Structure of a Tropical Cyclone Developed in a Three-Dimensional Numerical Simulation Model

Abstract

A three-dimensional, 11-level, primitive equation model has been constructed for a simulation study of tropical cyclones. The model has four levels in the boundary layer and its 70?70 variable grid mesh encloses a 4000-km square domain with a 20-km resolution near the center. Details of the model, including the parameterization scheme for the subgrid-scale diffusion and convection processes, are described. A weak vortex in the conditionally unstable tropical atmosphere is given as the initial state for a numerical integration from which a tropical cyclone develops in the model. During the integration period of one week, the sea surface temperature is fixed at 302K. The central surface pressure drops to about 940 mb, while a warm moist core is established. The azimuthal component of mean horizontal wind is maximum at about 60 km from the center at all levels. A strong in-flow is observed in the boundary layer. At upper levels, a secondary radial-vertical circulation develops in and around the region of negative mean absolute vorticity. In the same region, the azimuthal perturbation of horizontal wind is pronounced. At the mature stage, the domain within 500 km radius is supplied with kinetic energy for asymmetric flow by both barotropic and baroclinic processes. At 60 km radius, the temperature perturbation field is maintained by condensation-convection heating at upper levels and by adiabatic temperature change due to vertical motion at lower levels. An area having an eye-like feature is found off the pressure center. Structure of spiral bands in the outer region is extensively analyzed. The phase relationship among the pressure, horizontal motion, vertical motion, temperature and moisture fields is discussed. The spiral band behaves like an internal gravity wave. Once the band is formed in an area surrounding the center, it propagates outward apparently without appreciable further supply of energy, as far as the present case is concerned.