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UW ATMS 211 - Lecture Notes

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Lecture 15This week (Week 5):Continue describing another aspect of the“climate of the present”, namely, atmospheric motions.Today: Intro to concepts; Hadley circulation.Next week:Start “climate of the past”.Causes of air motionCauses of air motion 1) Vertical motion-Positive buoyancy (warm air rises)-Negative buoyancy (cold air sinks)2) Horizontal motion- Pressure gradient force (air tends to move from high to low pressure)- Friction, which slows down air movement- Effect of Earth’s rotation (“Coriolis effect”)Air circulation terminology- convection/subsidence (vertical)- convergence/divergence (horiz.)- conservation of matterUltimate cause is the solar energy distributionLayout of planet Earthname latitude rangeTropics 0 to 30ºExtratropics 30 to 90º Subtropics ~30º Midlatitudes 30-60º Polar Regions 60-90º portion of Earth surface 50% 50% 37% 13%Ocean 70%Land 30%Note:Most of Earth's surface: Tropics and/or Ocean. (These are themajor components that a climate model needs to get right.)Unequal distribution of solar energy with latitude: Fig 4-1Recall: Flux is energy per unit surface area: (normal to the beam): W/m2Net energy as a function of latitude: Fig 4-2Satellite measurement of net energy: EIN - EOUTdata source: Earth Radiation Budget ExperimentEarth as a Heat EngineWith no atmosphere or ocean currents, low latitudes would continue to warmand high latitudes would continue to cool.Result is "Circulation"(warm currents poleward, cool currents equatorward)Atmosphere and ocean currents remove heat from Tropics and transport it tohigh latitudes. (Also from warm to cool regions on smaller scales - e.g.land/sea breezes.)heat flow inatmos. & oceansPoleEq.energy INenergy OUTThese currents cannot flow in one direction only - air and water would "pileup".Heat transport from low to high latitude: Fig 05_16Mechanisms: (i) circulation of the troposphere(ii) surface ocean currentsNote: These two are intimately connected.Heat transport from low to high latitude: Fig 5-16Questions:1. What transports most heat in the tropics (ocean or atmosphere)?2. What transfers most heat in the midlatitudes?Tropics Midlatitudes Polar regions1. Oceans transportmore heat than theatmosphere in tropics2. Atmospheretransports mostof heat inmidlatitudesTropical Circulations - Move heat from source regions to sink regions - Have enormous consequences for regional/seasonal weather - Three big ones... Hadley circulation• encompasses entire Tropics• moves heat from low latitudes (near Equator) to higher latitudes (near 30º) Monsoons• move heat between land and ocean• regional/seasonal Walker circulation• regional (but huge region)• moves heat from warm Western Pacific to cooler Eastern Pacific• strengthening and weaking of this is El Nino Southern OscillationGeorge Hadley (1685-1768)London lawyer and amateur scientist. Proposed the theory ofplanetary-scale circulation cells in a 1735 paper,"Concerning the Cause of the General Trade Winds"Trade winds (or “trades”) are easterly winds (i.e. blowingfrom the east) that occur most of the year in the tropicsGenerally trade winds blow: - from northeast in the northern hemisphere- from southeast in southern hemisphereWinds within 30° of the equator were mapped in 1686by Edmund Halley (of Halley’s comet fame).But it took George Hadley to explain them.Hadley Circulation - 1buoyancy rising and falling(think of rubber ball in water vs rock in water)density mass per unit volumeless dense fluid risesmore dense fluid sinksgas law (see p. 57)warm air is less denseEq.Hadley Circulation - 2pressure gradient force"gradient" refers to high and low pressure regionscause of horizontal air motionsinduces air to flow from high pressure to low pressureactual air motion is modified by:frictionEarth rotation (Coriolis force)Eq.LHadley Circulation - 3stratospherevery stable regionvertical motion is inhibitedacts as a lidEq.LstratosphereHadley Circulation - 4conservation of matter ... >>>>> CIRCULATIONEq.Lstratosphere30ºN30ºSThe Hadley CirculationHadley Circulation - Fig 4-3Horizontal motionsconvergence: coming togetherdivergence: spreading apartVertical motionsconvection: rising airsubsidence: sinking airconvectionIR satellite image ITCZ: Fig 04_07Hadley Circulation - convectionEq.Lstratosphere30ºN30ºSConvection• evaporation at surface• phase change (liquid to gas)• requires tremendous energy• energy carried up as latent heatspecific heat of water: 4.2 J/g/ºC "It takes 4.2 Joules of energy to heat 1 gram of water by 1ºC." latent heat of water: 2500 J/g"It takes 2500 Joules of energy to evaporate 1 gram of water."• rising air expands and cools• this causes water to condense when RH=100% (saturation)• clouds form• latent heat is released, causing the cloudy air to warm• becomes less dense and more buoyant• rises even faster >> towering cumulonimbus (thunderstorms)Convection and the energy budgetHuge amounts of energy (and moisture) are transported into the atmosphereby convection.Hadley Circulation - subsidence & desertsEq.Lstratosphere30ºN30ºSSubsidence & Subtropical ‘High’• sinking air compresses and warms• this suppresses cloud formation• absence of rain - desertsAtacama desertin northern Chile, one of the driestplaces on Earth.Hadley Circulation - convergenceEq.Lstratosphere30ºN30ºSLow-level Convergence• air forced upwards: "ITCZ" (Inter-Tropical Convergence Zone)• horizontal motion modified by- friction - Earth's rotation (Coriolis Force)• Coriolis: wind (or ocean current) veers right in N Hemi.non-rotating


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UW ATMS 211 - Lecture Notes

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