Asymmetric Boundary Layer Dynamics in Tropical Cyclones

This thesis aims to investigate asymmetric boundary layer dynamics of tropical cyclones using convection-permitting ensemble forecasts to address the relatively slow progress of intensity forecast improvements. Tropical cyclones are regularly approximated as being axisymmetric, although storm-scale asymmetries and smaller-scale stochastic processes can contribute to intensity change. This thesis presents contributions to the study of asymmetric tropical cyclone boundary layers, including improved use of observations, development of full-physics model parametrisations, and development of new theories to explain asymmetric dynamics. First, we develop a novel framework for evaluating the 3D structure of tropical cyclones in forecasts, focusing on azimuthal variability, rather than the simple surface based metrics typically used operationally. Next, we show that reducing the turbulent mixing in the free troposphere of asymmetric, sheared tropical cyclones can improve intensity forecasts by contributing to a more efficient downdraft ventilation weakening process, which offers a mechanism by which free-tropospheric turbulence can impact intensity change. Finally, we examine the asymmetric boundary layer wind structure of a landfalling tropical cyclone, showing that large-scale vorticity gradients can produce external forcing which leads to the asymmetric re-distribution of winds, and rainbands can also exert their own influence on the primary and secondary circulations. This thesis demonstrates the significance of different asymmetric boundary layer structures in tropical cyclones, and their impact on flow and intensity, which contributes to the expanding research on representing complex storm dynamics.