Budget Analysis of a Tropical Cyclone Simulated in an Axisymmetric Numerical Model

Abstract

A tropical cyclone simulated in an axisymmetric numerical model is analyzed in detail from various aspects in order to deepen the understanding of the basic mechanisms of its evolution. Namely, the budget equations of temperature, moisture, relative angular momentum, vorticity, radial-vertical circulation, and kinetic energy are investigated for the different stages of the development of a tropical cyclone. The spatial distributions of each term in the budget equations are shown and their role in the following processes are discussed. In the pre-deepening stage of a large weak vortex in a conditionally unstable atmosphere, a solenoidal field is formed as a result of a delicate heat budget which depends on the static stability and the moisture content. The baroclinicity field thus established drives the system into a deepening stage. A positive feedback process builds up a warm moist core, accelerates the radial-vertical circulation, and intensifies the moist convection. A net outflow of mass from the central region and a resultant drop of central surface pressure take place during this period. The relative angular momentum of the inner column as a whole increases through convergence of relative angular momentum. In terms of relative vorticity, intensification and shrinking of the vortex is due to the combined effects of advection, horizontal convergence and twisting. At the end of the deepening stage, conditional instability in the central region is neutralized. The moment due to Coriolis force acting on the intensified azimuthal flow counterbalances the baroclinicity vector, so that the acceleration of radial-vertical flow ceases. Concentration of relative angular momentum and vorticity in the central region also levels off. In the budget of these quantities, the role of both vertical and lateral steam becomes important. In the troposphere, except the upper part and the boundary layer, the gradient wind relationship is established between the pressure field and the azimuthal flow. In the mature stage, the status in the inner region is quasi-stationary while that of the outer area keeps changing slowly. The importance of evaporation at the central area for the maintenance of an intense tropical cyclone is demonstrated in an additional experiment.