Abstract/Summary

Tropical cyclone (TC) intensification is challenging to represent in numerical models because it results from complex interactions between multi-scale processes, some of which are poorly understood. This thesis investigates TC development in an idealised axisymmetric model in terms of moist Available Potential Energy (APE) density, and suggests how diagnostics based on moist APE budgets could be applied to forecasting and global climate models to study the links between model parameterisations and the intensity of TCs. The first full budget of moist APE density for the atmosphere in any context is constructed for the axisymmetric model, using the model’s initial environmental sounding as a reference state. The main source of moist APE relative to the environment is the surface latent heat flux occurring in the low-level radial inflow. This APE is then transported inwards and converted into kinetic energy in the eyewall. The budget also reveals that APE density can be discontinuous in time and space in the moist atmosphere if air parcels possess multiple levels of neutral buoyancy, in which case a reservoir of latent APE may exist. The best APE reference state for predicting the domain-integrated rate of kinetic energy generation in a TC is shown to be a vortex in thermal wind balance. Surface fluxes are still the chief source of APE when this reference state is used. Precipitation and subgrid mixing also contribute to the kinetic energy generation. The intensification of TCs in the model, in terms of total power dissipation, is associated with an increase in the efficiency with which surface fluxes generate moist APE, even when the WISHE feedback is cut off so that the surface fluxes themselves do not increase. A two-part WISHE-efficiency feedback is proposed to drive intensification. It is also demonstrated that moist local available energetics can be used to derive a theory of potential intensity