE GRADIENT FORCE

Is a function of theIs a function of the pressure differencepressure difference and airand airdensity:density:

F = (1/F = (1/)) p/p/nn

wherewhere

F = Pressure gradient force per unit massF = Pressure gradient force per unit mass

= Density of air= Density of air

p = Pressurep = Pressure

n = Distance normal to isobarsn = Distance normal to isobars

Newtons law states:Newtons law states:

force=mass times accelerationforce=mass times acceleration

F is force per unit mass, equivalent to acceleration.F is force per unit mass, equivalent to acceleration.


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APPARENT FORCESAPPARENT FORCES

IN THE ATMOSPHEREIN THE ATMOSPHERE

CoriolisCoriolis ForceForce

Centrifugal Force/CentripetalCentrifugal Force/Centripetalaccelerationacceleration


CORIOLISEFFECT


CORIOLIS EFFECTCORIOLIS EFFECT

Weather map shows that winds goWeather map shows that winds go

around a low pressurearound a low pressure area, not acrossarea, not across

the isobars.the isobars.

This deviation is a result of theThis deviation is a result of the Earth'sEarth's

rotationrotation and has been named theand has been named the

corioliscoriolis effect (effect (corioliscoriolis force)force)

This force always acts at right angles toThis force always acts at right angles to

the direction of motion.the direction of motion.


CENTRIFUGAL FORCE


THE MECHANISM OF GEOSTROPHIC FLOW


GEOSTROPHIC FLOWGEOSTROPHIC FLOW

A balance is reached between theA balance is reached between thecorioliscoriolis force and the pressureforce and the pressuregradient force.gradient force.

If there is no friction, this occursIf there is no friction, this occurswhen the parcel of air is parallel towhen the parcel of air is parallel tothe isobars.the isobars.

At this point there is no net force onAt this point there is no net force onthe air parcelthe air parcel -- no accelerationno acceleration -- ititnow moves with constant velocity.now moves with constant velocity.


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GEOSTROPHIC FLOW GEOSTROPHIC FLOW

Under these idealized conditions theUnder these idealized conditions the

airflow is said to be inairflow is said to be in geostrophicgeostrophicbalance.balance.

Wind generated is called theWind generated is called thegeostrophicgeostrophic wind.wind.

Normally only applies to winds aloft.Normally only applies to winds aloft.

Surface winds are subject to frictionSurface winds are subject to friction..


Fig. 6-15, p. 172


GRADIENT WINDSGRADIENT WINDS

Winds around centers of high or lowWinds around centers of high or lowpressure follow curved paths in order topressure follow curved paths in order tostay parallel with the isobars.stay parallel with the isobars.

These winds are calledThese winds are called gradient windsgradient winds..

Low pressure centers are calledLow pressure centers are calledcyclonescyclones -- rotation is counterrotation is counter--clockwiseclockwise

the same as the earth.the same as the earth.

Centers of high pressure are calledCenters of high pressure are calledantianti--cyclonescyclones..

In southern hemisphere the directions ofIn southern hemisphere the directions offlow are reversed.flow are reversed.


PHYC 40050 Environmental Physics PHYC 40050 Environmental PhysicsFig. 6-24,

p. 181


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SURFACE WINDSSURFACE WINDS

Friction affects winds only close to theFriction affects winds only close to the

earth's surface.earth's surface.

Here we must balance three forcesHere we must balance three forces --corioliscoriolis, pressure gradient and friction., pressure gradient and friction.

Net effect is to induce aNet effect is to induce a net inflownet inflow around aaround acyclone, an effect known ascyclone, an effect known as convergenceconvergence..

Around an anticyclone we get aAround an anticyclone we get a net outflownet outflow,,divergencedivergence..


The General Circulation


HADLEY CELLHADLEY CELL


GLOBAL CIRCULATIONGLOBAL CIRCULATION

George Hadley first suggested in 1735George Hadley first suggested in 1735

the general concept of atmosphericthe general concept of atmospheric

circulationcirculation a single cell, to explain thea single cell, to explain the

existence of the easterly winds at theexistence of the easterly winds at the

surfacesurface

Cold air at poleCold air at pole -- high pressure athigh pressure at

surface. Warm air at equatorsurface. Warm air at equator -- lowlow

pressure at surface. Pressure gradientpressure at surface. Pressure gradient

force at surface will move air from poleforce at surface will move air from pole

to equator at surface. Return path atto equator at surface. Return path at

high altitudes.high altitudes.

CoriolisCoriolis force produces easterliesforce produces easterlies


Fig. 7.10

Composite (clouds, surface temperature (colors))

image. Note the line of clouds along the ITCZ


Threecell

model


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But in reality we have three cells, withBut in reality we have three cells, with

boundaries at about 30boundaries at about 30 and 60and 60 latitudelatitude..

This results in sinking air at 30This results in sinking air at 30N and 30N and 30S.S.But sinking suppresses cloud developmentBut sinking suppresses cloud developmentand precipitation. Hence most of the worldsand precipitation. Hence most of the worldsdeserts occur along these latitudes.deserts occur along these latitudes.

These are the horse latitudes.These are the horse latitudes.

Circulation at the surface moves from 30Circulation at the surface moves from 30

to the equatorto the equator easterly winds.easterly winds.



Between 30Between 30 and 60and 60 the circulationthe circulation

at the surface is from 30at the surface is from 30 to 60to 60,,

giving rise to westerly winds.giving rise to westerly winds.

Between 60Between 60 and the Poles, the flowand the Poles, the flow

is from the Pole to 60is from the Pole to 60, leading to, leading to

easterly winds at the surface.easterly winds at the surface.

Upwelling at the equator and at 60Upwelling at the equator and at 60

precipitation.precipitation.

The Doldrums.The Doldrums.

PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics

PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics


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JET STREAMSJET STREAMS

Conservation of Angular Momentum:Conservation of Angular Momentum:

As the parcel of air moves from the equator to 30As the parcel of air moves from the equator to 30 atathigh altitudes, its velocity increases to conservehigh altitudes, its velocity increases to conserveangular momentum.angular momentum.

At the same time theAt the same time the corioliscoriolis force acts to produce aforce acts to produce astrong westerly windstrong westerly wind the subthe sub--tropical jet streamtropical jet stream

A similar jet forms at 60A similar jet forms at 60 the polar jet, or polarthe polar jet, or polarfront jetfront jet


Cloud band from Pacific to Florida follows Sub-tropical jet


SUBTROPICAL AND POLAR JET STREAMS INRELATION TO THE THREE CELLS


WESTERLIESWESTERLIES In the upper troposphere there is highIn the upper troposphere there is high

pressure over the equator, and low pressurepressure over the equator, and low pressure

over the poles.over the poles.

This produces a net flow from the equator toThis produces a net flow from the equator to

the poles.the poles.

This flow plus theThis flow plus the corioliscoriolis force producesforce produces

westerlieswesterlies..

Winds areWinds are geostrophicgeostrophic Pressure gradient increases with altitude.Pressure gradient increases with altitude.

Thus so does the wind speedThus so does the wind speed

Jet streams are part of theJet streams are part of the westerlieswesterlies


DISHDISH--PAN EXPERIMENTPAN EXPERIMENT

PHYC 40050 Environmental PhysicsFig. 7-14, p. 199


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500 MB WINDS

PHYC 40050 Environmental PhysicsFig. 7-16, p. 200

(A) Zonal flow pattern air flows nearly parallel tolatitudes.

(B) Meridional flow pattern.

(C) Combination of the two flows.


WAVES IN THE WESTERLIESWAVES IN THE WESTERLIES

Dish pan experiment.Dish pan experiment.

C. G.C. G. RossbyRossby..

Waves along the jet streams are knownWaves along the jet streams are knownasas RossbyRossby waves.waves.

Three to six of them around the globe.Three to six of them around the globe.

The air flow along the edge of theThe air flow along the edge of thewaves can be rapid, however the waveswaves can be rapid, however the waves

move slowlymove slowly about 15about 15 per day.per day. Higher jet stream speeds in the winter.Higher jet stream speeds in the winter.

Jets shifts south in the winter, north inJets shifts south in the winter, north inthe summer.the summer.


WESTERLIES AND THE HEAT BUDGETWESTERLIES AND THE HEAT BUDGET

Major function of atmospheric dynamics isMajor function of atmospheric dynamics isto move heat from the equator to the poles.to move heat from the equator to the poles.

But how can winds move heat when theBut how can winds move heat when thepredominate wind direction is zonal (E to W,predominate wind direction is zonal (E to W,or W to E)?or W to E)?

The meanderings of the jet streamsThe meanderings of the jet streamscontinually mix cold and warm air, thuscontinually mix cold and warm air, thustransporting heat.transporting heat.


POLEWARD TRANSPORT OF HEAT BY THE

OCEANS AND ATMOSPHERE


Mean position of the ITCZ

in January and July


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general circulation of atmosphere

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