Dynamical Aspects of Typhoon Formation

Abstract

An attempt is made to explain the dynamical mechanism on formationof typhoons.The process of formation is regarded as transformationprocess or destabilization process of a large-scale pre-existingdisturbance into a violent free convection.Chapter 1 is devoted to a general discussion on various free and forcedconvections in the atmosphere and their development. Especially,the growth rate of free meridional circulations in a circular vortexunder the unstably and/or the stably stratified atmosphere is obtainedas a function of their horizontal scales. It is shown that the existenceof radial temperature gradient with weak but stable stratificationcauses a situation in which the first horizontal mode of the convectionin the vortex develops most rapidly. The mechanism of instabilityis essentially the same as that observed in violent Hadley type convectionin rotating dishpan experiments, which has been discussed theoreticallyby Kuo.In Chapter 2, an explanation of the process taking place in actualtyphoon formation is made, where the whole process is divided intothree stages:(i)the existence of dynamically caused vertical motionwith an easterly wave, (ii) the excitation of numerous cumulus convectionswithin the disturbance and the warming at the center of disturbancedue to liberation of latent heat of condensation, (iii) the transformationof the disturbance into a large violent free convection due to radialtemperature gradient produced.The conclusion is that the condensation process does not contributeto induce the primary vertical circulation in typhoons even at theformative stage, but that its importance lies in creation and maintenanceof the warm core of the typhoons.