Numerical and Observational Studies of UTLS Potential Vorticity Dipoles in Midlatitude and Tropical Cyclones

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Midlatitude cyclones (MCs) and tropical cyclones (TCs) account for the major storms that greatly impact society. Their complex evolution can be influenced by atmospheric mesoscale motion in the upper troposphere and lower stratosphere (UTLS). The investigator has found that strong convective updraft within these cyclones can induce counter rotating air circulations, known as potential vorticity (PV) dipoles, in the UTLS. Near the top of the storms, these dipoles create horizontal flow enhancement (or "jetlets") and strong perturbations that can significantly modify the storm evolution and precipitation amount. This study aims to examine these ULTS features related to MCs and TCs. Emerging results can improve our ability to forecast major storms, helping to minimize their impacts. In particular, the negative anomaly of the PV dipole is inertially unstable and can promote precipitation and alter the evolution of mesoscale and synoptic systems. Coupled to the dipole orientation, the jetlets appear to form as a result of the difference in horizontal speed in the updraft compared to that of the surrounding UTLS air. The investigator hypothesizes that, in MCs, the jetlet will oppose the ambient flow, with the negative PV anomaly lying to the left of the flow. While in TCs, the jetlet will be in the same sense as lower tropospheric cyclonic flow, with the inertially-unstable member lying radially outward. To test this hypothesis, the proposal will carry out numerical and observational studies of the PV dipoles in the UTLS for a wide range of MC and TC cases, and will explore their dynamics, occurrence, and relationship with severe weather. The investigation will focus on four linked areas: (1) dynamics of PV dipoles, (2) climatological aspects of PV dipoles, (3) UTLS PV and extreme flooding events in the Upper Midwest, and (4) comparison of PV dipoles in mesoscale numerical simulations with auxiliary observations.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.