简介：Leaderpropagationisafundamentalissueinlightningphysics.Thepropagationcharacteristicsofpositiveleadersandnegativeleadersaresummarizedandcomparedbasedondatafromhigh-speedcameraandelectromagneticfieldinrocket-triggeredlightningandtower-initiatedlightningdischarges;availablechannelbasecurrentdatarecordedinrocket-triggeredlightningarealsoused.Thenegativeleaderspropagateinasteppedfashionaccompaniedbymanybranches.Thestemsaheadofthenegativeleadertipdeterminethemanneranddirectionoftheleaderpropagation,andeventhebranchingandwindingofthelightningchannel.Theimpulsivecurrent,electromagneticfield,andrelatedopticalimagessuggestthatthepositiveleadermaydevelopinastep-likefashionatitsinitialstageoftriggeredlightning.However,thesteppingprocessesofthepositiveleaderareobviouslydifferentfromthoseofthenegativeleader.Tower-initiatedlightningrevealedthatthemostconspicuouscharacteristicsofthestepwisepositiveleaderinvolvetheintermittentbrush-likecoronazoneinfrontoftheleadertipandtheluminosityenhancementofthechannelbehindthetip.Inrockettriggeredlightningflashes,thechargetransferredduringanindividualstepforthenegativeleaderwasnearlyanordergreaterthanforthepositivecounterpart.Thesuccessivestreamersaheadoftheleadertipareessentialforbothnegativeandpositiveleaderpropagation,andthestemscouldbeformedfromoneormorestreamersinthepreviousnegativestreamerzonewiththemainleaderchanneldim.High-resolutionobservationoftowerlightningalsorevealedanewtypeofbidirectionalrecoilleader,withpolaritycontrarytothetraditionalone,traversinginnegativechannelsassociatedwithtower-initiatedandrocket-triggeredlightning.

简介：Irregularsurfaceflattening,whichisbasedonaboundaryconforminggridandthetransformationbetweencurvilinearandCartesiancoordinatesystems,isamathematicalmethodthatcanelegantlyhandleirregularsurfaces,buthasbeenlimitedtoobtainingfirstarrivalsonly.Bycombiningamultistageschemewiththefast-sweepingmethod(FSM,themethodtoobtainfirst-arrivaltraveltimeincurvilinearcoordinates),thereflectedwavesfromacrustalinterfacecanbetracedinatopographicmodel,inwhichthereflectedwavefrontisobtainedbyreinitializingtraveltimesintheinterfaceforupwindbranches.Alocaltriangulationisappliedtomakeaconnectionbetweenvelocityandinterfacenodes.Thenajointinversionoffirst-arrivalandreflectiontraveltimesforimagingseismicvelocitystructuresincomplexterrainsispresented.Numericalexamplesallperformwellwithdifferentseismicvelocitymodels.Theincreasingtopographiccomplexityandevenuseofahighcurvaturereflectorinthesemodelsdemonstratethereliability,accuracyandrobustnessofthenewworkingscheme;checkerboardtestingillustratesthemethod’shighresolution.Noisetolerancetestingindicatesthemethod’sabilitytoyieldpracticaltraveltimetomography.Furtherdevelopmentofthemultistageschemewillallowotherlaterarrivalstobetracedandusedinthetraveltimeinversion.