A Comprehensive Observational Examination of the Physical Processes that Link Tropical Cyclone Vortex Alignment to Future Intensity Change

The vast societal impacts of rapidly intensifying tropical cyclones, or hurricanes, are exacerbated not only due to poor predictions that create challenges for emergency management decisions, but they also pose a significant threat to coastal populations as these storms quickly become more destructive. Prior to undergoing rapid intensification, which is defined as a quick or sudden increase in the strength of the storm, hurricanes develop from weaker tropical cyclones that initially are more susceptible to detrimental influences from the surrounding environment.

A significant environmental influence is vertical wind shear, which is a change in the direction or strength of the wind with increasing height, and can negatively impact the development of hurricanes. Vertical wind shear can cause a displacement of the mid–upper level storm circulation from the low-level circulation. This misalignment of centers allows more unfavorable, dry air in the surrounding environment to enter a storm’s inner core, a deterrent to further intensification.

Therefore, previous work strongly supports the notion that a key step for tropical cyclone intensification is achieving an alignment of centers throughout the depth of the atmosphere. However, it is still not well understood how or why some initially misaligned storms transition toward a more vertically-aligned state and rapidly intensify. In some recent, notable U.S. landfalling hurricanes, uncertainty over the processes associated with this structural transition has caused considerable forecast challenges, particularly with intensity change.

This project will use a comprehensive radar dataset to provide insights on the processes associated with the alignment of centers prior to and during rapid intensification.

Previous observational case studies and idealized modeling simulations have identified multiple pathways through which a misaligned tropical cyclone vortex can transition toward an aligned state. However, it is widely believed that convectively-driven diabatic processes are important in the alignment process, regardless of the pathway. Because all tropical cyclones feature convection to some degree, it is unclear how precisely precipitation structures associated with vortex alignment events differ from storms that remain misaligned.

This uncertainty is a key motivator for this study. Furthermore, because these different alignment pathways have only been documented either in idealized modeling systems or in observational case studies of individual storms, robust conclusions as to how precipitation, particularly in weak tropical cyclones, interacts with the surrounding environment and affects the vortex alignment pathway in nature remains ambiguous.

Therefore, this project aims to build on previous work by contextualizing the vortex, precipitation, and environmental characteristics associated with a given alignment pathway. The overall goal of this research is to investigate weak tropical cyclones using a novel airborne and ground-based Doppler radar database, which provides the most comprehensive observational database of tropical cyclone structure to date.

This assessment of precipitation structure at high resolution will help reveal the three-dimensional structure, from which the physical processes responsible for vortex alignment can be determined.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.