Recent Observational Support for Schubert’s Tropical Cyclone Conceptual Frameworks

In seminal work, Wayne Schubert and co-authors developed key conceptual frameworks for our understanding of tropical cyclone dynamics, including novel potential vorticity (PV) concepts and potential radius coordinates. These frameworks generalized dynamical concepts from their mid-latitude origins such as barotropic instability and semi-geostrophic coordinates to the rotating tropical atmosphere. While these important concepts and theory have demonstrated qualitative agreement with observations in many studies, quantitative validation has been largely limited to numerical modeling studies due to difficulties in observing tropical cyclones. New observational capabilities have recently provided additional support for the validity of these frameworks for understanding tropical cyclone intensity and structure change.

Aircraft observations collected as part of the Office of Naval Research Tropical Cyclone Intensity (TCI) field experiment and NOAA Intensity Forecasting Experiment (IFEX) allow for calculation of axisymmetric PV at high resolution throughout the troposphere during the rapid intensification and weakening of Hurricane Patricia (2015). The analyses reveal the formation of a “hollow tower” PV structure as Patricia rapidly approached its peak intensity, consistent with previous theoretical and modeling studies. Patricia’s rapid weakening phase corresponded to a breakdown of this structure, and suggest a rearrangement of the PV in a manner consistent with mixing at the eye-eyewall interface. Further insights into Patricia’s structural and intensity changes are obtained by viewing the PV in isentropic and potential radius coordinates.

Satellite and radar imagery showed evidence of an evolving polygonal eyewall as Hurricane Michael (2018) underwent rapid intensification (RI) during its approach to Florida. While snapshots of polygonal eyewalls have been observed in previous hurricanes, the corresponding evolution of wind asymmetries has never been quantitatively deduced due to spatial and temporal sampling limitations from radar and satellites. Here we present the first observational evidence of the evolving wind field of a polygonal eyewall during RI to Category 5 intensity by deducing the axisymmetric and asymmetric winds from single-Doppler radar observations. A spectral time decomposition of the radar-retrieved winds suggests that the propagation speeds of the asymmetries are consistent with linear vortex Rossby wave theory on an amplifying vorticity gradient. These recent observations from Hurricanes Patricia (2015) and Michael (2018) provide new support for conceptual frameworks that have been developed by Schubert and co-authors over the past few decades.