Theformationofatropicalcycloneistheresultofaprocessinwhichaninitialdisturbanceevolvesintoawarm-corelow-pressuresystem;however,theoriginoftheinitialdisturbanceandthefeaturesoftheinitialfieldsareoverlookedinmostexistingtheories.Inthisstudy,basedonFY-2CbrightnesstemperaturedataandtheJapanreanalysisdataset,theoriginandevolutionofthetropicaldisturbancethatbecameTyphoonFung-Wong(2008)wereexamined.Theresultsdemonstratedthattheinitialdisturbanceemergedwithinasaddle-typefieldwithlargeverticaltroposphericwindshear.Theverticalwindsheardecreasedwiththeadjustmentoftheuppercirculation;moreover,accompaniedbyconvectionoverthewarmsectionaroundtheuppercoldvortex,itprovidedfavorablethermalanddynamicconditionsforthedevelopmentofatropicalvortex.Duringitsdevelopment,thezoneofassociatedpositiverelativevorticitystrengthenedanddescendedfromthemid-tropospheretolowerlevels.Thisrapidstrengtheningoflower-levelvorticitywasduetoincreasingconvergencerelatedtotheintensificationofthepressuregradientsouthwestofthesubtropicalhigh.ThisindicatedthattheuppercoldvortexandWestPacificsubtropicalhighplayedveryimportantrolesinthiscase.
