Dynamical Aspects of Typhoon Formation

[摘要] An attempt is made to explain the dynamical mechanism on formation of typhoons.The process of formation is regarded as transformation process or destabilization process of a large-scale pre-existing disturbance into a violent free convection.Chapter 1 is devoted to a general discussion on various free and forced convections in the atmosphere and their development. Especially, the growth rate of free meridional circulations in a circular vortex under the unstably and⁄or the stably stratified atmosphere is obtained as a function of their horizontal scales. It is shown that the existence of radial temperature gradient with weak but stable stratification causes a situation in which the first horizontal mode of the convection in the vortex develops most rapidly. The mechanism of instability is essentially the same as that observed in violent Hadley type convection in rotating dishpan experiments, which has been discussed theoretically by Kuo.In Chapter 2, an explanation of the process taking place in actual typhoon formation is made, where the whole process is divided into three stages:(i)the existence of dynamically caused vertical motion with an easterly wave, (ii) the excitation of numerous cumulus convections within the disturbance and the warming at the center of disturbance due to liberation of latent heat of condensation, (iii) the transformation of the disturbance into a large violent free convection due to radial temperature gradient produced.The conclusion is that the condensation process does not contribute to induce the primary vertical circulation in typhoons even at the formative stage, but that its importance lies in creation and maintenance of the warm core of the typhoons.

[学科分类] 大气科学