A Tale of Two Vortex Evolutions: Using a High-Resolution Ensemble to Assess the Impacts of Ventilation on a Tropical Cyclone Rapid Intensification Event

Abstract

Abstract The multiscale nature of tropical cyclone (TC) intensity change under moderate vertical wind shear was explored through an ensemble of high-resolution simulations of Hurricane Gonzalo (2014). Ensemble intensity forecasts were characterized by large short-term (36-h) uncertainty, with a forecast intensity spread of over 20 m s−1, due to differences in the timing of rapid intensification (RI) onset. Two subsets of ensemble members were examined, referred to as early-RI and late-RI members. The two ensemble groups displayed significantly different vortex evolutions under the influence of a nearby upper-tropospheric trough and an associated dry-air intrusion. Mid-to-upper-tropospheric ventilation in late-RI members was linked to a disruption of inner-core diabatic heating, a more tilted vortex, and vortex breakdown, as the simulated TCs transitioned from a vorticity annulus toward a monopole structure. A column-integrated moist static energy (MSE) budget revealed the important role of horizontal advection in depleting MSE from the TC core, while mesoscale subsidence beneath the dry-air intrusion acted to dry a deep layer of the troposphere. Eventually, the dry-air intrusion retreated from late-RI members as vertical wind shear weakened, the magnitude of vortex tilt decreased, and late-RI members began to rapidly intensify, ultimately reaching a similar intensity as early-RI members. Conversely, the vortex structures of early-RI members were shown to exhibit greater intrinsic resilience to tilting from vertical wind shear, and early-RI members were able to fend off the dry-air intrusion relatively unscathed. The different TC intensity evolutions can be traced back to differences in the initial TC vortex structure and intensity. Significance Statement Despite recent advances, tropical cyclone intensity forecasts struggle to accurately predict episodes of rapid intensification. Such forecasts become increasingly challenging when a storm is embedded within an environment of moderate vertical wind shear. This study uses an ensemble of high-resolution simulations to examine how environmental influences can affect the tropical cyclone vortex and precipitation structure, which, in turn, modulate the intensity of the storm and the onset of rapid intensification. We propose a feedback that exists where slightly weaker and less resilient vortices are more susceptible to ventilation from dry, environmental air, aided in part by differential advection from the tilted circulation, resulting in a degradation of vortex organization and a delayed onset of rapid intensification.