Explorations in Meteorology 49 Lab 11 Answer Key

ANSWER KEYPart I: Locating Geographical Features

1. TheNationalWeatherService’sStormPredictionCenter(www.spc.noaa.gov)hasissuedatornadowatchonawarmspringday.Thewatchcoversa largeareabelievedtobeatriskforsevereweather, includingtornadoes.Lateintheafternoon,StormSpotterAseesastormthathasnumerousupdraftsorientedinaperfectlyuprightfashion.StormSpotterBseesastormwithasingle,rotatingupdraftthattiltsdownwind.Which, ifeither,of thestormspottersshould interprettheirstormsasan immediatethreat?WhattypeofthunderstormsdidSpotterAandSpotterBview?

Anordinarythunderstorm(sometimesreferredtoasan“airmassthunderstorm”)ischaracterizedbyoneormoreupdraftsthataretypicallyorientedinanuprightfashion.Thesupercellthunderstormischaracterizedbysingledominant,rotatingupdraftthattiltsdownwind.Hence,SpotterAislookingatanordinary,multi-cellthunderstorm,whereasSpotterBislookingatasupercell.Giventhatsupercellsarethemostprolificproducersofsevereweather,SpotterA’sstormisnotanimmediatethreat(thoughitcouldbecomeasupercelllater),whereasSpotterB’sstormis.

TypeofstormviewedbySpotterAOrdinary,multi-cellthunderstorm

TypeofstormviewedbySpotterBSupecellthunderstorm

2. Thunderstormsformafterrisingaircoolssufficientlyforwatervaportocondense,formingacloud.Basedon this requirement,what is thenameof the levelwhereyouwouldexpect tofind thebaseofa typicalthunderstorm?

LevelofbaseofthunderstormtheLiftingCondensationLevel(LCL)orConvectiveCondensationLevel(CCL)

3. Thunderstormsrequirerisingairfortheirdevelopment.Namethreesourcesofrisingmotionforthunderstormdevelopment.

Sourceofrisingair(#1)Fronts,drylines,oroutflowboundaries

Sourceofrisingair(#2)Solarheating

Sourceofrisingair(#3)Orographiclift(i.e.,terrain)

Explorations in Meteorology 50 Lab 11 Answer Key

Figure 6 – Air Temperature (in °F) and Wind Vectors for 10 November 2004at 11:00 AM CST (1700 UTC), as Measured by the National Weather Service

Figure 7 – Dewpoint Temperature (in °F) and Wind Vectors for 10 November 2004at 11:00 AM CST (1700 UTC), as Measured by the National Weather Service

4. BothacoldfrontandadrylineareprominentinFigures6and7––plotsoftemperature,dewpoint,andwinddatafrom2:00PMCST(2000UTC)on10November2004.Identifythelocationofthecoldfrontanddryline.Ontheappropriatemaps,drawthecoldfrontwithabluelineandthedrylinewithabrowndashedline.Also,identifyanycirculationsinthewindfieldandlabelthemappropriately.

Explorations in Meteorology 51 Lab 11 Answer Key

5. Thunderstormsoftenformalongsurfaceboundariessuchasdrylines,coldfronts,andwarmfronts.Generally,oneboundaryalonecanbefavorableforstormdevelopment,andthe intersectionofmultipleboundariesisevenmore favorable.Basedon this statementandyour results fromquestion4, identifyand labelonFigure6or7thelocationwhereyoubelievestormswillforminitially.Inaddition,identifyandlabelthelargerregionwhereyoubelievestormsmayoccurwithinthenexthour.Brieflyexplainyourchoiceof locations.(Note:Thunderstormsformedwithin30minutesofthetimethedatausedinFigures6and7werecollected.Donotworryaboutthemovementoftheboundarieswithtime.)

Thunderstormswill form initiallywherethere isstrongsurfaceconvergence,sufficientmoisture (TD>55ºF),andwarmtemperatures.Thunderstormswilldevelopinregionswithsufficientmoisture,warmair,andmoderateconvergence.

6. Atyourlocalsurfaceobservationstation,athunderstormwithfrequentlightningapproachesfromthewest.Atthesametime,thestationreportseasterlysurfacewinds.Thisthunderstormlikelyisinwhatstageofitslifecycle?Justifyyouranswer.

Ifthestormismovingfromthewesttowardtheobserver,themainupdraftistothewestoftheobserver.Itispossiblethattheupdraftisimmediatelyoverhead.Eitherway,easterlywindsimplythatairismovingintothestormbeforebeingliftedintotheupdraft.Thus,thestormcannotbedissipating.Becausefrequentlightningisaccompanyingthestorm,itmustbewellestablished.Thethunderstormislikelyinitsmaturestage.

7. One of the prominent features of the typical thunderstorm is the outflow at the surface created by thethunderstorm’sdowndraft.Basedonthetemperatureandwinddatafrom6:30PMCDTon17August1994(Figure8),circletheregionwhereyouexpectthunderstormactivitytobelocated.Explainhowyoumadeyourdecision.Also,howdoesyourselectedlocationcomparetotheradarreflectivitydata(Figure9)fromKTLX(theNEXRADradarnearOklahomaCity)at6:30PMCDT?

Thunderstormactivityislocatedinsouth-centralOklahoma,asidentifiedbyacoldpocketofairthatexistsacrossthatlocation.ThunderstormactivityshouldbelocatednorthandeastofLawtonbasedonthepuredivergentsignatureintheMesonetwindfield.ComparingthistobasereflectivitydatafromtheKTLXradar,stormsareinsouth-centralOklahoma.Theyseemtobedisplacedfromthecenterofthetemperatureminimum.Thisspatialseparationresultsfromthefactthatthestormshavemovedsouthward,andthusareontheleadingedgeofthetemperatureminimum.

Figure 8 – Air Temperature (in °F) for 17 August 1994 at 6:30 PM CDT (2330 UTC),as Measured by the Oklahoma Mesonet

Explorations in Meteorology 52 Lab 11 Answer Key

8. Using the information provided in Figures 8 and 9, what stage in their life cycle are the thunderstormssouthwestofKTLX?Explainyourreasoning.

Theyappeartobeinthedecayingstagebecause(1)cool,outflowairisapparentontheMesonettemperaturemapand(2)thestormsdonotexhibithighreflectivity(i.e.,50dBZandlower).

9. Useonesentencetostatetheprimaryimpactofthunderstormsontheverticaldistributionoftemperatureintheatmosphere.(Hint:Howdoestheverticaltemperatureprofilewithinathunderstormcomparetotheverticaltemperatureprofileoftheambientenvironment?)

Thethunderstormsacttostabilizetheatmospherebecausetheverticaltemperatureprofileoftheatmospheresurroundingthestormsisconditionallyunstable,whereasinsidethestormsthelapserateshavebecomemoistadiabatic.

10. Violenttornadoesaresignificantlymorelikelytodevelopfromsupercellthunderstormsasopposedtoothertypesofthunderstorms.Explainwhyyouthinkthisisthecase.

Tornadoesaregenerated,inpart,byrotationinathunderstorm.Thestrongerandlongerlastingthestormrotation,thehigherlikelihoodthatatornadowillbeviolent.Supercellthunderstormsalwaysrotateand,inmanycases,theyexhibitstrongrotationoverseveralhours.Thus,violent tornadoesaremorecommon insupercell thunderstormsthanotherthunderstorms.

11. Many people assert that the reason that the Southern Great Plains has so many tornadoes during thespringtimeisbecausecoolairfromthenorthmeetswarmairfromthesouth.Explainwhythisstatementisincorrect,usingtheknowledgethattornadoesarerareinCalifornia,wherecoolairfromthePacificOceanmeetswarmairfromfartherinland(e.g.,DeathValley).

Conditionsthatarefavorableforthedevelopmentofseverethunderstormsincludestronginstabilityresultingfromthepresenceofabundantsurfacemoistureandsteeplapserates.Theseconditionsoccurwhenwarm,moistairisnearthesurfacewithcold,dryairaloft.ThisscenariooccursfrequentlyintheSouthernGreatPlainsbecauseoftheinfluenceoftropicalmoisturefromtheGulfofMexico(i.e.,windsadvectmoisturenorthward)andcold,dryairfromtheRockies(i.e.,upper-levelwesterlies).InCalifornia,ontheotherhand,surfaceairfromthePacificOceanismodifiedbythecool,Californiacurrentthatisoffshore.Thus,anoceanicbreezeprovidescoolmoistair(i.e.,alowermoisturecontentbecauseof its temperatureandsaturationvaporpressure).Abreeze from farther inland (e.g.,DeathValley) isalmostalwaysdry.Thus,thestronginstabilitynecessarytoproduceseverethunderstormsrarelyoccursinCalifornia.Asaresult,fewtornadoesoccurinthisregion.

12. (Advanced Students/Meteorology Majors) In a shortparagraph, explainwhyaSouthernPlainsdrylinetendstomoveeastwardduringthedaybutwestwardduringthenight.

Withoutsignificantsynopticforcing,theSouthernPlainsdrylinetendstomoveeastwardduringthedayandwestwardduringthenight.Theterraininthisregionslopesupwardtowardthewestandseparateswarm,dryairthatmovesofftheelevatedterrainofthesouthernRockiesandMexicanPlateaufromGulfmoisturethatmovesinlandaroundthewesternsideoftheBermudaHighduringthespring.Duringtheday,solarradiationbeginstheprocessofconvectivemixingwiththedrierairaloft–especiallywestofthedrylinewhereskiesoftenareclear.Themixingprocessresultsintheerosionoftheshallowmoistlayerbydryair.Theeffectistogivetheimpressionthatthedrylineismovingeastwardwhen,inreality,itis‘developingeastward’becauseofaslowerosionofthemoistlayer.Aftersunset,withtheendofconvectivemixinganddryairentrainment,thedrylinestalls.Becausesurfacewindstendtobecomemoresoutheasterlyduringthelateafternoonintotheeveninghours,themoistlayerreturnsanddeepenswestward,givingtheimpressionthatthedrylineisretreating.

Explorations in Meteorology 53 Lab 11 Answer Key

13. (Advanced Students/Meteorology Majors) Describe (or sketch) a vertical profile of temperature andmoisturethatwouldprovideanexcellentenvironmentforaheatbursttooccur.

Anexcellentenvironmentincludeswarm,moistairatthelowlevelstosupportconvection,weakwindshearthroughoutthe troposphere (so that stormshave time todevelop raindropsaloft), anda significant layerofdry air in themid-levels.Thelayerofdryairisimportantfortheraindropstocompletelyevaporateandforcompressionalwarmingoftheevaporatedairtooccurbeforethedowndraftreachestheground.

14. (Advanced Students/Meteorology Majors)Thunderstormsthatforminawarm,moist,tropicalenvironmentoftenarecharacterizedbytorrentialrainfallandaminimumoflightning.Explainwhyyouthinkthismaybethecase.

Torrentialrainfallismadepossiblebythefactthatamaritimetropicalatmosphereislikelyverywarmandverymoist.Lightningrequiresthebuildupoflargedifferencesinelectricalchargeinacloud.Typically,theselargechargedifferencesrequirethepresenceoficecrystalsinthecloud.Inthetropics,however,showersandthunderstormsformwithfewornoicecrystalsbecauseoftheverywarmnatureoftheatmosphere.

15. (Advanced Students/Meteorology Majors)Drylinesoftenserveasafocusforconvection––thatis,stormstendtoformalongadrylinewhenoneispresent.Explainwhydrylineshavethischaracteristic.

Awindshiftacrossadrylineactstoincreaseconvergenceandverticalmotion.Inturn,theformationofstormsbecomesmorelikely.

16. (Advanced Students/Meteorology Majors)Severalstations intheOklahomaMesonetmeasureda largeheatburston23May1996.Usethetemperatureanddewpointmapsfrom1:30AMCDT(Figures10and11)todeterminewherewouldyouexpectaheatbursttobelocated.Outlineandlabeltheareayouselected.Foraheatbursttooccurshortlyaftermidnight,whatphysicalprocesslikelywasresponsiblefortheunusuallyhightemperatures?

TheheatburstislocatedsouthwestofOklahomaCity(nearChickasha,OK).Giventhetimeofday,solarheatingiszero.Baseduponamapofdewpointtemperature,thelikelyculpritfortheabnormallyhightemperatureswouldbewarming(anddrying)resultingfromtheadiabaticcompressionofadowndraftfromacollapsedthunderstorm.

Figure 10 – Air Temperature (in °F) for 23 May 1996 at 1:30 AM CDT (0630 UTC),as Measured by the Oklahoma Mesonet