Hadley Circulation In Action MET 200 Lecture 12 Global .

Hadley Circulation in ActionMET 200 Lecture 12 !Global Winds:The General Circulation ofthe Atmosphere1Scales of MotionPrevious Lecture Local Winds 2What balance of forcesoperates at these scales?Scales of MotionEddiesSea BreezeMountain-ValleyCirculationsChinook - SnowEaterDrainage Wind Katabatic WindPhenomena with large length scales occur over longtime scales and vice versa.34

Hawaii has Combined Sea Breeze andMountain - Valley CirculationsKona Sea-breeze FrontIn Hawaii, the sea-breeze and mountain-valley circulationsare combined to produce an island scale circulation that canbe quite vigorous, especially when trade winds are light.Hawaii sea breeze has insufficient kinetic energy toovercome the large altitude of the Big Island’s volcanoes.56Chinook Downslope WindsLecture 12 ! ! Main source of heating is compression during downslope flow General Circulationof winds at thesurface and aloft Idealized 3-cellmodel of the winds ITCZ & Monsoons Subtropical & PolarJet Streams!Global Winds– Key is loss of moisture on upwind slope so downslope heating occurs athigher dry adiabatic rate Latent heat release from condensation during upwind ascentalso contributes– If condensed water is removed as precipitation on upwind slope78

Global Wind CirculationGlobal Wind CirculationThe circulations of theatmosphere and oceans areultimately driven by differentialsolar heating and the localradiation imbalance betweenincoming solar (short wavelength)radiation and outgoing terrestrial(long wavelength) radiation.Early explorers were very familiar with the global circulationand used their knowledge in planning their voyages.9Radiation Budgetat the top of the Earth’s Atmosphere10Global Surface TemperatureRed Line is incoming radiation from the sunBlue Line is outgoing radiation emitted by the earth1112

A Single Cell Convection ModelDoes the Earth Exhibit a Single Cell? Solar heating leads to formation of a convection cell in each hemisphere Energy transported from equator toward poles What would prevailing wind direction be over N. America with this flowpattern on a rotating earth?1314Idealized 3-Cell ModelHadley Circulation in ActionA schematic of theEarth’s weathermachine bringingwarm moist airnorthward andcold dry airsouthward (latentand sensibleheat).Polar CellFerrel CellHadley CellHadley CellFerrel Cell1516

Idealized 3-Cell ModelKey features of 3-Cell Model Hadley cell- driven by differential heating by the sun- air rises near equator and descends near 30 - explains deserts, trade winds, ITCZ, and subtropical jet Ferrel Cell- driven by heat transports of winter storms- air rises near 60 and descends near 30 - explains surface westerlies from 30 -60 , and polar jet Polar Cell- driven by radiational cooling- air sinks over the pole and rises near 60 - explains surface easterlies from 60 - pole- explains why polar regions are as dry as deserts Weak winds found near Equator (doldrums), 30 degrees (horselatitudes), and over poles. Boundary between cold polar air and mid-latitude warmer air is thepolar front1718General Circulation - JulyThe Real World is More Messypresence of continents, mountains, and ice fieldsalters the general circulation from the ideal 3-cell model.During winter, highs form over land; lows over oceans. Vice versaduring summer. Consistent with differences in surface temperature.The1920

General Circulation - JanuaryGeneral Circulation - JulyDuring winter, highs form over land; lows over oceans. Vice versaduring summer. Consistent with differences in surface temperature.The general circulation shifts N and S with the sun.21Surface Pressure & Wind over the North PacificBased on historical ship reports50 N22Trade WindsTrade winds are the most common winds over Hawaiianwaters, accounting for 70% of all winds in Hawaii.Winter40 These persistent winds, which blow from a NE to ENEdirection, became known as trade winds centuries agowhen trade ships carrying cargo depended on the broad beltof easterly winds encircling the globe in the subtropics forfast passage.30 20 10 0 50 NWinds blow from each of the other quadrants (SE, SW, andNW) 10% of the time.Summer40 30 20 10 0 120 E140 160 180 160 140 120 100 Wm/s2324

Surface Pressure & Wind in Summer and Winter50 NFrequency of Trade Wind DaysWinter40 Percentage of Days30 20 10 0 50 NSummer40 30 JanuaFe ry"bruary"March"April"May"June"July"AuguSept st"embeOc r"toNo ber"vemberDe"cember"20 100"90"80"70"60"50"40"30"20"10"0"10 0 120 E140 160 180 160 140 120 100 Wm/sMonthBased on ship reports25Strong Trade Wind Days (25-33 kt)26Trade Winds12"Though often refreshingly cool, strong, gusty trade windscan cause problems for Hawaii.Blowing from the NE through East direction, these strongtrades funnel through the major channels between theislands at speeds 5-20 knots faster than the speeds overthe open ocean.8"6"4"In addition, terrain enhancement of trade winds can causeeven greater acceleration to more than hurricane er of Days10"Month2728

Evolution of Trade Wind InversionHadley Cell,Cumulonimbus,and Marine Stratus2930Intertropical Convergence ZoneThe Monsoon The Monsoon is– Seasonal– Common in eastern and southern Asia– Similar to huge land/sea breeze systems40 N180 0 40 NJuly180 20 20 JulyJuly0 0 JanuaryJanuaryJanuaryJanuary20 20 40 S0 July40 SThe Intertropical Convergence Zone (ITCZ) shiftssouthward in January and northward in July. Why?3132

The MonsoonCherrapunji received 30 feet of rain in July 1861!The MonsoonDuring winter strong coolingproduces a shallow highpressure area over SiberiaSubsidence, clockwisecirculation and flow out fromthe high provide fair weatherfor southern and easternAsiaDuring summer, air over thecontinent heats and rises,drawing moist air in from theoceans. Convergence andtopography produce liftingand heavy rain.Cherrapunji received 30 feet of rain in July 1861!During summer, air over thecontinent heats and rises,drawing moist air in from theoceansConvergence andtopography produce liftingand heavy rain.3334Jet StreamsSubtropical Jet StreamFast air currents in the uppertroposphere, 1000’s of km’slong, a few hundred km wide,a few km thickTypically find two jet streams(subtropical and polar front)at tropopause in NHThe subtropical jet streamresults from the Coriolisacceleration of thepoleward branch of theconvection (Hadley) cell.Rotation speed around the Earth’s axis500Rotation speed (m/s)Equator400New York City30020010000 20 40 Latitude60 80 N NorthPoleRotation speed due to rotation of the earth3536

Pressure Patterns and Winds AloftWesterly Jet Streams Maximum ( 60 m/s) near Japan& secondary max (40 m/s) alongthe US east coast.The subtropical jet streamresults from the Coriolisacceleration of thepoleward branch of theHadley(convection) cell. Responsible for longer returnflights to Japan from NorthAmerica.How does the polar jetstream form?Wind velocity (m/s) at 300hPa in January, viewed fromthe North Pole37Heating, Pressure Patterns, and Winds38Cool the left column & Warm the right columnThe heated columnexpands500 mb level500 mbThe cooledcolumn contractsN 1000 mboriginal 500 mb level500 mbEarth’s surface1000 mb SStart with two columns of air with the sametemperature and the same distribution of massNS1000 mb391000 mb40

Air moves from high tolow pressure at the surfaceAir moves from high to low pressure in middleof column, causing surface pressure to change.Where wouldrising motion be?LowHighNoriginal 500 mb levelNS1003 mbLowHighHighLow1003 mb997 mboriginal 500 mb levelS997 mb4142Constant Pressure ChartsWhat have we just observed?Constant pressure charts are often used by meteorologists.Constant pressure charts plot variation in height on a constantpressure surface (e.g., 500 mb). Starting with a uniform atmosphere at rest, we heat thetropics and cool the poles. The differences in heating cause different rates ofexpansion in the air (warm air takes up more space). The differing rates of expansion result in horizontalpressure gradients (differences). The pressure gradients produce wind. This is a simple model of how the atmosphere turnsheating into motions. In this example a gradientbetween warm and cold airproduces a sloping 500 mbpressure surface. Pressure decreasesfaster with height in acolder (denser) air mass. Where the slope of thepressure surface issteepest the heightcontours are closesttogether.4344

The Polar Jet StreamThe Polar Jet Stream0 mb0 mb200 mb400 mb600 mb800 mbImagine the atmosphere is a ‘block’ ofair that pushes down with 1000 mb ofpressure at the bottom.200 mbThe block starts out at a uniformtemperature – the thickness of theatmosphere is the same everywhere.400 mbNow we make the block cold on thenorth side (polar night) and warm onthe south side (tropical sun).600 mbThe 1000 mb pressure surface is stillflat – there is the same amount offluid above the surface whether youare on the cold side or the warm side.But above the surface a pressuregradient appears, which drives wind.800 mbL1000 mbCOLDNWARM1000 mbNSSHYX4546The Polar Jet StreamThe Polar Jet Stream0 mb200 mb400 mb0 mbThe 1000 mb pressure surface is stillflat – there is the same amount offluid above the surface whether youare on the cold side or the warm side.But above the surface a pressuregradient appears, which gets strongeras you go up. So the wind getsstronger as you go up.600 mb800 mb800 mbLS400 mb600 mb1000 mbN200 mbThe 1000 mb pressure surface is stillflat – there is the same amount offluid above the surface whether youare on the cold side or the warm side.But above the surface a pressuregradient appears, which gets strongeras you go up. So the wind getsstronger as you go up.HL1000 mbYNXSHYX4748

Polar Front Jet StreamThe Polar Jet Stream0 mb200 mb400 mbThe 1000 mb pressure surface is stillflat – there is the same amount offluid above the surface whether youare on the cold side or the warm side.But above the surface a pressuregradient appears, which gets strongeras you go up. So the wind getsstronger as you go up.600 mb800 mbL1000 mbNSHYPolar front jet streamforms along polar frontwhere strong thermalgradient causes a strongpressure gradientStrong pressure gradientforce and Coriolis forceproduce strong west windparallel to contour lines.The jet stream is nearly ingeostrophic balance.X49Polar Jet Stream and the Thermal Wind50Deriving the Thermal Wind RelationshipIf we differentiate the geostrophic wind,(where f is the Coriolis parameter, k is the vertical unit vector, andthe subscript "p" on the gradient operator denotes gradient on aconstant pressure surface) with respect to pressure, and integratefrom pressure level from p0 to p1 , we obtain the thermal windequation:Substituting the hypsometric equation,one gets a form based on temperature,.The jet stream associated with the polar front owes it existence to thedifferential solar heating from equator to pole. Thus, the jet is strongerin winter than in summer and moves north and south with the sun.5152

Thermal Wind in Component FormThermal Wind and AdvectionCOLDCOLDV500mbV850mb850-500mb VTThermal Wind850-500mbVT Thermal WindV500mbV850mb(a)uT -R/f ( T/ y)p ln(P1/P2)WARM(b)WARMWind vectors at 500 and 850 mb and the wind shear vector orthermal wind for (a) cold advection: wind direction is backing withheight and (b) warm advection: wind direction is veering with height.vT -R/f ( T/ x)p ln(P1/P2)5354Polar Jet Stream - Shifts with the Sun5556

Waves form along JetTroughs and Ridges Contour lines areusually not straight.– Ridges (elongatedhighs) occur where airis warm.– Troughs (elongatedlows) occur where air iscold.Temperature gradients lead to pressure gradients.Height contours decrease in value toward cold air.5758Waves determine Track and Intensityof Winter StormsPressure patterns and winds aloftAt upper levelswinds blow parallelto the pressure/height contours.Troughs are cold,ridges are warm.Temperature gradient across the polar frontdetermines the strength of the polar jet matics/jetcore2.html5960