Improved understanding of tropical cyclone lifecycle in realistic simulations through the perspective of eyewall ventilation and boundary layer mass flux

An improved understanding and forecast of tropical cyclone intensity changes requires in part a better diagnosis of their internal dynamics. Recent contributions notably relate the evolution of maximum tangential winds to the relative changes in eyewall and boundary layer mass fluxes. The present work investigates the interaction between the boundary layer inflow, eyewall convection and maximum wind evolution using realistic, high-resolution numerical simulations with a full-physics flow configuration. An in-depth analysis of a two-weeks long simulation of Hurricane Irma (2017) shows that intensification occurs when the increase of the boundary layer inflow is met with a concurrently increasing upward mass flux in the eyewall. Conversely, maturity and decay exhibit a gradual decline of this eyewall upward mass flux, and the subsequent generation of outflow above the boundary layer near the eyewall. Novel diagnostic variables are designed to quantify the upward mass flux ascending out of the boundary layer, and the fraction of this mass flux successfully ventilated to the upper troposphere by the eyewall convection. The joint evolution of these two quantities is shown to go with intensity changes, consistently over the eight simulations. The virtuous cooperation between an increasing boundary layer inflow and a positive mass flux ventilation in the eyewall is systemically associated with intensification, while the opposite detrimental cooperation characterizes intensity decays. A minority but notable amount of simulated life cycle phases show conflicting boundary layer and eyewall ventilation dynamics, and depart from the classical axisymmetric vortex model that conditions intensification on the import of cyclonic absolute vorticity above the boundary layer. These cases exhibit different pathways of intensification or decay, but remain constrained by the classical model, in part due to their inherently transient nature. This study highlights the importance of considering the eyewall ventilation along with boundary layer dynamics to diagnose the intensity variations of tropical cyclones.