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Chapter 4

Chapter 4.pdf

29 Pages
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Department
Chemistry
Course Code
CHEM 302
Professor
Michael Wheeler

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40CHAPTER 4ATMOSPHERIC TRANSPORTWesawinchapter3thatairmotionsplayakeyroleindeterminingthedistributionsofchemicalspeciesintheatmosphereThesemotionsaredeterminedbythreeprincipalforcesgravitypressuregradientandCoriolisWepreviouslysawinchapter2thattheverticaldistributionofmassintheatmosphereisdeterminedbyabalancebetweengravityandthepressuregradientforcewhentheseforcesareoutofbalancebuoyantmotionsresultwhichwillbediscussedinsection43InthehorizontaldirectionwheregravitydoesnotoperatetheequilibriumofforcesusuallyinvolvesabalancebetweenthepressuregradientforceandtheCoriolisforceandtheresultingsteadyflowiscalledthegeostrophicflowBelow1kmaltitudethehorizontalflowismodifiedbyfriction with the surfaceTheatmosphereisconsiderablythinnerinitsverticalextentscaleheight7kmthaninitshorizontalextentThelargestscalesofmotionareinthehorizontaldirectionandformthebasisforthegeneralcirculationoftheatmosphereWewillconcernourselvesfirst with these horizontal motions41GEOSTROPHIC FLOWLargescalemovementofairintheatmosphereisdrivenbyhorizontalpressuregradientsoriginatingfromdifferentialheatingoftheEarthssurfacerecallourdiscussionoftheseabreezeeffectinsection25AsairmovesfromhightolowpressureonthesurfaceoftherotatingEarthitisdeflectedbytheCoriolisforceWebeginwithanexplanationoftheCoriolisforceandthengoontoexaminethebalancebetweenthepressuregradientandCoriolisforces411Coriolis forceConsider an observer fixed in space and watching the Earth rotateFromtheperspectiveoftheobserveranobjectfixedtotheEarthatlatitudelistravelinginacircleataconstanttranslationalspeedinthe longitudinal direction2pRlcosv41EtoBostonwefindv1250wheret1dayFigure41Forl42E411kmhWeareoblivioustothisrapidmotionbecauseeverythinginourframeofreferencethesenotesyourchairistravelingatdecreaseswithincreasinglatitudeitisthesamespeedNotethatvEthis latitudinal gradient that causes the Coriolis forceRcosllR Figure 41 Spherical geometry of EarthConsidernowanobserverOfixedtotheEarthandthrowingaballatatargetTTobeginwiththesimplestcaseimaginetheobserverattheNorthPoleandthetargetatalowerlatitudeFigure42IttakesacertaintimeDtfortheballtoreachthetargetduringwhichtimethetargetwillhavemovedacertaindistanceDxasaresultofthe Earths rotation causing the ball to miss the targetball trajectoryviewed byrotating observer inOT new positionnOOnnDxballnqTT old positionball trajectory timetDtviewed by observerotimetofixed in space Figure 42 Coriolis effect for rotating observer at North PoleTherotatingobserverattheNorthPoledoesnotperceivethetargetashavingmovedbecauseeverythinginhisherframeofreferenceismovinginthesamewayHowevertheshotmissedFromtheperspectiveofthisobservertheballhasbeendeflectedtotherightofthetargetSuchadeflectionimpliesaforcetheCoriolisforce
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