Mechanisms and Variability of Tropical Cyclone Formation under the Upper-Tropospheric Influence

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).Hurricanes and tropical storms have a variety of different pathways to development. Significant attention has been paid to storms developing from tropical waves crossing the very warm waters of the tropical Atlantic Ocean. However, less attention has been paid to tropical transition (TT) storms, which form over cooler sea surface temperatures and under stronger winds aloft. These complex storms are nonetheless potentially very impactful to society because they develop closer to the southeastern US coastline and are generally less predictable. This award is for the study of these tropical transition storms. The intended output from this study will be improved predictive skill of TT storms which is important for protection of life and property. The project will also support graduate students, ensuring the training of the next generation of scientists.Tropical Transition (TT) storms form due to the interaction of an upper-level trough and a low-level disturbance outside the Main Development Region (MDR), usually in cooler waters. Two competing influences impact TT storms; high vertical wind shear that hinders tropical cyclone development, and quasi-geostrophic ascent that promotes convection and tropical cyclogenesis. This project will test a set of hypotheses centered on the dynamic, thermodynamic, and convective evolution leading to the formation of a TT storm using observational data (track, reanalysis, satellite) and high-resolution numerical modeling simulations using the WRF-ARW model. The four hypotheses to be tested are: H1: Convective evolution at the first stage of TT is largely controlled by the balanced ascent of the 3D synoptic flow, which is affected by both the upper-level and the low-level disturbances, H2: The interaction between an upper-level potential vorticity (PV) anomaly and a low-level disturbance creates a pouch-like structure (i.e., a quasi-closed Lagrangian circulation) in the lower and mid-troposphere, H3: The formation of the pouch-like structure marks the onset of the second stage of TT, during which the tropical cyclone formation proceeds with the same mesoscale processes as in MDR storms and differential moistening within the pouch plays an important role in convective organization, and H4: A new Genesis Potential Index (GPI), which considers both the upper- and lower-tropospheric conditions, will better represent TT storms than the current GPI formula and can help better understand and predict the variability of TT storms.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.