Development of Asymmetries in a Three-Dimensional Numerical Model of the Tropical Cyclone1

Abstract

Notable asymmetric features of an early experiment with a three-dimensional hurricane model were spiral bands of convection and large-scale asymmetries (eddies) in the outflow layer. Using an improved version of the model, we describe the formation and maintenance of these features in greater detail in this paper. The spiral bands in the model propagate cyclonically outward in agreement with bands in nature. The breakdown of symmetry into a chaotic pattern of eddies in the outflow region is shown to be the result of dynamic (inertial) instability, with the eddy kinetic energy derived from the kinetic energy of the azimuthal flow. This instability does not contribute to the overall intensification of the model storm, however. We observe a curious anticyclonic looping of the vortex center in these experiments. This looping appears to be associated with asymmetries in the divergence pattern associated with the eddies in the outflow layer. This paper also summarizes improvements made in the original version of the model. In contrast to the earlier model, the current version contains an explicit water vapor cycle. A staggered horizontal grid is used to provide a higher resolution in evaluating the pressure gradient forces. Some of the pragmatic assumptions made in the earlier model, notably those involving horizontal diffusion of heat and momentum, have been eliminated in the current version.