简介：AquantitativediagnosisiscarriedoutfortheupwardbranchofalocalmeridionalcirculationoversouthernChina(SC)duringtheabnormalsnowstormswithseverefreezingrainfrom10Januaryto3February2008.Thediagnosticstudyshowsthattheupwardbranchismainlyassociatedwiththezonaladvectionofwesterlymomentumandmeridionaltemperatureadvectioninsteadofthelatentheating(whichiscommonlythedominantfactorinmanyotherstormcases).Thecorrespondingweatheranalysesindicatethat(1)thezonaladvectionofwesterlymomentumrepresentstheeffectoftheupper-leveldivergenceontheanticyclone-shearsideintheentranceofa200hPawesterlyjetwithawestwarddeviationfromitsclimatologicallocationoversouthwesternJapan;(2)themeridionaltemperatureadvectionrepresentstheinteractionbetweenthemid-lowerlayer(850to400hPa)warmadvectionoverSC(aheadoftemperatureandpressuretroughswiththelattertroughdeeperthantheformerintheBayofBengal)andcoldadvectionovernorthChina(steeredbyanunderlyingflowat500hPa);(3)therelativelyweakvaportransport(comparedtothatofspring,summerandautumn)fromtheBayofBengalandtheSouthChinaSeatoSCandtheexistenceofatemperatureinversionlayerinthelowertroposphereoverSCdiminishtheeffectoflatentheating.Withthesignificantincreaseofvaportransportafter24January,theroleoflatentheatingisupgradedtobecomethethirdpositivecontributortotheupwardbranchoverSC.