Warm-rain-process-convection暖雨過程對流課件

HeavyrainfallandflashfloodFlashFloodsFlashfloodsarethenumberoneweather-relatedkillerinovermuchoftheglobeNearlyhalfofallflashfloodfatalitiesareautorelatedFlashFloodDefinitionIntensePrecipitationSmallfast-responsebasins“Asituationwherethecausativeprecipitationandthesubsequentrunoffareoccurringonthesamescales.”

NATOAdvancedStudyInstitute(1999)FlashfloodforecastingTheflashflood-aprototypicalhydro-meteorologicaltopicAmeteorologicalevent:rainfallAhydrologicalsettingBasinresponsetimesRunoffcharacteristicsWorkingwithhydrologists?Flashflood“guidance”isanestimateoftherainfallneededforflashfloodingtooccurFloodconsiderationsLargeriversliketheYangtzerespondtotherainfalloveraperiodofdaysFlashfloods-responsetorainfallisveryrapid:periodofperhaps10sofminutesSmallbasinsincomplexterrainarethosemostlikelytohaveflashfloodsUrbanfloodingcanhaveimportantimpacts-highrunoff.20070718JinanFlashFloodingEventJinanisthecapitalcityofShandongprovince.During12hperiodfrom14:00BST(BeijingStandardTime)18Julyto02:00BST19July2007,theaveragerainfallfrom123weatherstationsforwholeShandongprovinceis62mm.Itscapitalcity,Jinan,hasexperiencedmostsevererainfall.The12hrainfallindowntownareais140mm,mostoftherainfallwasprecipitatedduring3hperiodfrom17:20to20:20BSTon18July2007.25peoplewerekilledbytheflashfloodinginJinancity.AFFForecastingstrategyObtainflashfloodguidancefromhydrologists:theminimumrainfallperunittimeneededfortheoccurrenceofaflashfloodBeginwiththequestion:“Whatistheprobabilityofrainfallapproachingthatvaluetoday?”Precipitationin

FlashFloodsAccumulation5-50cmover1-12hoursDurationMostlylessthan3hoursIntensity

Arid/semi-aridclimates7.5-20cm/hour(avg.11.5cm/hour)Humidclimates12.5-30cm/hour(avg.19cm/hour)CourtesyMattKelschPrecipitationin

FlashFloodsEnhancedIntensityPrecipitationefficiencyTropical,maritimeconnectionDeep,above-freezinglayerLow-leveljet,rapidmoisturereplenishmentLow-levelfocus(terrainandboundary)/class/FLOAT_2001/index.htmThelowleverjetThelowleveljetThelowleveljetfromsouthwestLowleveljetfromeastandmiddleleveljetfromsouthwestBasinCharacteristics

inFlashFloodsSmall9-150km2(avg.55km2)70%mountainouswatersheds>50%alteredUrbanizationDeforestation,firescarsThin/impermeablesurfacelayer,Clay,rocks(lowinfiltration)Pavement(noinfiltration)Or,thick,saturatedsoillayersCourtesyMattKelschFlashFloodForecastingInsomecasesthehydrologyisthedrivingforce,andsignificantflashfloodscanhappenwithunimpressiverainfall.“FlashFloodforecastingrequiresmorethanaccurateforecastsofexcessiveaccumulation.”

-MattKelsch(2000)FlashFloodForecastingKnowrunoffcharacteristicsSmall,fast-responsebasinsUrbanbasinsDeforestationandfirescarareas.HailremainingonedayafterCheyenne,WYflashfloodof

1August1984FlashFloodForecastingKnowledgeofrelationshipofstormspeedanddirectiontostreamflowcanbecriticalFlashfloodforecastingWhathappenstothewateronthegroundisjustasimportant,andinsomecasesmoreimportant.Knowyourbasins!RainfallAmountBasically,theamountofrainfallthatfallsoveranareadependson:TheINTENSITYoftherainfallTheSIZEoftherainfallareaThePROPAGATIONoftherainfallareaRainfallAmount(cont.)Additionalconsiderationsdetermininghowmuchrainwillfallinclude:Howmuchmoistureisavailable?Willtheprecipitationbeconvectiveornot?Willcellstrainoverthesameareaornot?Willnewstormsformorcontinuetoformupstream?(regeneration)Willaboundaryorlocaltopographyprovidelocalenhancement?FlashFloodThreatfromWeaklyshearedCellsRainfallcontributionstotheflashfloodthreatare:therainfallrate,anditsduration.Rainfallrateisdependenton:theupwardmoistureflux,andespecially,precipitationefficiencyFlashfloodthreatfromweaklyshearedcellsUpwardmoisturefluxdependsonUpdraftstrengthMoisturecontentofairenteringupdraftPrecipitationefficiencyisthefractionoftotalmoistureingestedbytheupdraftthatfallsbacktothegroundasrainfall.Environmentsfavoringhighprecipitationefficiencyaremostlikelyassociatedwithflashfloodreports FlashfloodthreatfromweaklyshearedcellsFavorableconditionsforhighprecipitationefficiency:Highmid-levelrelativehumidity(littledryentrainment)Deepwarmcloudlayer(increasewarmcollisioncoalescencemicrophysics)Wideupdrafts(reducedryentrainment)SeedingBasicprinciplesChappell’sLaw-Part1Rainfallrate,R

EwqE=precipitationefficiencyw=ascentrateq=watervapormixingratioEffiencyschematicWhatcontrolsefficiency?EvaporationofcondensedwaterEnvironmentalhumidityWindshearExportationofcondensedwaterintoanvil,typicallyintheformoficeMultiplecellsEarlycellsmaybeinefficientLatercellsdevelopinanewenvironmentspectrumofclouddropletsizesLargerarealcoverageofMPSFlashfloodthreatfromweaklyshearedcellsWarmclouddepthincreasescollisioncoalescenceresultinginexcessiverainfallratesStormtopmaynotbehighFlashfloodthreatfromweaklyshearedcellsAsignatureofadeepwarmcloudLookforsignificantincreaseinreflectivityfromthefreezingleveldowntotheLCLRadarwillunderestimaterainfallpotentialofthesestormsifbeamrisesabovetheLCL17:53BST0.5°reflectivity(yellowthinlinerepresentthepositionofverticalcrosssection)17:53BSTThesooneryoucankeyinonwarmrainprocessdominatedconvection,thequickeryoucanswitchtotropicalZ/Randhelpdetectflashflood-producingrainAnticipatingDominantWarmRainProcessConvectionEnvironmentalassessmentismostimportant,thensupplementwithreal-timedatasignaturesAssesstheenvironmentJinan2007071808Jinan2007071814Jinan2007071820Xintai2007071808Zhengzhou2007071808ComparisonofwarmrainconvectionwithintenseconvectionWarmRainProcesses

Real-TimeDataSignaturesLookforreflectivitystructures,satellite,andlightningdataHighreflectivityconcentratedbelowthein-cloudfreezinglevel(lowechocentroid)IRSatellite:Cloudtops“warm”LackofsignificantlightningWarmRainProcesses

RadarSignaturesKansasTurnpikeFlashFloodAug30,2003LEC17:53BST0.5°reflectivity(yellowthinlinerepresentthepositionofverticalcrosssection)17:53BST對流單體6時04分和6時10分剖面圖WarmRainProcesses

ARecentExampleCrossSectionthroughWarm-RainSupercellBasicprincipleChappell’sLaw-Part2DurationoftherainfallSizeoftherainfall“footprint”SpeedofmovementoftherainfallareaNotthesameasstormmovement!IndividualconvectivecellscanmovethroughaconvectivesystemDurationandintensityofrain1.0度速度圖FlashfloodthreatfromweaklyshearedcellsFF-typedurationscanresultfromindividualstormsthathave:Slowmovement(<10kts)Largeareasofhighrainfallratesalongthemotionvector,celltraining,orAlloftheseRainfalldurationsfromindividualstormsdependon;(1)slowmovementand(2)largeareaofhighrainfallrates.070021.01.27.591070021.02.27.591070021.01.19.591070021.02.19.591070021.03.27.591070021.03.19.591070021.00.37.591070021.00.41.591070021.00.57.591072422.01.27.591072422.02.27.591072422.01.19.591072422.02.19.591072422.03.27.591072422.03.19.591072422.00.37.591072422.00.41.591072422.00.57.591074206.01.27.591074206.02.27.591074206.01.19.591074206.02.19.591074206.03.27.591074206.03.19.591074206.00.37.591074206.00.41.591074206.00.57.591075355.01.27.591075355.02.27.591075355.01.19.591075355.02.19.591075355.03.27.591075355.03.19.591075355.00.37.591075355.00.41.591075355.00.57.591081703.01.27.591081703.02.27.591081703.01.19.591081703.02.19.591081703.03.27.591081703.03.19.591081703.00.37.591081703.00.41.591081703.00.57.591FlashfloodproducingconvectionSustainedconvectionisalwaysmulticellularSupercellstypicallyarenotefficientOftenmoverapidlySheared,highkineticenergyenvironments,oftenwithdeeplayersoflowhumidityabovetheboundarylayerNonsupercellstorms-bothdiscreteandlinearlyorganized“CellTraining”PRODUCESASEQUENCEOFCELLS,REPEATEDLYPASSINGOVERTHESAMEAREACellTrainingorRegenerationFactorsthatcanleadtoechotrainingorregenerationinthesameareainclude:Aslowmovinglow-levelboundaryorfrontAquasi-stationaryLLJAquasi-stationaryareaofupper-leveldivergenceAlow-levelboundaryalmostparalleltothemeanflow(sourceofmoistureconvergence)Alackofstrongverticalwindshear

TrainingechoTrainingechoSystemMovementNotsurprisingly,slowmovingsystemsusuallyproducetheheaviestrainfall!Forveryshort-termforecasts,extrapolationbasedonradarandsatelliteprovideprimaryguidanceForlongertermforecasts,modelsprovidedecentguidanceButyoustillneedtoconsidermodelcharacteristicsandlimitationsCellMovementandPropagationSystemmovementdependsoncellmovementandpropagationIndividualconvectivecellsusuallymoveataround90%ofthemeanwindwithaslightdeviationtotherightPropagationisaffectedbyhowfastnewcellsformalongsomeflankofthesystemUsingRadarImagery(cont.)AnimatingimageryisusefulforDeterminingcell/systemmovementDeterminingifcellsaretrainingDiscussionQuestion

Whataresomeoftheradarlimitationsthatcanadverselyimpactyourdecisionmakingduringaheavyprecipitationevent?

AttenuationAnomalousPropagation(AP)–beampropagationRange–radarhorizonGroundclutterPoorcalibrationBeamblockageImproperZ/Rrelationship(algorithm)Hailcontamination(algorithm)BelowbeamaffectsBrightbandcontaminationPowerfailure,equipmentfailure

RainfallEstimationLimitationsBrightbandContaminationRadarCenteredArchofHigherRainfallAccumulationsonprouduct.OverestimaterainfallButraretoaffectconvectiveflashfloodingeventsRainf