Balanced dynamics in tropical cyclones

Abstract

The Earth’s atmosphere is generally a stochastic dynamical system in which physical processes take place within a wide range of temporal and spatial scales. Dynamical systems associated with small-scale perturbations often have a characteristically low predictability due to our lacked understanding of various physical processes and their feedback mechanisms. In contrast, the atmospheric motion at the meso- to synoptic-scales tends to possess a slow manifold with more predictable behaviours. It is this slow manifold component, often referred to as the balanced flow to be distinguished from the fast smaller-scale motion, that is more of our interest because it plays a critical role in evolution of common weather systems such as midlatitude baroclinic disturbances, mesoscale convective systems (MCSs), or tropical cyclones (TCs). Any weather system that differs too far from a balanced state will undergo a brief period of rapid adjustment, i.e., the so-called adjustment processes. From the balanced perspective, the evolution of mesoscale and larger-scale systems can be viewed as a series of continuous balanced adjustments, often referred to as the quasi-balanced dynamics. Herein, the quasi-balanced dynamic is simply that the mean flows are in a near-balanced state, but the (weak) superimposed perturbation flows are not, which are often related to the development of secondary circulation beyond the framework of balanced appropriations.