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ThermallyDrivenFlows

Atmos 6250:MountainMeteorologyJimSteenburgh

DepartmentofAtmosphericSciencesUniversityofUtah

jim.steenburgh@utah.eduPhoto:greatmontanabeertrek.com

Reading

Zardi,D.,andC.D.Whiteman,2013:Diurnalmountainwindsystems.MountainWeatherResearchandForecasting:RecentProgressandCurrentChallenges.T.Chow,S.deWekker,andB.

Snyder,Eds.,Springer,35–119.

Discussion

• Whydoterrainheterogeneities(mountains,valleys,slopes)leadtocirculationsinthepresenceofuniformsurfaceheatingorcooling?

• Canyouprovidesomeexamplesofthermallydrivenmountain,valley,orslopecirculationsinUtah?

TypesofMountainWinds

• Dynamicallydrivenflows producedbytheinteractionofthelarge-scaleflowwithtopography

• Thermallydrivenflows (a.k.a.diurnalmountainwinds)producedbyhorizontalcontrastsinheatingandcoolingthatarisefromtopographicandland-surfacecontrasts

• Frequentlycombinedtosomeextent

TypesofMountainWinds

• Dynamicallydrivenflows producedbytheinteractionofthelarge-scaleflowwithtopography

• Thermallydrivenflows (a.k.a.diurnalmountainwinds)producedbyhorizontalcontrastsinheatingandcoolingthatarisefromtopographicandland-surfacecontrasts

• Frequentlycombinedtosomeextent

ThisLecture

ThermallyDrivenMountainWinds• Slopeflows(upslopeanddownslope)

– Producedbyhorizontaltemperaturecontrastsbetweenairoveravalleyandairoveritssidewalls

• Valleyflows(up-valleyanddown-valley)– Producedbyhorizontaltemperaturecontrastsalongavalleyaxisorbetween

airinthevalleyandoveranadjacentplain

• Cross-valleyflows– Producedbyhorizontaltemperaturedifferencesbetweentwoopposingvalley

sidewalls

• Mountain-plainwindsystem– Producedbyhorizontaltemperaturedifferencesbetweenairoveramountain

massifandairoverthesurroundingplains

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ThermallyDrivenMountainWinds• Diurnalmountainwindsystems

– ThermallydrivenwindsystemsassociatedwiththediurnalcycleofheatingandcoolingattheEarth’ssurface

– Exhibitdiurnalreversal

• Anabaticflows– Upslope,upvalley

• Katabaticflows– Downslope,downvalley

• Anabaticandkatabaticflowsdonotnecessarilyexhibitdiurnalreversal– Katabaticmorecommonlythananabatic

SlopeandValleyFlows

Whiteman(2000)

DaytimeNighttime*Diurnalreversalcommon,butnotrequired(e.g.,glacialflows)

CrossValleyFlow

BaderandWhiteman(1989),Whiteman(2000)

Mountain-PlainWindSystem

H

L

H

L

Daytime

Nighttime

Regional-scalecirculationbetweenmountain/plateaumassiff andsurroundinglowlands

Mountain-PlainWindSystem

Reiter & Tang (1984); Whiteman (2000)

MeanJuly850mbheight(0500MST) MeanJuly850mbheight(1700MST)

ReiterandTang(1984),Whiteman(2000)

Multiscale Interactions

Whiteman(2000)

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Multiscale Interactions

Whiteman(2000)

SlopeFlows

Photo:http://oldbearnews.blogspot.com/

Discussion

• Lateintheday,flowsthatmovedirectlydowntheterraingradienttypicallydevelopalongslopes

• Whatmechanismsdrivetheseflows?

• Whydon’ttheycontinuetogetstrongerduringthenight(inmostinstances,theydon’t)

SlopeFlows

• Gravityorbuoyancyflowsthatfollowtheterraingradient

• Drivingforceisbuoyancygeneratedbythecoolingorwarmingofairadjacenttotheslope

• Bestdevelopedinclear,undisturbedweather

• Difficulttofindinpureform

SlopeFlows

Whiteman(2000)

Along-SlopeForceBalance

S

n

u =velocityparalleltoslope(alongs),positivedownslope

ρ0 =referencedensitywithoutcooling

Pa =Ambientpressure

θ’=perturbationθ nearslope

θ0 =referenceθ

α=slopeangle

u’w’=turbulentmomentumflux

DuDt

= −gθ 'θ0sinα − 1

ρ0

∂(p− pa )∂s

−∂(u 'w ')∂n

HorstandDoran(1986),Whiteman(2000)

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Along-SlopeForceBalance

S

n

DuDt

−1ρ0

∂(p− pa )∂s

−gθ 'θ0sinα

−∂(u 'w ')∂n

DuDt

= −gθ 'θ0sinα − 1

ρ0

∂(p− pa )∂s

−∂(u 'w ')∂n

Along-slopeacceleration

Buoyancycomponentalongslope

Along-slopepressuredifferencegradient(differencerelativetoambient)

Turbulentmomentumflux(a.k.a.,friction)

HorstandDoran(1986),Whiteman(2000)

SlopeFlows

U

B

PGFF Assumingsteadystate,buoyancybalancedby:

1.Friction/entrainment2.Pressuregradient(relativetoambient)producedbychangesindepthofcoldpoolalongslope

DuDt

= −gθ 'θ0sinα − 1

ρ0

∂(p− pa )∂s

−∂(u 'w ')∂n

HorstandDoran(1986),Whiteman(2000)

SlopeFlows• Typically1–5ms-1,butspeedincreaseswithslopelength

– Antarctica15–30ms-1

• Decreaseindepthwithincreasingstaticstability

• Downslopeflows– Strongestatorjustfollowingsunset

• DeepeningcoldpoolcreatesadversePGFlateratnightandlimitsbuoyancyeffect

• Convergeintogullies,basins,etc.

• Upslopeflows– Strongestmidmorning– Increaseindepthasonemovesupslope– Convergeoverhigherground(e.g.,ridges,plateaus,etc.)

SlopeFlows• Verysensitivetospatialvariationsinsurfaceradiation– Persistentcloudcover,timeofsunrise/sunset– Slopeaspect,topographicshading,etc.

Whiteman(2000)

UpslopeFlowaboveForest

Whiteman(2000)

Heatingincanopylayercanproduceupslopeflowaboveshallowdownslopeinthesnow-cooledsub-canopylayer

ValleyFlows

https://cleanairfairbanks.files.wordpress.com/2011/01/helios-fire_2011-1-16_nh_1453_duringfire.jpg

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ValleyFlows

• Drivenbystrongerheatingandcoolingofthevalleyatmospherecomparedtotheadjacentplain

• Bestdevelopedinclear,undisturbedweather

Whiteman(2000)

Discussion

• Basedonthecrosssectionspictured,whichofthevalleysbelowwouldhaveastrongerdaytimeup-valleyflow?

ValleyWinds

ValleyVolumeEffectValley-to-plain

temperaturecontrastsdevelopbecauseairvolumeinvalleyissmallerthanthatinplain

Valleydoesn’thavetoslopetoproducealong-valley

winds

Whiteman(2000)

SeasonalEffectonDiurnalReversal

Reiteretal.(1983),Whiteman(2000)

NocturnalValleyExitJet• Windstendtobehighernearmouthofavalleybecause– Windsacceleratewithdistancedownthevalley– Horizontalpressuregradientlargestnearexitwherethereisanabrupttransitionfromhighpressureinvalleytolowpressureoverplain

– Bycontinuity,windspeedincreasesascoldvalleyairsinksandfansoutoverplain

• Greatestimpactfordeepvalleysthatendabruptlyataplain

NocturnalValleyExitJet

ExitjetfromAustria’sInnValley

Pamperin andStilke (1985),Whiteman(2000)

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ValleyWindVariability

• Along-valleytemperatureandwindvariabilitycanbeproducedby– Contrastinshort- orlon-waveradiationlossesorgainsalongvalley

– Rateofconversionofnetradiationtosensibleheat• Concerns:Heterogeneoussnowcover,soilmoisture,vegetation

Whiteman(2000)

ValleyShape

• Along-valleytemperatureandwindvariabilitycanbeproducedbychangesinvalleyvolume– Heatingofairinvalleyisproportionaltovalleyvolume(Whiteman1990)

– AreaswithconvexvalleysidewallsheatmorequicklythanU-shapedareas

Whiteman(2000)

Discussion

ThisphotoofupperLittleCottonwoodCanyonwastakenshortlyaftersunriseAssumingcalmlarge-scalewinds,whatlocalflowsdoyouexpect?

Cross-ValleyWinds

• Blowacrossvalleywhenairononesidebecomeswarmerthanothersid)• Compensatoryflowsinoppositedirectionaloft• Culprits:Unequalsidewallexposuretosun,heterogeneoussnowcover,

vegetation,soilmoisture

Whiteman(2000)

Cross-ValleyWinds

BaderandWhiteman(1989),Whiteman(2000)

TheDiurnalPBLCycle

• Nighttime(decoupled)period• Morningtransition• DaytimeCBL• Eveningtransition

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DiurnalPBLEvolutionoveraPlain

Whiteman(2000)

DiurnalPBLEvolutionoverValley

Whiteman(2000)

NighttimeDecoupledPeriod

• ValleyflowdecoupledfromresidualCBLaloft

• Downvalleyanddownslopewindsprevail

Whiteman(2000)

Discussion

• WhatisgoingonintheWasatchWeatherWeeniesbannerbelow?Describethecloudsandhypothesizewhytheylookthewaytheydo

MorningTransition• CBLbeginstodevelopoverheated

slopes

• Upslopeflowsdevelopalongvalleysidewals,thenshallowup-valleyflowalongvalleyaxis

• Topofinversionsinksduetocompensatingsubsidence

• Downvalley flowpersistsinshrinkingstable-corealoft

• StablecoreeventuallydestroyedbyCBLgrowthandup-valleyflowextendsthroughvalleydepth

• Valleymayeventuallycouplewithambientsynopticfloworplain-mountaincirculation

Whiteman(2000)

MorningTransition

Whiteman(1980)

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DaytimeCBL• Convectionoccursaboveall

slopes

• Upslopeandup-valleyflowspredominatewithinvalley

• Windstransitiontothelarge-scaleflowabovethevalley

• Windsinthevalleymaybecometurbulentandupperwindsmayoccasionallychannelintovalley

Whiteman(2000)

EveningTransition

• Beginsinlateafternoon• Downslopeflowsbeginfirstinshadedareasonsidwalls andvalley

floor,thenoverallsidewals• Valleyinversionwithdown-valleyflowgrowsindepth

Whiteman(1986)

DiurnalCycleHodograms

RightBankwindsturnclockwise

LeftBankwindsturn

counterclockwise

Hawkes(1947),Whiteman(2000)

Discussion

Howwelldothemeansummerwindsatthestationsabovematchthetheoryonthepriorslide?Explaindiscrepancies(Fullandhalfbarbdenote1and0.5ms-1,respectively)

Stewartetal.(2002)

DiurnalCycleofValleyDivergence

Convergence

Divergence

October2000

CourtesyMikeSplitt

BasinCirculationsNASA

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Discussion

Describethediurnalevolutionofwindsyouexpectinabasin.HowaretheyDifferentthaninavalley?Why?

NASA

BasinCirculations• Unlessperturbedbythelarger-scaleflow,enclosed

terrainfeaturesdevelopslopeflowsbutweakalong-basincirculations– Slopeflowstypicallyweakenasbasincoldpooldeepens

• Theabsenceofalong-basinadvectionandassociatedturbulenceresultsinalargerdiurnaltemperaturerange– Overnightminimumsareespeciallylow

• Basincoldpoolscanbediurnal(i.e.,developatnight)orpersistfordaysorweeks

Examples

Whiteman(2000)

Examples

CourtesyDaveWhiteman

PeterSinks

Clements(2001)

Utahall-timerecordlowof-57°C

NorthSink

Clements(2001)

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NorthSink

Clements(2001)

Vegetation“inversion”

Perimeter

Clements(2001)

Instrumentation

Clements(2001)

SurfaceTemperature

Clements(2001)

BigdiurnalcycleColdestairinbottomofbasin

Airtemperatureincreasesupwardalongbasinsidewalls

VerticalStructure

• Gradualcoolingofbasinairmass throughnight

• Downslopeflowearly,thenwindsbecomeweak

• Inversionattopofbasinstrengthens

Clements(2001)

Summary• Largespectrumofthermallydrivenflowsincomplexterrain

– Slopeflows– Valleyflows– Cross-valleyflows– Mountain-plaincirculations

• Typicaldiurnalevolutionunderlightforcingincludes– Nocturnalregime

• Downslopeanddown-valleyflows– Morningtransition

• Rapiddevelopmentofslopeandshallowup-valleyflows• Gradualerosionofelevateddown-valleyflowandstablecorealoft

– Afternoonregime• Upslopeandup-valley;possiblecouplingwithlarge-scaleflowaloft

– Eveningregime• Rapidonsetofdownslopethendownvalley withgradualdeepeningofcoldcore

• Basins– Weakslopeflowsinitiallyineveningthatweaken– Attainverycoldnocturnaltemperaturesduetolimitedflowsandturbulence

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ReferencesBader,D.C.,andC.D.Whiteman,1989:Numericalsimulationofcross-valleyplumedispersionduringthemorningtransitionperiod.J.Appl.Meteor.,28,652-664.

Clements,C.B.2001:ColdAirPoolEvolutionandDynamicsinaMountainBasin.UniversityofUtahM.S.Thesis,100pp.

Hawkes,H.B.,1947:Mountainandvalleywindswithspecialreferance tothediurnalmountainswindsoftheGreatSaltLakeregion.Ph.D.dissertation,OhioStateUniversity,312pp.

Horst,T.W.,andJ.C.Doran,1986:Nocturnaldrainageflowonsimpleslopes.Bound.-LayerMeteor.,34,263–286.

Pamperin,H.,andG.Stilke,1985:NocturnalboundarylayerandLLJinthepre-alpineregionneartheoutletoftheInnValley.Meteor.Rundsch.,38,145–156.

Reiter,E.R.,andM.Tang,1984:Plateaueffectsondiurnalcirculationpatterns.Mon.Wea.Rev.,112,638–651.

Stewart,J.Q.,C.D.Whiteman,W.J.Steenburgh,andX.Bian,2002:AclimatologicalstudyofthermallydrivenwindsystemsoftheUSIntermountainWest.Bull.Amer.Meteor.Soc.,83,699-708.

Whiteman,C.D.,1980.BreakupofTemperatureInversionsinColoradoMountainValleys.Ph.D.dissertation,ColoradoStateUniversity. AvailablefromUniversityMicrofilms,AnnArbor,MichiganorasAtmosphericSciencePaperNo.328fromDepartmentofAtmosphericScience,ColoradoStateUniversity,FortCollins,Colorado80523.

Whiteman,C.D.,1986:TemperatureinversionbuildupinColorado'sEaglevalley.Meteor.Atmos.Physics,35,220-226.

Whiteman,C.D.,2000:MountainMeteorology:FundamentalsandApplications.OxfordUniversityPress,355pp.

Zardi,D.,andC.D.Whiteman,2013:Diurnalmountainwindsystems.MountainWeatherResearchandForecasting:RecentProgressandCurrentChallenges.T.Chow,S.deWekker,andB.Snyder,Eds.,Springer,35–119.