AnumericalsimulationofatorrentialraineventoccurringintheJiang-HuaiValleyofChinafrom22-24June1999isperformedandanalyzedbyusingthePSU/NCARMM5mesoscalenon-hydrostaticmodel.Thehigh-resolutionmodeloutputdataareutilizedtodiagnosethedoublefrontstructure,andthedistributionsofpotentialtemperature,equivalentpotentialtemperature,andspecifichumidityinthevicinityoftheMeiyuFrontSystem(MYFS)intheJiang-HuaiValley.TheresultsshowthatboththepotentialtemperaturegradientandthespecifichumiditygradienthaveimportantimpactsonthetwostrongequivalentpotentialtemperaturegradientzonesassociatedwiththedoublefrontstructureoftheMYFS,butthelatter(moisturegradient)ismoreimportant.Inaddition,thetendencyequationofspecifichumiditygradientistheoreticallyderived.Itshowsthatvariationsofthespecifichumiditygradientarerelatedtotheadvection,convergence/pergence,horizontalandverticalvorticities(secondarycirculation)effectsandthegradientofwatervaporsource/sink.Asanexample,thebudgetofthemeridionalcomponentofthetendencyequationisselectedanddiagnosedbyusingtheabovemodelsimulationdataofthetorrentialrainevent.Itisshownthatthevariationofthespecifichumiditygradientaveragedthroughoutthesimulationismainlycontrolledbytheconvergence/pergenceeffect,thesecondarycirculationeffectassociatedwiththehorizontalvorticities,andthewatervaporsource/sinkeffect.Sincethewatervaporsource/sinkisoftenformedfromthephasechangeprocessesofwatervaporintheairandthusdirectlyassociatedwithcloudandprecipitationmicrophysicsprocesses,thevariationofthespecifichumiditygradientiscloselyrelatedwithcloudandprecipitationmicrophysicsandthedistribution,developmentandevolutionofcloudandrainfallsystems.ThedoublefrontstructureoftheMYFSprovidesanadvantageousenvironmentalconditionforthedevelopmentandmovementofthemesoscaletorrentialrain
