A Detailed Analysis of Typhoon Formation

Abstract

The formation of typhoon Doris 1958 which appeared in the MarshallIslands area is described from the stage of an easterly wave undera large high-level anticyclonic cell to the stage of an incipientvortex. It is revealed that the initial easterly wave possessed afairly large horizontal extension of strong wind region, togetherwith a systematic vertical motion. The formation of a thick moistlayer and the variation of thermal structure of the disturbance fromof a cold- to of a warm-core type are clearly shown by the analysis.Quantitative discussions of the three-dimensional distributions ofvorticity, horizontal divergence and vertical velocity are made.The mechanisms of increase of vorticity at low levels, and of maintenanceof negative vorticity at high levels are also discussed.The analysis reveals that the warming of ascending air was the resultof liberation of latent heat of condensation. This, coupled withthe cooling of surrounding air due to evaporation, created a horizontaltemperature gradient of the warm-core type. The importance of thebaroclinity thus formed for typhoon development may be supportedby th observed fact that rapid fall of the sea-level pressure tookplace after the completion of the warm-core structure.Based on the results of analysis, a model of typhoon formation isdescribed, in which the whole process of the formation is dividedinto three stages. Some preliminary remarks on dynamical processesin these stages are also mentioned.