Jun 27, 1998 - Using data from the 11 cm, CSU-CHILL multiparameter radar, the simultaneous ..... vide information on the size, shape, orientation, and thermo- dynamic phase of .... at 500 mbar in excess of 20 m s -⢠with weak temperature and ...... NLDN95: A combined technology upgrade of the U.S. national network ...
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 103, NO. D12, PAGES 13,979-14,000, JUNE 27, 1998
Electrical and multiparameter radar observations of a severe hailstorm
Lawrence D. Carey and StevenA. Rutledge Departmentof AtmosphericScience,ColoradoState University,Fort Collins
Abstract. Using data from the 11 cm, CSU-CHILL multiparameterradar, the simultaneousevolutionof the radar-inferred precipitation structureand electrical characteristicsof a severehailstormis investigated.We comparethe substormpoint dischargecurrent,intracloud(IC) lightningflashrate, cloud-to-ground(CG) lightning flash rate, groundstrike location,and flash polarity to the progressionof precipitation typesand amounts.This studyis unique in that it presentsmultiparameterradar observationsof a thunderstormcomplexwhich exhibitedan extremelyhigh IC-to-CG ratio (IC/CG, 20-70) and predominantlypositiveCG lightning(over 74%) after it became severe,producinglarge hail and weak tornadoes.In particular,we investigatethe reported relationshipbetweenlarge hail and positiveCG lightning.Although a direct correlationis found betweena rapid increasein IC/CG, the first positiveCG lightningflashes,and the initial productionof hail aloft, the temporal and spatialbehaviorof large hail and positive CG lightningappearto be anticorrelated,as broad peaksin the positiveCG flashrate lag relative maxima in the fallout of large hail by up to 30 min. The majority of positive ground strikeswere adjacentto the main precipitationcore in a region of light rain and little or no hail at the surface.Aloft• radar data indicatedthat ice c•t•l• ,•,•,-• aligned verticallyin a strongelectricfield. Corona point observationsat the surfaceindicate that theseregionsadjacentto the convectivecore were characterizedby net positivecharge aloft. Possiblemechanismsto explaintheseobservationsare discussed. 1.
Introduction
More than 90% of cloud-to-ground(CG) lightningin warmseasonthunderstormsare of negativepolarity [Fuquay, 1982; Orvilleet al., 1987;Reap and MacGorman, 1989;Orville, 1994; Orvilleand Silver,1997]. As a result, the number of negative CG lightningflashestypicallydominatesthe number of positive CG flashesat any given stageand location in an ordinary (i.e. nonsevere)summertimethunderstorm.Exceptionsto this situationhave been noted in a few scenariosduring the warm season.(1) The dissipatingphase(i.e., verticalmotionscharacterizedby precipitation-filleddowndraftsonly) of someordinary thunderstorms can be dominated by positive CG flashes,althoughthe flashdensityis usuallysmall[e.g.,Fuquay, 1982].(2) The trailingstratiformregionof mesoscaleconvective systems(MCSs) [e.g., Rutledgeand MacGorman, 1988; Rutledgeet al., 1990;Engholmet al., 1990; Stolzenburg, 1990] andthe anvilregionof someseverestorms[Rustet al., 1981a,b] are dominatedby positiveCG lightning,while the deep convectiveregionsof these stormsare still characterizedby predominantlynegativeCG lightning.The positiveCG flashesin these storms occur over a large area during the course of severalhourssuchthat the positiveCG flashdensityis typically
low(2 cm) and sometimestornadoesare often characterizedby a predominanceof positive CG lightningfor extendedperiodsof time (->30 min) duringthe mature phase [e.g., Rust et al., 1985; Reap and MacGorman, 1989; Branick and Doswell, 1992; Curran and Rust, 1992; Seimon,
1993; MacGorman and Burgess,1994; Stolzenburg,1994]. In these severe thunderstormsthe positive CG lightning flashes are clusteredin time and spacein or near convectiveregions similar to negative CG flashesin ordinary storms and have comparableflash densities. There are currentlyfew detailed casestudiesof the coevolving precipitation structure and lightning behavior in severe convection.MacGorman [1993] reviewedearlier studieswhich suggestedthat many severe stormsare characterizedby enhancedintracloud(IC) lightningflashratesand high percentagesof positiveCG lightningdischarges.More recently,Branick andDoswell[1992]and CurranandRust [1992]investigated the occurrenceof dominant positive CG lightning activity in low-precipitation(LP) supercelIswhich producedlarge hail and tornadoes.Seimon[1993] analyzedvisualand radar observations of a tornadic F5 supercellwhich produced primarily positive cloud-to-ground lightning during the development phaseand which exhibiteda reversalin dominantCG polarity from positiveto negativeat the time of tornado touchdown. MacGorman and Burgess[1994] and Stolzenburg[1994] have surveyeda large number of cases(15 and 24, respectively)of predominantlypositive CG lightning producingstormswith lightningmappingnetworks,storm reports, and limited radar data. These studieshave confirmed an intriguing and robust one-waycorrelationbetweenhigh positiveCG flash densities and severeweather. Although not all severe stormsproduce predominant positive CG lightning, the majority of storms
13,979
13,980
CAREY AND RUTLEDGE: RADAR OBSERVATIONSOF A SEVEREHAILSTORM
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Figure1. Depiction of theobservational network usedin thisstudy. Locations of the11cm,dual-polarized CSU-CHILL, andDenverWSR-88D radarsare indicated.The positionof the mobilecoronapointsensoris shownabout19km to the southwest of the CSU-CHILL radar.The maximumoperatingrangeof thefiat plate
antenna(locatedat the CSU-CHILLradar)andthe coronapointsensor(locatedabout19 km to the southwest of theCSU-CHILLradar)aredepicted bydashed ringsof 40 and15km,respectively. Thelocation oftheclosest NLDN (National Lightning Detection Network) LPATS(lightning position andtracking system) station isdepicted bya triangle. Hatched areasrepresent regions of stormechoexceeding 55dBZat 1812and 1952 MDT.
periods.In addition,the intracloudlightwhichare characterized by highpositiveCG flashdensities (in ing 6 min averaging near1 s-• andtheIC/CGexceeded 20. excess of negativeCG flashdensities) duringthe maturephase ningflashratepeaked and are associatedwith severeweather suchas large hail and tor- At the sametime,the stormproducedmultipletornadoes nadoes. large hail up to 5 cm in diameteras verifiedby chasevans However, the abovestudieshave demonstratedthat there is workingwith the CSU-CHILL radar [Hubbertet al., 1997]. collectedby the CSUalsosignificant variabilityin the kinematic,microphysical, and Multiparameterradar measurables electricalattributesof thesestorms.As suggested by MacGor-
man andBurgess [1994]"... the nextobviousstepin studying positivegroundflashes in severestormsisto examinerelationshipswith stormevolutionmorecompletely." Thereforethe purpose of thispaperisto presentdetailedobservations of the coevolving precipitation structure,surfaceelectricfield, and lightningactivityassociated with a severehailstormoccurring
CHILL radar allowedinferencesregardinghydrometeortypes
(suchaslargehail) andprecipitation amounts (bothrainand hail rates)to be made. As a result,we are able to investigatethe reportedassocia-
tion of positivegroundflashesandenhanced intracloudlightningflashratesto the occurrence of severeweather,suchas largehail, usinguniquemultiparameter radaranalyses of the in eastern Colorado on June 7, 1995. Over 3 hours of CSU- three-dimensional precipitationstructure.In section2, we methodsusedin CHILL multiparameter radar (S band, 11 cm), observationsbrieflyreviewthe datacollectedandanalysis radartechniques with high temporalresolution(-
