A minimal axisymmetric hurricane model

Abstract

Abstract Solutions of an axisymmetric version of the minimal three‐dimensional numerical model of a tropical cyclone developed by Zhu et al. (2001) are described and compared with those of the three‐dimensional model. Vortex evolution is similar in the two models during the early stages of intensification, but the period of rapid intensification occurs earlier in the axisymmetric model due to the higher effective resolution obtained using a staggered grid. There are marked differences at later times, when, in the three‐dimensional model, asymmetric structures develop. The findings are compared with those of an earlier study by Anthes (1972). The axisymmetric model is used to investigate certain fundamental aspects of tropical‐cyclone dynamics, including the emergence of a region of supergradient winds in the boundary layer and the evolution of regions satisfying necessary conditions for inertial and barotropic instability. Supergradient winds develop in the boundary layer within a radius of about 100 km of the vortex axis at an early stage of evolution and appear to be a natural feature of the vortex boundary layer. The development of flow regions satisfying necessary conditions for inertial and barotropic instability occur later, and may be attributed inter alia to the upward transfer of air with relatively high angular momentum, from the boundary layer to the middle and upper layers, by the secondary circulation of the vortex, and the downward transfer of air with relatively low angular momentum to the middle layer. A linear analysis of a two‐layer slab‐symmetric flow suggests why inertial instability does not occur in the axisymmetric model. Barotropic instability does not appear to be the mechanism responsible for the growth of asymmetries in the calculations using the three‐dimensional version of the model. Copyright © 2002 Royal Meteorological Society.